Framework Silicates
& Building the Crust
JD Price
Sil
icat
e S
truct
ure
Sili
ca
te S
tructu
re
(SiO2)
Zeolites
Natrolite - mineralien-verkauf.de
Zeolites have cage-like
structures that store water (or
other large molecules).
Typically found in amygdales
within basaltic rocks.
Natrolite Na2[Al2Si3O10] 2(H2O)
Thomsonite NaCa2Al5Si5O20
6(H2O)
Chabazite CaAl2Si4O12 6H2O
Natrolite
Water is located
in open
structures
within the
lattice. But note
the framework
of Si-O
tetrahedra.
SiO2
Quartz | Coesite | Stishovite
mineral.galleries.com
De minuscules cristaux de quartz ont une
disposition radiale autour de la coesite : ceci
montre que le quartz se forme au d師riment de la
coesite (LPA)
High birefringence Stishovite in coesite, synthetically grown
by J. Mosenfelder, CalTech.
Cristobalite nodules
within a vitrophere -
Snowflake obsidianRockhoundblog.com
gemandmineral.com
Tridymite crystal, from
deposit associated with
Mono Lakes volcano.
SiO2
Image from Blackburn and Dennen, 1988
Silica Polymorphs
Beta quartz form
from Bishop Tuff,
CA. Note that
crystal is actually
alpha quartz
CalTech
SiO2Agate | Jasper | Chert |
Flint | Chalcedony
Agate is name applied
banded rocks made of
microcrystalline quartz,
typically made of fibrous
quartz, called
chalcedony. Colors result
from impurities within the
crystals
Chert and flint are
homogenous chalcedony
- often related to
fossilizationLace agate / www.lhconklin.com
Silica saturation in water is quite low at
ambient conditions, pH=7, but increases
rapidly with T. Note that amorphous silica
has a higher solubility than quartz, Rimstidt
& Cole 1983.
• In the absence of organic ligands or fluoride,quartz solubility is relatively low in natural waters.
• Below pH 9, the dissolution reaction is:
SiO2(quartz) + 2H2O(l) ! H4SiO40
• Quartz is frequently supersaturated in naturalwaters because quartz precipitation kinetics areslow.
• Quartz saturation does not usually control theconcentration of silica in low-temperature naturalwaters.
opal
Image from Klein and Hurlbut, 1985
Opals are made of
spheres of silica and
water - not exactly a
mineral.
Feldspars
Plagioclase Feldspar
Anorthite (CaAl2Si2O8)
Alkali Feldspar
Albite (NaAlSi3O8)
Orthoclase (KAlSi3O8)
Images from mineral.galleries.com
Pla
gio
cla
se S
olid
So
luti
on Liquid
Liquid +
Plag
Plag
M
T
T
Co
mm
on
Feld
sp
ar
Tern
ary
Feldspar Twinning
Image from Blackburn and Dennen, 1988
Feldspathoids
Leucite (KAlSi2O6)
Nepheline (NaAlSiO4)
Sodalite (Na4Al3Si3O12Cl)
Image from mineral.galleries.com
dkimages
Foids
Nephaline vs. Albite
NaAlSiO4 NaAlSi3O8
1 Si per NaAl 3 Si per NaAl
Leucite vs. Orthoclase
KAlSi2O6 KAlSi3O8
2 Si per NaAl 3 Si per NaAl
Lower Si than feldspar counterparts.
Arise from melts that are low in SiO2
undersaturated.
Igneous nomenclature“archaic” texture discrimination
boundary at 1mm.
An alkali feldspar and quartzdominant rock with an averagegrainsize >1 mm is a granite, <1mm is a rhyolite.
> 1mm Phaneritic
< 1 mm Aphanitic
Glassy - vitrophyric
Rhyolite
Dacite
Andesite
Basalt
Phonolite
Granite
Diorite
Tonalite
Gabbro
Anorthosite
Foid Syenite
After Strekeisen, 1978
Rock names
Mean grain size < 1mmMean grain size > 1mm
Igneous rocks are the end products of partial
melting in the earth, so their compositions are
determined by the chemical processes
involved in melting earth materials
Source, conditions, and degree of melting
Partial melting in S.W. Maine
Tectonic thinning of the crust
Tectonic increase of
crustal thickness
(after Burnham and Davis,
1974)
H2O
and
melting
(after Burnham and Davis,
1974)
H2O
and
melting
IUGS Phaneritic
Phaneritic rock
classification -
note that the
scheme
precludes the
coexistence of
foids and SiO2.
Aphanitic rock
classification -
compare with
the previous
slide.
This is a nice
diagram, but
the fine-grained
nature of these
rocks can
impede its use.
IUGS Aphanitic
IUGS TAS
If you really want to classify a volcanic rock - powder
it and analyze its composition. How does this relate
to the previous slide? What of plutonic rocks?
Again – our understanding
of the earth falls off with
depth.
Mafic (Mg-Fe rich) Rocks
Coarse: Gabbro, Anorthosite Fine: Basalt
Minerals: Largely olivine (isolated), pyroxene (single-
chain), and feldspar (framework).
Origin: Partial melting of the mantle
Oceanic crust, oceanic islands, the moon.
Univ. North Carolina, Atlas of rocks, minerals,
and textures website
Anorthosite
Univ. North Dakota, Plutonic Images
Basalt
Gabbro
Univ. North Carolina, Atlas of rocks, minerals,
and textures website
Plume theory
Earth is hottest at its
core, the release of heat
may be convective -
mass transferred. Some
masses may move as
buoyant ductile bodies.
Diagram by J. Tarney
End of the ride… if such plumes exist, they would
become more or less stalled at the transition to
brittle mantle.
Diagram by J. Tarney
If most of the heat needed
for melting is deep within
the earth, why are there
volcanoes?
Magma is driven upwards.
Parameters - density, viscosity
Pd = Ph + Po - Pvis - Sh
Hogan et al., 1998
Ascent of Magmas
Kilauea, Hawaii
March, 1996
20-406Figure 20.19
A cross section of a shield
volcano
Generally low Si
Low volatile
High rate
Mauna Kea, Hawaii
Pu’u Hulu
Mauna Loa, Hawaii
Pahoehoe
Oceanic rifting
Continental Rift
Example: the East African Rift
http://jsc.nasa.gov
STS-32
Rifting - Birth of an ocean
Diagram by J. Tarney
Diagram by J. Tarney
Cartoon to left shows
some general attributes
of rift formation
Presumably, the tension
that drives rifting is not
well understood.
Intermediate rocks (less Fe-Mg, more Na-K-Si-Al)
Coarse: Granodiorite, Tonalite Fine: Andesite, Dacite
Minerals: Feldspar (framework), pyroxene (single
chain), amphibole (double chain), and quartz
(framework)
Origin: liquid fraction of crystallizing mafic liquids.
Location: Convergent tectonic settings
Tonalite
Subduction: Island Arcs
Example: the Aleutians
Subduction: Continental Arcs
Example: the Cascades
Mount Saint Helens:
May 18, 1980 0832
Q: What is a
volcanic dome?
View from South Rim, June 1991
Crater Lake
Oregon
Felsic (K, Na, and Si rich) rocks
Coarse: Granite
Fine: Rhyolite
Minerals: Feldspar and quartz (framework), with small
amounts of amphibole (double chain) and mica (sheet).
Origins: Partial melting of crustal rocks and liquid
fraction of crystallizing intermediate liquids.
Location: continental magmatism
Granite
Univ. North Carolina, Atlas of rocks,
minerals, and textures website
Rhyolite
Vitrophyre
©Pam Gore, GPC
Felsic rocks
Barker, 1983 after
Bowen, 1928
Fractional Crystallization
Pegmatite Pegmatites are
typically coarsely
crystalline - may
contain rare
minerals. Most
result from
extreme
fractionation.
Slow cooling
Low nucleation
Lots of H2O