Microsoft Word - Front Matter V.doci
Soapstone Vessels in the Ohio River Valley and Determining their Source of Origin
Using Visible/Near-infrared Reflectance Spectrometry
degree of Masters of Arts
by
Approved by
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Spectral Variations……………………………………………………….51
IV. Data Analysis………………………………………………………………...68
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Soapstone Sources……………………………………………………….87
VIII. Conclusions……………………………………………………...………..190
REFERENCES CITED…………………………………………………………………197
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LIST OF FIGURES
1. Modal Mineralogy from Thin-section Analysis at the Vermont Marble Company
Quarry, Rochester, VT………………………………………………………..…19
2. Modal Mineralogy from Thin-section Analysis at the Newfane Soapstone Quarry,
Newfane, VT…………………………………………………………………….19
3. Modal Mineralogy from Thin-section Analysis at the Osborne Soapstone Quarry,
Blanford, MA……………………………………………………………………20
4. Modal Mineralogy from Thin-section Analysis at the Frost and Goodrich Quarries,
Grafton VT………………………………………………………………………20
5. Mean Mineralogy from Thin-section Analysis of Six New England Soapstone
Quarries………………………………………………………………………….22
8. Schematic of Intimate and Areal Mixtures……………………………………………60
9. Baseline Variability and Hull-Quotient Correction…………………………………...73
a. Baseline Variability in Three “Actinolites”………………………………...…73
b. Baseline Correction by Hull-Quotient Subtraction……………………………73
10. Representative Amphibole Spectra………………….……………………………….76
11. Derivation of a Linear Offset
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a. The Effects of Varying Baseline Slope on Apparent Band Position………...79
b. First Derivative Correction of Baseline Slope……………………………….79
12. Derivation and Feature Amplification
a. Two Gaussian Curves………………………………………………………..80
b. Feature Amplification Due to Derivation……………………………………80
13. Derivative Window Size……………………………………………………………..82
14. Map of Soapstone Sources..………………………………………………………….89
15. Detector Offsets…………………………………………………………………….108
16. Magnitude of Detector Offsets as a Function of Time……………………………..109
17. Methods of Compensating for Detector Offsets
a. Uncorrected Detector Offsets……………………………………………....111
b. Offsets Corrected by Averaging Method…………………………………...111
c. Offsets Corrected to First Detector…………………………………………111
d. Offsets Corrected to Middle Detector………………………………………111
18. Spectral Water Features in a Chloritic Soapstone……………………………..……115
19. Spectral Water Features in a Talcose Soapstone…………………………………...115
20. Comparison of Spectral Water Features in Soapstone and Soil……………………120
21. Time-lapsed Drying Experiment……………………………………………………118
22. Time-lapsed Drying Experiment II-Ratio of Wet/Dry Spectra……………………..123
23. Low Temperature (350°-725°C) Heat Alteration at the SR Quarry………………..123
24. High Temperature (850°-1025°C) Heat Alteration at the SR Quarry……………...120
25. Low Temperature (350°-725°C) Heat Alteration at the YC Quarry………………126
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26. High Temperature (850°-1025°C) Heat Alteration at the YC Quarry……………..126
27. Low Temperature (350°-725°C) Heat Alteration at the GQ Quarry……………….128
28. High Temperature (850°-1025°C) Heat Alteration at the GQ Quarry……………...128
29. Reflectance Maxima Histogram of the GQ Quarry………………………………...131
30. Spectral Effects Due to Sooting……………………………………………….……132
31. Spectral Effects Due to Different Grain Sizes in a Sample of Chlorite ………...….134
32. Intimate Mixtures of Different Proportions of Powdered Talc and Chlorite……….136
33. Intimate Mixture of Talc and Chlorite (2000-2500 nm Region)…………………...138
34. Band Positions and Depth in an Intimate Talc-Chlorite Mixture
a. Position of 1400 nm Band……………………………………………………139
b. Position of 2300 nm Band…………………………………………………...139
c. Position of 2400 nm Band……………………………………………………139
d. Depth of 2450 nm Band……………………………………………………...139
35. Comparison of Intimate and Areal Mixtures of Talc and Chlorite…………………140
36. Intimate Mixtures for Different Proportions of Powdered Talc and Magnetite..…..142
37. Comparison of Spectra for Pure Powdered Talc and Tremolite. ………... ………..144
38. Intimate Mixture of Powdered Talc and Tremolite (1200-2500 nm)……...……….144
39. Plot of Correlation Coefficients for the Talc-Chlorite Mixtures………………..….145
40. Examples of Specular Reflection…………………………………………………..147
41. Spectra of Non-soapstone Artifacts………………………………………………...150
42. Spectral Effects of Mineral Orientation in Anthophyllite…………………………..152
43. Map of Ohio Artifacts………………………………………………………………172
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44. Classification Results for Artifacts from the Ohio Valley Using the Quarry
Match Index (QMI)…………………………………………………………..…186
45. Classification Results for Artifacts from the Ohio Valley Using the Modified
Quarry Match Index (MQMI)…………………………………………………..187
LIST OF TABLES
1. Greenschist to Amphibolite Metamorphism at Four Ultramafic Bodies in New
England ……………..…………………………………………………………..18
2. The Precision Index and Sample Sizes Required for Parametric Statistics…………...30
3. Quarry Sample………………………………………….……………………………..90
6. Demonstration of the Method for Determining Match Indices……………………...153
7. Summary of the Results of Correlation Analysis……………………………………155
8. Coefficient of Variation for Source Samples………………………………………...158
9. Correlation Results (Green Tree Artifacts)…………………………………………..166
10. Correlation Results (Eastern Pennsylvania)………………………………………...167
11. Correlation Results (New England)………………………………………………..168
12. Revised Correlation Results (New England)………………………………………169
13. Radiocarbon Chronology for Important Transitional Archaic Sites in the Ohio River
Drainage………………………………………………………………………..174
14. Projectile Point Frequencies at Mountaintop Sites in Boone County, West
Virginia………………………………………………………………………...180
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ACKNOWLEDGEMENTS
I was truly surprised and impressed with the professionalism and the willingness
of so many people to cooperate with me on this thesis. Their dedication to advancing
knowledge, if even just a little, was a constant source of inspiration. Regretfully, space
limits those that I can mention here and I hope those I haven’t will understand that their
contributions are no less appreciated. However, several individuals deserve special credit.
First and foremost I am genuinely grateful to my advisor, Mark Seeman, not only
for sharing his wisdom and navigating me through this thesis, but at times for exhibiting
extraordinary patience, and when I needed it the most, encouragement. I couldn’t have
done this without Mark’s assistance, or indeed, his persistence. I also want to sincerely
thank Joseph Ortiz, of the Kent State Geology Department, for introducing me to the
capabilities of reflectance spectrometry and for fielding my constant barrage of questions
with equanimity and insight. The learning curve would have been much steeper if not for
Joe’s ability to simplify and elucidate the sometimes highly technical concepts involved.
My other committee members, Richard Meindl and Olaf Prufer, both provided valuable
suggestions and I appreciate the time and effort they expended in helping me improve
this final product.
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do this including Susan Haskell (Peabody Museum of Archaeology and Ethnology,
Harvard University), Brian Redmond and Mark Kollecker (Cleveland Museum of Natural
History), Richard George (Carnegie Museum of Natural History), Kevin Smith
(Haffenreffer Museum, Brown University), Chris Stevenson (Virginia Division of
Historical Resources), and Stephen Warfel (The State Museum of Pennsylvania). I would
especially like to thank a few people for providing selfless assistance beyond the call of
duty, including Alison Eichelberger (North Museum of Natural History and Science,
Lancaster, Pennsylvania) who acted as my tour guide at the Georgetown Quarry, Paul
Inashima (Washington Suburban Sanitation Commission) for field-collecting vital
geological samples, and James Richardson (Carnegie Museum of Natural History) for
driving all the way from Pittsburgh just so that I could analyze artifacts from his
museum’s collections.
Others that provided important information, documents, or samples were Mike
Anslinger, Jeb Bowen, Wayne Clark, Bob Converse, Doug Hooks, Jeanine Kreinbrink,
Bob Maslowski, Wayne Mortine, Ben and Pam Mullenix, Mike Murphy, James Payne,
Matthew Purthill, Ken Sassaman, David Smolinski, and Frank Vento. Ernest Carlson,
Rodney Feldman, and David Waugh of the Kent State Geology Department provided
other facilities and assistance.
Finally I would like to thank my family and friends for putting up with my
incessant preoccupation with soapstone for the past two years. Maybe now that I am done
with this thesis we can talk about something you find more interesting.
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Introduction
The origins of pottery, and by extension, stone bowls, have been of perennial
interest to archaeologists in the eastern United States and elsewhere around the world
(Barnett and Hoopes, 1995). Due to their weight, bulk, and fragility, these artifacts are
usually considered non-portable equipment and are thought to reflect the increasingly
sedentary lifestyles of those that adopt them (Catlin et al., 1982; Benison, 1999:303;
Smith, 1986:29-30; Custer, 1984; Brown, 1989:216). Sedentism in turn, has implied a
radical shift in subsistence base from the mobile hunter-gatherer existence that
characterized most of human prehistory, to settled life dependent upon localized and
predictable food resources, especially domesticated crops. This transition was the basis
for what V. Gordon Childe (1936) famously dubbed the “Neolithic Revolution” and has
been a subject of considerable interest ever since (e.g. Binford, 1968; Flannery, 1969;
Bender, 1985). Some of the effects ascribed to a reduction in mobility include “changes
in food storage, trade, territoriality, social and gender inequality, male/female work
patterns, subsistence, and demography as well as cultural notions of material wealth,
privacy, individuality, cooperation, and competition” (Kelly, 1992:43-4). Therefore, the
archaeological expectation is for durable vessels to be accompanied by other measures of
sedentism, complexity and agriculture.
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When the first attempts were made to impose a systematic temporal framework on
eastern United States prehistory (McKern, 1939; Griffin, 1946, 1952; Willey, 1966)
ceramics, above all material culture, were perceived to be diagnostic markers separating
Archaic hunter-gatherers from later Woodland horticulturalists. Other features such as
earthen mound-building and complex mortuary ritualism were also proposed as part of
the overall Woodland trait-complex. However, the advent of radiocarbon dating and the
accumulation of dates in the latter half of the Twentieth Century eventually led to the
revelation that several complex cultural groups previously assigned to the Woodland
Period were in fact much older (e.g. Poverty Point, Indian Knoll, Glacial Kame). It was
also during this time that American archaeology underwent a significant theoretical
reorientation that fostered an interest in the processes of cultural change. As there is
perhaps no better example of change in Eastern United States prehistory than that
representing the Archaic-Woodland transition, naturally the Late Archaic has become a
focal point for scholarly research.
Innovations attributed to the Late Archaic Period (ca. 6000-2500 B.P.) include
for the first time evidence for the use of durable containers made of ceramics and stone
(Custer, 1984; Sassaman, 1996), long-distance trade networks (Goad, 1980; Winters,
1968; Jeffries, 1996), complex mortuary ritualism (Abel et al., 2001; Pleger, 2000),
earthen and shell mound-building (Gibson, 2001; Russo, 2004), intensive shellfish
exploitation in some coastal and riverine settings (Braun, 1974; Turnbaugh, 1975; Catlin
and Custer, 1982), canoe use (Witthoft, 1953; Wheeler et al., 2003), incipient horticulture
(Yarnell et al., 1993), and a proliferation of new artifact types including ground stone
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tools, woodworking tools, personal adornments, smoking pipes, and the widespread
distribution of certain diagnostic bifaces called broadspears (Witthoft, 1953; Turnbaugh,
1975; Cook, 1976). Although still controversial we can also provisionally add
fundamental changes to settlement patterns with a greater emphasis on riverine and
coastal resources for both subsistence and communications.
It is now clear that the link between sedentism, pottery, and agriculture, as was
assumed in the earlier formulations of the Woodland Period, is extremely idiosyncratic.
Sedentary or semi-sedentary groups have been documented in the absence of agriculture
(Brown, 1985; Widmer, 1988) and the use of durable containers is widely known among
mobile hunter-gatherer groups (Arnold, 1985; Witthoft, 1953; Barnett and Hoopes, 1995;
Eerkens et al., 2002; Hoffecker, 2005). Yet despite a lack of any substantial agricultural
component to the Late Archaic subsistence strategy, there is still compelling evidence for
increasing social complexity. Soapstone vessels and broadspears have warranted
particular attention both for their pan-regional distribution during the Terminal or
Transitional Archaic (~4500-2500 B.P.; Witthoft, 1953; Ritchie, 1965) and for the
rapidity with which they became dispersed.
Broadspears, as the name suggests, are broad-bladed stemmed bifaces. The large
size and asymmetry of many broadspears suggest they were probably intended to be
hafted and used as knives (Cook, 1976). Other flaked artifacts in the toolkit at this time
include smaller more symmetrical broadspears (probably projectile points), cruciform
drills, scrapers, and strike-a-lights all with the same basic stemmed hafting design
(Dincauze, 1972:41; Ritchie, 1965:151). The earliest recognized broadspear is the
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Savannah River stemmed point which has been dated along the Georgia and South
Carolina coasts to between 4200-3800 B.P. (Ken Sassaman, personal communication).
The appearance of similar tools at the Hornblower II site on Martha’s Vineyard has been
dated to 4140±100 (Ritchie, 1969:54-55) and has been taken by Dincauze (1972) as the
earliest phase of the Transitional Period (Susquehanna Tradition) in New England.
Farther up the coast broadspears have been dated to about 3800 B.P. at the Turner Farm
site in Maine’s Penobscot Bay (Bourque, 1995:244-253), and at 4000±180 BP and
4010±180 BP in New Brunswick (Deal, 1986:72-8). Along with other dates summarized
in Turnbaugh (1975), the data supports a remarkably rapid appearance of broadspears up
and down the East Coast, all within just a few centuries. Soapstone bowls, often
associated with broadspears, don’t get added to the toolkit until approximately 3700 B.P.
(Kraft, 1970:55; Klein, 1997; Sassaman, 1999), although there is some controversy over
the earliest dates (c.f. Truncer, 2004; Sassaman, n.d). They too became widely adopted
more or less simultaneously throughout their range.
Explanations for this phenomenon can be distilled into two opposing viewpoints
with different emphases on the mechanisms of dispersal. The first stems from the
observation that soapstone vessel weight would have posed a substantial transportation
burden. The first person to put this concept to print probably was McGuire who in a
wonderful piece of hyperbole observed that soapstone vessels, “could not have been
transported any great distance from where they were manufactured, except with a greater
exertion of strength than was probably ever made” (McGuire, 1883:592). McGuire
posited canoes as the transportation mechanism as did the influential William Henry
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Holmes some years later (Holmes, 1893:106), but it really wasn’t until Witthoft (1953)
formalized the concept of the Transitional Archaic in eastern Pennsylvania that soapstone
vessels became viewed as part of a larger “riverine adaptation”. According to Witthoft,
“When people began to depend on canoes for transportation or to settle down in
permanent farming villages, pottery and stone cooking vessels became desirable. In the
Transitional Period, there developed a canoe-using way of life, in which people traveled
the streams feeding upon fish, wildfowl, and game killed at the waterfront. As this way of
life spread in the Northeast, the soapstone pot spread with it, carried hundreds of miles
from their places of origins in canoes. It would appear, from the distribution of sites and
the way the people carried stone for vast distances, that such scouring of the rivers by
fishers and hunters involved traveling great distances, and that the prosperity of such
communities depended upon their being able to hunt over hundreds of miles of river
every few months. In this fashion, the tool types and the economic pattern may have
spread very rapidly throughout the area” (Witthoft, 1953:25).
Mouer et al. (1981) proposed a very similar scenario for the James River in
Virginia, and along with Turnbaugh (1975) they saw Late Archaic cultural innovations as
stemming from the Savannah River area and spreading by a series of coastal and riverine
migrations. This idea was further developed in New England by Dincauze (1972) who
argued for an intrusion of people from the Susquehanna region forming one contiguous
cultural area extending from eastern Pennsylvania to southern Maine prior to ca. 3000
B.P.
The opposing view does not disagree with the fundamental proposition that
Transitional Archaic settlement patterns had shifted to aquatic resources, but that
mobility had become bounded by territorial constraints brought about by population
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pressures (Catlin et al., 1982). Under these conditions of reduced mobility access to
important raw materials and constricted foraging territories fostered an increasing
dependence on social alliances to obtain necessary goods and to share resources during
times of stress. In this context, innovations such as stone bowls served to underwrite
social interactions in addition to their utilitarian functions (Sassaman, 1996:71).
The common denominator that these explanations (exchange, logistical mobility,
out-migration) share is that they are all risk-management strategies that mobile hunter-
gatherers have at their disposal to overcome short-term resource stresses (Brown,
1985:206). More chronic stress such as that caused by environmental change or
overpopulation would eventually require more extreme adaptive responses such as
sedentism. Since population growth and environmental change are relatively gradual
processes, at least compared to the fluorescence of soapstone vessels, and since they are
unlikely to have been pervasive for all cultural regions and habitats, it stretches the
imagination to assume that the “existence of a widespread and integrated exchange
system” (Catlin et al., 1982:134) is solely responsible for their dispersal. Indeed, despite
the primacy ascribed to the Susquehanna Soapstone Culture as the regional progenitor for
such cultural diagnostics as soapstone bowls and broadspears, there is precious little
evidence for exotic materials reaching the Susquehanna basin itself. For instance,
Witthoft (1953) estimates that over 99% of all Susquehanna broadspears are made from a
certain purplish/gray rhyolite found in just two counties in south-central Pennsylvania.
This lithic parochialism is also true for much of western Pennsylvania (George, 1991;
Fobes, 1959) and southern New York (Ritchie, 1965). Other exotic raw materials such as
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copper and marine shell which were common in southern and mid-western exchange
systems, are likewise absent among soapstone using people of the north, at least prior to
the Orient phase in New England.
One possible explanation for this is provided by Klein (1997) who sees the
Potomac River as the demarcation line between two major regional ceremonial exchange
spheres. The southern sphere was apparently driven by Poverty Point’s appetite for
soapstone and is typified by Sassaman’s (1996) work in the Savannah River basin. In the
South soapstone bowls were used for serving food and drinks in some form of “pan-
regional ritual activity linked by exchange ties” (Klein, 1997:147). On the other hand, the
northern sphere was affiliated with Great Lakes ceremonialism where soapstone played a
less important ritual role. Klein is unfortunately vague on how these exchange system
functioned or what raw materials were involved except to say that, “soapstone from the
Middle Atlantic quarries moved through this Southeastern exchange sphere, while the
New England quarries supplied the Northeast and Midwest” (Klein, 1997:147). The
primary evidence for this assertion is derived from the appearance of Marcey’s Creek
ceramics in the Potomac basin. As the earliest pottery in the Mid-Atlantic region,
Marcey’s Creek Ware bears a striking resemblance to the soapstone vessels in use at the
time and is even frequently tempered with soapstone (Manson, 1948). Klein sees the
collapse of the southern ceremonial exchange network as the impetus for Marcey’s Creek
ceramics in an attempt to continue traditions predictaed on the trade of soapstone bowls.
The fact that soapstone continues to be used in New England, and even flourishes in
ritual contexts (burials), is taken as confirming evidence for a disconnect between the
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northern and southern exchange spheres.
The scholarly disagreement on this subject is largely due to a paucity of raw data
from habitation contexts. Most of what we know about soapstone vessel function and
style has been derived from quarry samples (e.g. Haynes, 1883; Holmes, 1890; Bushnell,
1940; Dunn, 1945; Fowler, 1956, 1968, 1969, 1975; Dixon, 1987; Ward and Custer,
1988). Truncer has identified three deficiencies, or priority research areas that need to be
addressed in order to assess the relative merits of the numerous hypotheses that have been
proposed. These include a need for better chronological control, functional analysis, and
provenancing (Truncer, 1999:290). To these we can add distributional analysis which has
been the foundation of both major interpretive frameworks advanced in the last decade
(Truncer, 1999; Sassaman, 1996), but still remains poorly understood in most areas. As
Truncer puts it, “By analyzing the spatial and temporal distributions of steatite vessels
throughout the entire region of their occurrence, all variation becomes informative,
contributing to a more complete understanding of why they were made when and where
they were. Distribution edges or margins can be particularly revealing” (Truncer,
2004:507). Geologically soapstone is confined to a relatively narrow band in the
Appalachian Mountains and adjacent piedmont running from Alabama to Labrador.
Prehistoric exploitation in the eastern United States is restricted to quarries from
Massachusetts south. Therefore, the distribution edges to which Truncer alludes are areas
outside the immediate soapstone supply zone and include the Atlantic coastal plain,
northern New England, New York State, and the regions west of the Appalachian
Mountains.
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The purpose of this thesis is to investigate the occurrence of soapstone vessels in
one of these marginal areas, the Ohio River Basin. The Ohio River drains an area of over
a half a million square kilometers encompassing parts of eleven different states. With the
possible exception of a small area in northwest North Carolina and western Virginia,
soapstone does not occur naturally within the basin. Glacially transported deposits are
also exceptionally unlikely given the softness of soapstone and the great distances it must
have been conveyed from the nearest northern sources in Ontario (Prud’homme,1983).
Therefore, the occurrence of soapstone vessels in the Ohio basin indicates transport by
mechanisms of exchange or migration or both. Since relative and absolute dates are
scarce for soapstone vessels in the region, little more than a summary of the available
data can be offered at this time. Likewise, the small number of artifacts available for
study and the poor provenience for many of these is insufficient to draw significant
conclusions about their social and technological function. This type of study is better
suited to areas in the east where large collections can be examined and where raw
material is available for experimental analysis.
The most basic question, and one that needs to be addressed before any other, is
where were the soapstone vessels that are found in the Ohio River basin ultimately being
derived from? This will be examined geographically by looking for patterns in the
distribution of vessels and sherds found west of the Appalachian Mountains in
Pennsylvania, Ohio, West Virginia, Kentucky, and Indiana. Additionally, at present there
exists no effective means of determining soapstone provenance directly. Archaeologists
are increasingly turning to chemical or mineralogical approaches for identifying and
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characterizing prehistoric exchange and finding a method to do this with soapstone
should be a high priority. One technique that offers this potential is Visible Near Infrared
Reflectance Spectrometry (VNIRS). This method has only recently been applied to
archaeological materials (Emerson et al., 2002, 2003) and has yet to be tested on
soapstone. To examine this in more detail geological samples from 20 soapstone quarries
and artifacts from a number of collections will be analyzed to test the efficacy of VNIRS
for soapstone provenancing. Since this has not been attempted previously the bulk of this
thesis is devoted to methodological, statistical, and practical considerations necessary to
accomplish this.
Chapter 2
Geological Background
Soapstone Geology
Soapstone and steatite are two terms used to describe a suite of metamorphic
rocks used in antiquity to fashion a variety of artifacts including stone vessels. The two
terms are often used interchangeably, but the terminology adopted for the rest of this
study makes an archaeologically useful distinction between the two (after Gillson, 1937).
Soapstone is therefore any soft metamorphic rock with a greasy or soapy feel and a
massive or schistose texture. This may include various serpentinites, greenschists, and
amphibolites. Steatite, while technically a variety of soapstone, is distinguished by its
high talc content. Steatite grade soapstone was preferred where it was available,
presumably for its exceptional thermal and chemical characteristics and for the ease with
which it was fashioned into durable containers and other artifacts. However, all types of
soapstone were routinely used during the Transitional Archaic to fashion durable
containers.
Soapstones are most commonly formed from rocks having high silica and
magnesium contents such as siliceous dolomites and ultramafic igneous rocks. The actual
talc paragenetic sequence is highly variable due the widespread availability of suitable
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precursor rocks and the stability of talc (+ H2O) under most metamorphic conditions
(Evans and Guggenheim, 1988). However, according to Truncer et al. (1998), prehistoric
soapstone procurement was limited to talcose rocks derived from ultramafic sources.
Unlike talc rock of carbonate origins, which is favored commercially for its purity,
ultramafic soapstones contain appreciable amounts of impurities such as chlorite and
amphiboles. For instance, in the analysis of samples from six New England quarries it
was found that talc constituted an average of only 28.0 to 47.3% of the minerals present
in thin-section (Turnbaugh et al.,1984). Presumably, the inclusion of harder minerals in
an otherwise soft talc matrix confers better resistance to abrasion, while minerals with
acicular crystals can form felted textures increasing structural durability. Whether the
prehistoric preference for ultramafic soapstones represents a conscious recognition of
these advantages remains to be confirmed. Therefore, pending evidence to the contrary,
only ultramafic origins of soapstone are of archaeological relevance in the eastern United
States.
Bodies of ultramafic rock such as dunites, peridotites, harzburgites, and
pyroxenites are found throughout eastern North America in the Appalachian orogenic
belt extending from Newfoundland to Alabama. Their composition resembles mantle
rock, being enriched in olivines and pyroxenes, and can usually be differentiated from the
country rock in which they are embedded by their higher magnesium and iron contents
relative to silicon. Due to their extreme age (many are Precambrian) and repeated bouts
of deformation and metamorphism, their exact origin is not always discernible. Some
probably represent intrusive events while others have been interpreted as ophiolitic
13
terranes that have been obducted onto the continental margin (Drake and Morgan, 1981).
Regardless of their method of emplacement, many ultramafics become wholly or partly
serpentinized by heated solutions during metamorphism. As the name suggests,
serpentinite is a rock type largely composed of serpentine minerals (e.g. antigorite,
chrysotile) along with varying amounts of relict ultramafics (olivine, clinopyroxene), and
other minerals such as actinolite, chlorite, chromite, and magnetite (Drake and Morgan,
1981).
In regional metamorphism soapstone is formed by the metasomatic replacement
of deeply buried serpentinized ultramafics. These bodies are typically lens or pod shaped
and may vary in size from a few tens of meters to tens of kilometers in length.
Steatization occurs under moderate metamorphic conditions (greenschist and amphibolite
facies). At these temperatures and pressures serpentine becomes unstable and is altered to
talc and magnesite by an influx of SiO2 and CO2 from the surrounding country rock. In
later stages, magnesite may be altered into dolomite or calcite or be completely replaced
by talc. Talc may also undergo further reaction to form chlorite or amphibole. This leads
to the progressive growth of a series of reaction zones. The generalized sequence for
ultramafic metasomatism under greenschist to amphibolite facies has been described by
Sanford (1982) as follows:
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Based on the modal mineralogy, any rock between the ultramafic precursors and
the country rock (CR) might be considered a usable soapstone grade material. Although
Sanford only lists the dominant minerals in his generalized sequence, there are significant
deviations from this series at all four quarries he investigated. For instance, chlorite and
serpentine are each found in appreciable percentages throughout the talc-rich zones at
two quarries and there are also talc-amphibole associations at two quarries.
Soapstone Mineralogy
Since steatite grade soapstone is predominantly composed of talc it is worthwhile
to first look at talc mineralogically. Talc is a hydrous magnesium silicate,
Mg6(Si8O20)(OH)4 with a tri-octahedral, or 2:1 layer structure. In this arrangement silica
molecules (SiO4) share three basal oxygen atoms with adjacent silica molecules forming
an “infinite” sheet or layer of hexagonal, or more properly, di-trigonal silicate rings (for
discussion of this and other rare polytypes of talc see Deer et al., 1992). Appropriately,
this layer is called the tetrahedral layer after the silica tetrahedra. Two tetrahedral layers
are held together in octahedral coordination between the apical silica oxygens and a layer
of cations, magnesium in the case of talc. This layer is known as the octahedral layer.
This simple structure, an octahedral layer sandwiched between two tetrahedral layers, is
the basic chemical building block of talc. Since each 2:1 layer is electrostatically neutral,
adjacent layers are only held together loosely by van der Waal’s forces giving talc its
characteristic softness and soapy feel. This interlayer region may also contain small, but
15
sheets, or organic compounds” (Evans and Guggenheim, 1988; 225).
Although most talc approaches its ideal composition, a certain amount of cation
substitution can be expected in the octahedral layer (Fe+2,Fe+3,Al, Mn) and more rarely in
the tetrahedral layer (Al, Fe+3; Guggenheim and Evans, 1988). Between interlayer
impurities and cation substitutions there may be sufficient inter-source variability to
allow source discrimination in some cases. For instance, talcose rocks of mafic and
ultramafic origin can be expected to be enriched in certain transition metals (e.g. Ni, Cr,
Co, and Sc) relative to talc-carbonate rocks (Roe and Olson, 1983: 1282). However, it is
unlikely that the chemical variability of “pure” talcs would alone be sufficient to
discriminate between all, or even most sources. To test this would require appropriate
mineral separation techniques for bulk samples and sensitive chemical analysis. Similar
reasoning applies to other minerals present in soapstones.
For the purposes of lithic source analysis it is convenient to view rock
composition as a three-tiered hierarchy. At the finest level of discrimination are the minor
and trace-elements, usually defined by convention as comprising less than 1.0 and 0.1 wt.
percentage respectively (Leudtke, 1992). Sensitive geochemical techniques are required
to measure these elements and since they are not integral to the mineralogical structure of
the rock, they can effectively be treated as an independent source of information. In
soapstones, the relative immobility of trace-elements is deemed of significance since it
can serve as a geochemical signature of the parent igneous rock (Williams, 1977).
Therefore, trace-element analysis by Instrumental Neutron Activation Analysis (INAA),
16
particularly of the rare-earth Lanthanide series (La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy,
Ho, Yb, Lu), has been the method of choice for the majority of soapstone sourcing
studies to date.
The next highest level of the compositional hierarchy is mineralogical. This
includes major element geochemistry with which it is highly correlated. Theoretically,
soapstone mineralogy should be determined by the composition of its precursor rocks, the
type of metamorphism involved (contact, regional, hydrothermal), temperature and
pressure variables, and by the chemical fluxes and concentration gradients that drive
them (Sanford, 1982). The resultant suite of minerals should therefore be diagnostic of its
formational environment and highly susceptible to local perturbations in the conditions
mentioned above. It is surprising then that soapstone mineralogy has not been the basis of
more concerted investigation in provenancing studies.
Several reasons can be suggested for this situation. The first is purely historical.
Lithic sourcing theory in archaeology is largely driven by the successes of chert and
obsidian sourcing. Unfortunately, the nearly monomineralic composition of these raw
materials (almost pure cryptocrystalline quartz) virtually necessitates trace-element
analysis (Shotten and Hendry, 1979) and makes them particularly poor analogues for
soapstone. Another historical factor concerns the tremendous influence of the research
team of archaeologists and chemists at the University of Virginia in the 1970’s and 80’s
(Luckenbach et al., 1974, Allen et al., 1975, Luckenbach et al., 1975, Williams, 1977,
Allen et al., 1978, Allen and Pennell, 1978, Allen et al., 1983, Rogers et al., 1983, Allen
et al., 1984). Their prolific output on soapstone provenancing by INAA has appeared in
17
prominent journals and remain the most widely read and cited articles on the subject.
More recent research has cast some doubt on their methods and conclusions (Moffat and
Buttler, 1986; Truncer et al., 1998; Truncer, 1999) and will be examined more closely in
a later section. In Allen and Pennell (1978), they are dismissive of soapstone sourcing by
mineralogy because,
“On the basis of the different origins and mineralogies of the materials called soapstone,
one might expect that the different formations could be classified in terms of their
chemical and mineral contents. This has not been the case, and the reason can be
explained in geochemical terms. First of all, as pointed out above, there are several
distinct processes by which talcose rocks like soapstone can be formed. Although the
initial reactants may be very different, the products can be very similar in mineralogy and
composition. On the other hand, a single metamorphic episode may cause a series of
rocks varying in mineralogy but which can still be classified as soapstone”. (Allen and
Pennell, 1978: 232).
Unfortunately the authors do not cite any data or references to support their
assertion. In fact, there is very little quantitative data available to assess the true
mineralogical variability of soapstone sources in North America. The data that is
available tends to come from commercial talc deposits that have already been selected for
their high talc content and low occurrence of impurities. One rare exception is Sanford’s
(1982) thorough analysis of four modern soapstone quarries in New England. Each of
these quarries differs in important variables such as the composition of the ultramafic
precursor rocks, the type of surrounding country rock, and the degree of metamorphism
that they experienced (Table 1). The modal mineralogies for these quarries are plotted in
Figures 1-4 for comparison.
18
Table 1. Greenschist to Amphibolite Metamorphism at Four Ultramafic Bodies in New
England (after Sanford, 1982).
Newfane, VT
Epidote- Amphibolite
Grafton, VT Amphibolite 7.5-8.5 590-645° anthophyllite+ chlorite
quartz- plagioclase- biotite gneiss
Each graph represents a cross-section of the steatized ultramafic body occurring
between the unaltered ultramafic rock on the left-hand side and the country rock on the
right. An abrupt shift in mineralogy in each graph marks the transition from Sanford’s
talc or talc-carbonate reaction zone to the amphibole-chlorite reaction zone. Although
there are multiple lithologies present at each quarry, they can still be differentiated by
purely qualitative criteria (i.e. the presence/absence of minerals). Additionally, here the
carbonates (dolomite, calcite, magnesite and combinations thereof) were collapsed into
one category for simplicity, but in Sanford’s original data there were diagnostic
qualitative differences at all four localities in these as well. Chemically, even talc
displayed greater compositional variation among quarries than within them due to the
substitution of iron and aluminum for magnesium (Sanford, 1982:559). Factor in other
petrological observations such as the presence of certain rarer minerals (rutile, magnetite,
19
Fig 1. Modal Mineralogy from Thin-section Analysis at the Vermont Marble Company
Quarry, Rochester, VT (from Sanford 1982:fig. 2).
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
Distance to Ultramafic/Country Rock Contact (cm)
Talc Serpentine Carbonates Cr-Magnetite Chlorite Actinolite
Fig 2. Modal Mineralogy from Thin-section Analysis at the Newfane Soapstone Quarry,
Newfane, VT (from Sanford, 1982: fig. 3).
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
Talc Actinolite Chlorite Cr-Magnetite Carbonates
20
Fig 3. Modal Mineralogy from Thin-section Analysis at the Osborne Soapstone Quarry,
Blanford, MA (from Sanford, 1982:fig. 4).
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
Distance to Ultramafic/Country Rock Contact (cm)
Talc Serpentine Chlorite Cr-Magnetite Carbonates Anthophyllite
Fig 4. Modal Mineralogy from Thin-section Analysis at the Frost and Goodrich Quarries,
Grafton VT (Sanford, 1982: fig. 5).
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
Talc Chlorite Actinolite Carbonates Cr-Magnetite
21
pyrite, chalcopyrite, pyrrhotite, apatite, ilmenite, cubanite, and pentlandite), textures, and
grain sizes, and it is clear that at least in some cases the claims of Allen and Pennell may
have been premature.
None of the quarries investigated by Sanford had been exploited prehistorically.
The possibility still remains that prehistoric material preferences may not coincide with
geological variability. To the extent that this is likely to some degree, it remains
necessary to quantify the mineralogical variability at just those sources that had been
utilized. Only one study has attempted to do this (Turnbaugh et al., 1984) and their results
are reproduced in Figure 5. Although their sample sizes were small, the authors note that,
“even at this relatively gross level of analysis, highly significant compositional variation
appears to exist among soapstones from southern New England” (Turnbaugh et al.,
1984:137). Combined with Sanford’s data from northern New England, there is reason to
believe that mineralogical analysis may be sufficient to distinguish between at least some
soapstone sources.
After chemistry and mineralogy the third, and probably the most variable level in
the aforementioned hierarchy is the physical arrangement and size of the mineral grains
in the rock. Along with mineralogy, these are largely responsible for the physical and
visual characteristics of soapstone. Unfortunately, they are also difficult and time-
consuming to quantify. However, they do provide an additional layer of information
about the conditions of soapstone formation, even in cases where mineralogy is invariant.
22
Fig 5. Mean Mineralogy from Thin-section Analysis of Six New England Soapstone
Quarries (after Turnbaugh et al., 1984)
Bakerville (N=3)
“Steatite provenance research, however, has produced only modest, if not controversial,
methodological gains. Previous attempts to identify steatite artefact provenance remain
largely unconvincing because they fail to fully assess the chemical and mineralogical
variability both within and between steatite sources”. (Truncer et al., 1998: 23)
For the sake of convenience the analytical techniques used for lithic
characterization can be broadly classified as either geochemical or
petrological/mineralogical. Depending on the raw material, usually one or the other is
more appropriate. For some materials either can be effective, while in other cases a
23
combination of techniques may be warranted. For instance, obsidian is a nearly
monominerallic silica glass. Except for occasional flow lines, obsidian has no internal
structure. The lack of non-silica minerals or structure makes obsidian petrography of little
diagnostic value, thus necessitating a geochemical approach. On the other hand, chert,
which is also largely silica (cryptocrystalline quartz), has been formed under a wide
variety of circumstances throughout geological time. Although not without controversy,
macroscopic characteristics such as distinctive banding, colorations, and textures are
routinely used by archaeologists to assign chert artifacts to potential sources. Slightly
more objective approaches include the use of fossil inclusions or rare minerals identified
in thin-section (Leudtke, 1992). The value of petrographic analysis of chert is limited,
however, because diagnostic features are not present in every chert type that might be
encountered in an artifact assemblage. Also, for characteristics such as color the overlap
between potential sources has probably too often been underappreciated leading to
misidentifications. Again, the geochemical approach has been widely adopted as a
solution to these problems.
Soapstone shares one thing in common with chert. It is a macroscopically diverse
material, even within the confines of a single quarry, and there is considerable overlap in
visible qualities between sources. This has frustrated attempts to classify soapstone
macroscopically. Of course, there are numerous examples in the literature of chemical or
mineralogical descriptions of one or two artifacts or quarry samples. Though, while of
interest, such descriptions do not strictly qualify as lithic sourcing analyses which by
definition seek to quantify the variability within and between sources. This requires the
24
characterization of several samples from more than one source. In fact, prior to 1975
there had been only one attempt to characterize soapstones by any method. Bullen and
Howell (1943) used mass spectroscopy to examine major element concentrations from
seven New England quarries. Their results, however, were inconclusive since the total
sample size was small (N=11 plus one artifact) and was presented in interval scale
making it difficult to interpret or treat statistically.
Over three decades after Bullen and Howell’s initial study a new era in soapstone
research was launched at the chemistry department of the University of Virginia. Here
Ralph Allen and his colleagues and students would analyze an unprecedented number of
soapstone samples over the next decade using Instrumental Neutron Activation Analysis
(INAA; Luckenbach et al., 1974, Allen et al., 1975, Luckenbach et al., 1975, Williams,
1977, Allen et al., 1978, Allen and Pennell, 1978, Allen et al., 1983, Rogers et al., 1983,
Allen et al., 1984). Their prodigious output, however, is of more than just historical
interest. The perceived success of INAA, particularly using the Rare Earth Elements
(REE), has made it the de facto standard by which new soapstone provenancing
techniques are evaluated. Furthermore, some of the assumptions implied by the team’s
work including their very rationale for using REE-INAA, would, if true, effectively
preclude the use of petrological techniques including the one advocated in this study. In
order to assess this possibility it is necessary to critically examine the data and claims
derived from REE-INAA studies.
The basic concept of INAA is that each sample is bombarded by neutrons in a
nuclear reactor to produce radioisotopes. As these radioisotopes decay they emit gamma-
25
rays with energies that are characteristic of the elements that form them. These can be
measured by solid-state detectors with sensitivities in the ppm to sub-ppm range. The
allure of INAA is that it can measure many elements simultaneously (20-30 are typical in
archaeological applications), and it is considered among the most sensitive and precise of
the trace-element techniques. The utility of INAA is well established for cherts
(McGinley and Schweikert, 1979; Leudtke, 1978, 1979,1992) and obsidian (Capannesi et
al., 1990; Oddone et al., 1997), but what about for soapstone?
The key to success for any provenancing analysis lies in its ability to accurately
estimate the full range of variability in the attribute(s) of interest. A sample that does this
is said to be representative and allows one to make statistically sound inferences
regarding the true population from a much smaller sub-sample. Although this is a simple
concept to understand it is far more difficult to implement because there is no general
consensus on what constitutes the target population or of how to acquire a representative
sample from it once it has been identified. If we restrict the investigation to just the
quarry and its immediate environs there are several possible candidates for the target
population. The extant outcrop, which may show evidence of quarrying activity, is
probably of limited use. It provides only a glimpse of the potential variation and its
abandonment at this point may be indicative of its unacceptability as a raw material.
Although abundant, float and other erosional debris are difficult to relate to prehistoric
material preferences. That leaves manufacturing rejects, debitage and quarry tailings. Due
to extensive private collecting at most known quarries, rejects are not always available or
are available in insufficient quantities. Furthermore, Leudtke (1979) has recommended
26
not using artifacts found at quarries for sourcing since they may have been transported.
This is probably more of a concern for chert quarries where entire tool-kits could have
been abandoned during replenishment. Although Allen et al. (1984) have raised this very
concern with respect to soapstone in Labrador, the consensus remains that bowls were
rarely transported in an unreduced state (e.g. Holmes, 1893; Fowler, 1975; Ward and
Custer, 1988). In any case, it’s not clear that the few rejects that do remain at quarries are
in any way representative of those that have been removed. A better argument against the
use of manufacturing rejects, however, is that there simply is no reason to subject these
artifacts to destructive analysis when vast amounts of debitage and tailings would do just
as well in their stead.
Depending on how large and how extensively a quarry has been worked, the
mixture of quarry debris can range from just a small scatter to massive piles weighing
hundreds of tons (a cubic meter of soapstone weighs approximately 2.5-2.7 metric tons).
The typical weight of a sample submitted for INAA is somewhere between 150 mg
(Truncer et al., 1998) and 700 mg (Clark et al., 1990). Therefore, even for a “healthy”
quarry sample of N=30 the total weight of material analyzed is still only between 4.5 and
21.0 grams. This is a vanishingly small fraction of the actual material present at quarry
sites. For the necessary representativeness to be achieved we must either postulate an
extraordinary degree of homogeneity or effectuate a sampling strategy that can account
for REE variability. However, the latter can’t be done visually since trace elements are
invisible to the naked eye. As for whether soapstone is homogenous or not, it has already
been argued that this is not the case for mineralogy, but what about trace elements?
27
Information on trace element variability in soapstone is relatively scarce. One
study (Truncer et al.,1998) found that the average coefficient of variation (CV) for the
REE concentrations at eight quarries to be 125%, compared to just 34% for transition
metals. Individual scores ranged as high as 254.4% (La at the Lawrenceville quarry). This
is in good agreement with radiochemical-separation instrumental activation analysis
(RNAA) data from the Shetland Islands, where an average CV of 126% was documented
by Moffat and Buttler (1986). Unpublished chemical data from North Carolina and
Virginia soapstones yielded similar results using inductively coupled plasma-mass
spectrometry (CV=122%; Christopher Stevenson, personal communication).
The preferred method of presentation used by the University of Virginia
researchers was the chondrite-normalized REE plot for “typical” samples from each
quarry. However, in one publication they provide mean concentrations and standard
deviations for two elements (La, Lu) from 8 Labrador quarries (Allen et al., 1984:Table
II). From this data the calculated CV’s are just 17% and 18% (range=8-36%). These
abnormally low CV’s are probably the result of the selection process utilized rather than
the true variability at the quarries, and will be discussed later.
Notwithstanding the Labrador quarries, the high degree of variability in soapstone
REE chemistry suggests that sample sizes should be a concern. Leudtke has
recommended using samples of 30 or more for chert analysis (Leudtke, 1978), but the
CV’s for REE concentrations in chert are generally about half of what has been observed
in soapstone (Leudtke, 1992:Appendix B). Tykot (2004) considers sample sizes of 25-30
for homogenous sources to be sufficient and Truncer et al. suggest samples of 25 or more
28
for soapstone, because this is the “sample size required for multivariate statistical
analysis” (Truncer et al., 1998: 25).
Unfortunately, sample size is neither a matter of opinion nor of convention. In
much of biological and archaeological typology, CV’s are usually low enough that
multivariate statistical analysis can be accomplished by substituting the sample mean or
standard deviation for their respective population parameters without deleterious effects
(for sample sizes of 25-30). However, for REE geochemistry, this is rarely the case
because of its variability (Koch and Link, 1971a). Fortunately there are a number of
relatively simple ways for estimating the required sample size for geochemical lithic
analysis (Dennison, 1962; Size, 1987; Barnes, 1988; Gill et al., 2005). Using the
approach described by Gill et al. (2005) as an example, the sample size required to
€
≤ µ ≤ X − tβ s N−1
where µ is the true mean, X is the sample mean, s is the sample standard deviation, and tβ
is the user-defined confidence level from the Student’s t-distribution (β is taken here to
be 95%). The ratio of the upper and lower confidence intervals is labeled the precision
index, p,
X − tβ s N−1
This index is analogous to “choosing the maximum admissible relative error on the
determination of the population mean” (Gill et al., 2005: 34). For research applications
the authors recommend a precision index of 1.2 or better, corresponding to an error of
9.1%.
By rearranging the terms we can solve for N, the required sample size, in terms of
€



2
+1
Since tβ requires knowledge of the degrees of freedom (N-1), Equation 3 has to be solved
using numerical integration of the Student t probability density function, which is easy
enough to accomplish by trial and error (Sokal and Rohlf, 1995: 263) or with a simple
computer program (Gill et al., 2005).
Using data from Truncer et al. (1998) for transition metals (CV=35%) and REEs
(CV=125%), the sample sizes required for achieving precision indices of 1.2, 1.35, and
1.5 are calculated in Table 2. It can be seen that the average sample size used by Truncer
et al. (N=26) was probably sufficient to achieve a moderate level of precision for the
30
transition metals (p=1.35, relative error=15%), but was probably not adequate to estimate
the means for the REE. This is reflected by their posterior classification success rates, just
45% for REE and 59% for transition metals (Truncer et al., 1998).
Table 2. The Precision Index (p) and Samples Sizes Required (N) for Parametric
Statistics Using the Coefficients of Variation (CV) Determined Experimentally for
Soapstone by Truncer et al. (1998). See the Text for a Discussion of this Computation.
To achieve the same level of precision for the REE the sample would have to be
increased substantially (N=271), which the authors admit would be “prohibitively
destructive” for the mostly artifactual quarry samples used in their analysis (ibid; 39).
Not only is INAA potentially destructive, but it is expensive as well. So even using
geological samples or debitage in the requisite quantities is not satisfactory for the
analysis of multiple quarries, each requiring several hundred samples. For the earlier
REE-INAA analyses, the sample sizes were not always clearly stated. However, when the
data is supplied or can be inferred, the number of samples per quarry are especially low;
<2 (Allen et al., 1975), 4-15 (Moffat and Buttler, 1986), and ~9 (Allen et al., 1984).
Given even the lowest precision indices, these samples are not sufficiently large to
p t CV N 1.50 2.131 35% 14 1.35 2.064 35% 24 1.20 2.000 35% 59
1.50 1.976 125% 150 1.35 1.969 125% 271 1.20 1.963 125% 726
31
produce reliable estimates of either the mean or the standard deviation of REE
concentrations. Although their sample sizes have been criticized previously (Bishop and
Canouts, 1993; Truncer et al., 1998), attention is required of two other phenomena that
occurred in their REE curves.
The REE distribution curves used by Allen et al. are produced by dividing the
REE concentrations of soapstone samples by those of a chondritic meteorite standard.
This normalization removes the effects of nucleosynthesis and isolates a terrestrial REE
pattern that is indicative of the geochemical processes that have occurred in a particular
rock source since its early undifferentiated state (represented by the chondritic REE
values; Rogers et al., 1983). When plotted against the REE atomic numbers, the resulting
curve should produce a unique “fingerprint” for each source (Holland et al., 1981).
Furthermore, since REEs are considered to be largely immobile during metamorphism,
these patterns should be uniform throughout an ultramafic body and its serpentinized and
steatized products, regardless of mineralogy (Williams, 1977).
What was found in their earlier studies was that the absolute REE concentrations
varied within a source by a factor of two to six, but that the relative concentrations were
stable (Allen et al., 1975; Luckenbach, 1975; Rogers et al., 1983). This allowed the
comparison of curve shapes, but not absolute concentrations. In later work, however,
Rogers et al. (1983) found several different curve shapes at Peabody Point in Labrador. In
a small sample from this quarry (N=10) there were two distinct REE patterns for
soapstone along with the usual differences in absolute concentrations. A third pattern was
found for a chlorite-rich sample, and a fourth for several serpentinite samples originally
32
thought to be soapstone. This demonstrates that there is some fractionation of REE for
different minerals. The authors concede this but note that, “in soapstone there are several
minerals which appear to fractionate the REE in a similar manner, but there are variations
in the absolute amounts of REE accepted in each type of mineral lattice. Thus mixtures of
these minerals would have similar patterns while varying in absolute concentrations”
(Rogers et al., 1983: 190).
The strategy recommended for dealing with outliers, and apparently implemented
for all the Labrador quarries, was to remove any samples that did not conform to the
“typical” pattern for a source. Since both serpentines and chlorite are enriched in trace
elements compared to talc, these can be identified after REE data is collected. The
authors justify this selective culling by suggesting that serpentinite would have been
physically too hard to fashion into artifacts and that the chloritic sample may have been
from the margin of the reaction zone, although they don’t explain why that makes this
material unsuitable for use prehistorically. Furthermore, they explain that “the higher
concentrations of REE near the margin of the reaction zone was interpreted to be the
result of a deposition of trace elements derived from the more metamorphosed talc rich
zone” (Rogers et al., 1983:190).
In a rebuttal to the Virginia researchers, Moffat and Buttler (1986) have presented
evidence that ultramafic REE patterns fall into a “small number of definite classes”
according to their gross mineralogy and field relations (Moffat and Buttler, 1986: 111).
They conclude that it is “therefore probably inappropriate to look for any source-to-
source variation in steatite REE geochemistry as a product of any variation in ultramafic
33
REE geochemistry from body-to-body or association-to-association within a particular
geological terraine” (ibid; 111). Rather, when differences are observed it is due to
selective fractionation and REE mobility.
Mineralogically dependent fractionation can be expected on theoretical grounds
(e.g. Henderson, 1984), but the bulk of evidence suggests that most trace-elements are
immobile during greenschist metamorphism (Pearce and Cann, 1971; Goldsmith and
Force, 1978; Sanford, 1982). A more parsimonious explanation for the observed REE
concentrations, which hasn’t been considered in archaeological INAA analyses, is that
volume changes are a major contributing factor. There are considerable volume increases
associated with steatization (Sanford, 1982). Talc in particular requires an influx of
matter, primarily silica and FeO, but chlorite formation entails a loss of volume. Based on
a comparison of the presumably immobile Cr-magnetite content in the serpentine and
talc-carbonate zones from three New England quarries, Sanford reports anywhere from
160% to 1210% increase in volume (Sanford, 1982). There is some variation within the
zones, but generally there is a correlation between talc (and actinolite) and a volume
increase, and with chlorite and serpentine content and little or no volume increase. This
suggests that by mere dilution, the REE content in highly metamorphosed talc-rich
soapstones will be depleted compared to less metamorphosed chlorite or serpentine rich
soapstones. Indeed, given significant mineralogical variation within and between zones at
the same quarry, along with any fractionation effects, it would be highly unlikely to find
a single REE pattern that characterizes all the potentially utilizable soapstone found at a
specific locality.
34
In light of this, the method of REE pattern selection used previously (Rogers et
al., 1983) has to be viewed with skepticism. Serpentines are a normal constituent of many
soapstones. Turnbaugh et al. (1984) reported moderate amounts of serpentine at two New
England quarries (Oaklawn=15%, Horne Hill=12%), and Moffat and Buttler (1986)
reported its presence in all 12 of their samples from 5 Shetland quarries (1.8-30.9%). In a
review of the literature on soapstone/talc mineralogy discussed earlier, over 21% of the
localities contained serpentine minerals. As to whether serpentine is too hard to work,
this is probably not the real issue. The most common massive form of serpentine,
antigorite, has a Moh’s hardness of 2.5-3.5 (Deer et al., 1992). Although pure talc has a
hardness of one, soapstones may range from 1-2.5 due to the presence of non-talc
accessory minerals (Ward and Custer, 1988). In fact, bowls made of serpentine have been
reported (Fowler, 1956; George and Fischer, 1999), although their widespread use as
cooking vessels is probably limited by serpentine’s relatively low thermal decomposition
temperature (530°C at 1 kbar for antigorite; Deer et al., 1992). However, if present as an
accessory mineral in talcose soapstone this is less of a problem, and no doubt this mixture
of minerals would have been utilized prehistorically.
As for chlorite, it is nearly as common as talc in soapstone, particularly those of
ultramafic origin. Indeed, some soapstones are chlorite schists with talc as the accessory
mineral. Based on observations of many hundreds of artifacts in the course of this study,
it can be confidently stated that large chlorite crystals like those described by Rogers et
al. (1983) are common in finished soapstone vessels. So whether they originate from the
reaction zone margins or not, their presence is not sufficient reason to exclude such
35
samples. In the future an effort to avoid measuring solid chlorite crystals might be useful,
but the practice of excluding samples a posteriori based on their REE patterns is of
questionable utility and lacks real justification.
Problems and Prospects
Concerns about representative sampling are not unique to INAA. However, the
ability to overcome this problem in future analyses remains an issue due to cost and
destructiveness. Prudence dictates that alternative analytical techniques should be
explored, especially those with a realistic expectation of obtaining representative
samples. One obvious approach, which hasn’t been seriously exploited in the United
States, is simple visual inspection of hand samples. In a comprehensive analysis of
Shetland Island steatites, Buttler (1984) found that visual examination was more effective
at discriminating between sources than any other method including INAA. In New
England, Turnbaugh and Keifer (1979) found “distinctive variation” in color, texture and
mineralogy between six quarries, although their sample size was small (N=12). It’s
doubtful that visual characteristics alone could supply unique descriptions for each
quarry, but they certainly could narrow down the possibilities. The real problem is that
qualitative data is difficult to objectify and communicate. This is especially true when a
source material is as heterogeneous as soapstone.
The decision to use visible/near-infrared diffuse reflection spectrometry (VNIRS)
in this study is based on its ability to overcome some of the limitations of INAA and
36
visual classification. In isolation most of the minerals that can be expected in soapstones,
including many chemical isomorphs and structural polymorphs, produce unique VNIR
spectra. The physical factors responsible for texture tend to modify mineral spectra in
predictable ways as well. VNIRS is also highly sensitive to transition metals (Hunt,
1977), which Truncer et al. (1998) found to be useful for soapstone discrimination.
Perhaps most importantly, the technique is non-destructive, inexpensive, and fast,
allowing large numbers of samples to be analyzed. Outliers are also less of a concern if
non-parametric statistics are used for matching artifacts, so serpentines, unaltered
ultramafics and country rock can be included in the analysis without detriment.
37
technique that measures the interaction between electromagnetic radiation (light) and
material surfaces at wavelengths between 350-2500 nm. Subject to a number of factors
incident light is either reflected or absorbed by a surface. By measuring the intensity of
the reflected portion of light and plotting it as a function of wavelength, a pattern, or
spectrum, is generated. Reflectance spectra contain a great deal of information about the
physical and molecular composition of an analyte. Although VNIRS is generally not as
powerful as more traditional analytical techniques for identifying unknown materials or
for quantitative work (there are exceptions), its outstanding speed, non-destructiveness,
and low operating costs make it desirable for a number of applications. Today VNIRS is
widely used, especially in the food and pharmaceutical industries, where these qualities
are important (e.g. for online process monitoring and quality control).
In geology, the history of infrared spectrometry can be traced all the way back to
W. W. Coblentz’s pioneering work in the early 1900’s. However, it wasn’t until after
commercial IR-spectrometers became available in the 1950’s that mineralogical
investigation began in earnest. Major compilations of these early findings remain relevant
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today (e.g. Hunt and Salisbury, 1970; Hunt et al., 1973; Hunt, 1977). Since then a number
of improvements have been made in the technology including better diffraction gratings
and photon detectors, more powerful personal computers, and the introduction of fiber
optic cables. On the theoretical front, refinements in the theory of light scattering, long an
impediment to progress, now allow sufficient modeling for quantitative analyses (Clark
and Rousch, 1984). Another relatively recent advance is the establishment of large open-
source mineral spectral libraries like that at the USGS Speclab (Clark et al., 2003) and the
ASTER Spectral Laboratory (Jet Propulsion Laboratory,1999). Commercial libraries are
also available, notably the Specmin library (Hauff, 1993).
In addition to rapid data collection, VNIRS has the added advantage of being
capable of gathering reflectance information from distant surfaces. The remote sensing
application of reflectance data is called hyperspectral imaging or sometimes simply
imaging spectrometry, where “hyper” refers to the multidimensionality of the spectra and
“imaging” refers to the use of color-coded maps to display the data spatially. Over the
past couple of decades hyperspectral imaging has become a major driving force in
VNIRS. Although remote sensing has it’s own unique set of challenges (e.g.
environmental interference, viewing geometry) as well as terminology, it is of special
interest to lithic provenance analyses in the realm of data processing. Hyperspectral
imaging routinely handles massive datasets of millions of spectra. The use of large
spectral reference libraries and measurements of spectral similarity are two examples of
subjects from the remote sensing literature that are of direct applicability to lithic
sourcing.
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Despite the growing acceptance of VNIRS for classificatory purposes in other
fields, including geology, archaeologists have been slow to embrace this technology.
Most reviews of the analytical techniques used in archaeology do not mention VNIRS
(Shotton and Hendry, 1979; Harbottle, 1982; Herz, 2001; Tykot, 2004). One exception is
in Church (1994) who briefly mentions Purdy’s (1981) use of VNIRS to examine surface
patinas in chert, but concludes, “its utility in sourcing studies has not been investigated”
(Church, 1994; 52). As in Purdy’s study, VNIRS has occasionally been used in task
specific applications in archaeological materials analyses. Bacci (2000) summarizes some
of these in the context of conservation and Giardino et al. (1998) demonstrate the utility
of visible spectrometry for quantifying color in ceramics. One particularly interesting use
of NIR spectrometry has been for dating of sub-fossil wood. Yonenobu et al., (2003)
have found that the decay rate of cellulose in wood, and hence the spectroscopic features
associated with it, are significantly related in a linear fashion to absolute dates obtained
by dendrochronology.
The possibilities of NIR-dating notwithstanding, it has only been in the last few
years that the sourcing potential of reflectance spectrometry has really been explored.
The program for Ancient Technologies and Archaeological Materials (ATAM) at
University of Illinois (Urbana-Champaign) has recently been engaged in a
multidisciplinary collaboration to investigate several different types of archaeological
materials (Emerson and Hughes, 2001; Emerson et al., 2002, Emerson et al., 2003;
Wisseman et al., 2002). Using a short-wave infrared (SWIR; 1300-2500 nm range)
instrument called the Portable Infrared Mineral Analyzer (Integrated Spectronics, Inc.) or
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PIMA™ for short, the ATAM team has extensively investigated Mid-western pipestone
sources as well a Hopewellian pipes and Cahokian figurines. Their results demonstrate
that pipestone sources in Ohio, Minnesota, Illinois, Missouri and Arkansas are each
composed of distinctive mineralogical suites that can be detected and even quantified to
some extent by SWIR spectrometry. Other materials investigated at ATAM include bone
(Klepinger and Wisseman, 2002) and various materials used in artifact restoration such as
plaster, adhesives, wax, varnishes and shellacs, and paints (Wisseman et al., 2004). They
even speculate that ceramics could be analyzed by SWIR as long as firing temperatures
are not high enough to obliterate the hydroxl bonds found in clays. Finally, and most
importantly from the perspective of the present study, the ATAM team has announced the
possibility of investigating soapstones from the Southeast and Poverty Point sites
(ATAM Newsletter, 2005). In addition to complimenting the current study, that project
would expand coverage to a region that was not considered here and allow for the
comparison of different instruments and methods.
As far as is known, only one other study has examined the provenacing potential
of VNIRS with archaeological materials (Hubbard et al., 2005). In this study we
compared two visually indistinguishable Ohio cherts, Flint Ridge “chalcedony” and gray
Upper Mercer chert. Posterior classification was successful for 98.1% of the samples
(N=96). In the course of the analysis we also examined cherts, obsidians, rhyolites,
quartz, porcelanite and petrified wood from more than 50 different sources from around
the world (Hubbard et al, 2003). Differences, often quite distinctive, were observed in
many of the materials. However, for a significant portion of the samples the differences
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were subtle and often predicated on iron oxide content. More intensive analysis with
larger samples and from multiple outcrops is, however, justified from these preliminary
findings.
Although some of the attractions of this technique for lithic sourcing have
previously been mentioned, it is important to understand that every analytical technique
has advantages and disadvantages. There are several disadvantages that need to be
evaluated when considering VNIRS to source archaeological materials. First, it can only
detect those molecules that produce features in the visible and near-infrared portion of the
spectrum. This includes phosphates, nitrates, carbonates, sulfates and metal oxides and
hydroxides. Fortunately, these molecules are present in many of the rock-forming
minerals and are easily detected. Secondly, although touted as a non-destructive
technique, there are rare instances when some sample preparation may be required or
when some samples need to avoided altogether. Thirdly, the spectra of naturally
occurring rocks, particularly metamorphic and clastic sedimentary rocks, are rarely pure.
Polyminerallic rocks represent problems for interpretation because, “intimate mixtures of
several components produce a reflectance spectrum that is considerably more
complicated than a simple additive combination of individual spectral characteristics”
(Singer,1981:7967). For archaeologists familiar with the neat and orderly presentation of
geochemical data or the discreteness of spectral data from techniques like mass
spectrometry or even x-ray diffraction, they will find VNIRS spectra rather unappealing.
Multiple overlapping broad peaks often create slowly undulating curves and subtle
shoulder features. Visual inspection is not always sufficient to detect differences between
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spectra, especially when the samples are mineralogically similar. Mathematical
transformations like the first derivative, however, can help resolve and amplify
differences. The fourth disadvantage is that spectra contain hundreds or thousands of
points of data and in any significant sourcing study the number of spectra is also likely to
be in the thousands. This creates enormous computational requirements for traditional
multivariate statistical approaches. For the more sophisticated data analyses that are
becoming common a certain degree of computer programming may be also be required.
In spite of these disadvantages, none can truly be classified as limitations since all
can be overcome in one way or another. One real limitation, however, is the issue of
sensitivity. No matter how diagnostic a mineral (or trace element) is of a given material,
if it cannot be detected it is of little use for classification purposes. Sensitivity is only an
issue for mixtures since monominerallic samples are either infrared-active and detectable,
or they aren’t. In mixtures (e.g. soapstones), detection limits are determined by variations
in the minerals present, their proportions, and their absorption coefficients and particle
sizes. Also important is the spectral resolution of the instrument (Clark et al., 1990), the
amount of noise present, and to some extent the wavelength position(s) of characteristic
absorption features. Therefore, an instrument’s sensitivity to a particular mineral is
dependent on the mineralogical environment in which it is found.
To confuse matters further, there are two types of sensitivity. The most common
usage refers to sensitivity as the smallest amount of a mineral in a mixture that is required
for identification. Since most minerals are identified by the presence of several spectral
features, all of these must be present for an identification to be reliably made. The other
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definition of sensitivity can be equated with the detection limit. This is the smallest
amount of a mineral that is required to cause an observable effect on the spectrum. For
classification purposes it is the true detection limit that is of relevance and this will be
explored for various common soapstone minerals later in this study.
Daunting as the disadvantages are, they are still outweighed by the many
advantages of VNIRS. As has already been suggested, the sample sizes required to
characterize a lithic source are much larger than is usually assumed. Small sample sizes
are probably the most common laments and criticisms of lithic provenancing studies. The
problem is, as everyone recognizes, an issue of time and funding. At about $100-
$400/sample, INAA is prohibitively expensive to acquire representative samples even
from a fraction of the many hundreds of soapstone outcrops that have been documented.
Even thin-section petrography, at $10-$15/sample, can become expensive when
thousands of samples are being analyzed. There is no cost associated with VNIRS
operation, although the initial equipment outlay is moderate. Time is the other limiting
factor. The Labspec Pro ® instrument used in this analysis can obtain a spectral reading in
0.1 seconds and requires no sample preparation. Even with calibration procedures
between each sample and a 20 second scan time to average out random noise, it should be
possible to analyze 100-200 samples in a day (averaging 5 spectra per samp