Mineral StructuresMineral StructuresSilicates are classified on the basis of Si-O polymerism Silicates are classified on the basis of Si-O polymerism
the [SiOthe [SiO44]]4-4- tetrahedron tetrahedron
Mineral StructuresMineral StructuresSilicates are classified on the basis of Si-O polymerism Silicates are classified on the basis of Si-O polymerism
[SiO[SiO44]]4-4- Independent tetrahedra Independent tetrahedra NesosilicatesNesosilicates
Examples: olivine garnetExamples: olivine garnet
[Si[Si22OO77]]6-6- Double tetrahedra Double tetrahedra SorosilicatesSorosilicates
Examples: lawsonite epidoteExamples: lawsonite epidote
n[SiOn[SiO33]]2-2- n = 3, 4, 6 n = 3, 4, 6 CyclosilicatesCyclosilicates
Examples: benitoite BaTi[SiExamples: benitoite BaTi[Si33OO99]]
beryl Beberyl Be33AlAl22[Si[Si66OO1818]]
Mineral StructuresMineral StructuresInosilicates Inosilicates
[SiO[SiO33]]2-2- single chains single chains Inosilicates Inosilicates [Si[Si44OO1111]]4-4- Double chains Double chains
pryoxenes pyroxenoidspryoxenes pyroxenoids amphiboles amphiboles
Mineral StructuresMineral StructuresPhyllosilicates Phyllosilicates
[Si[Si22OO55]]2-2- Sheets of tetrahedra Sheets of tetrahedra PhyllosilicatesPhyllosilicates
micas talc clay minerals serpentinemicas talc clay minerals serpentine
Mineral StructuresMineral StructuresTectosilcates Tectosilcates
[SiO[SiO22] 3-D frameworks of tetrahedra: fully polymerized ] 3-D frameworks of tetrahedra: fully polymerized TectosilicatesTectosilicates
quartz and the silica minerals feldspars feldspathoids zeolitesquartz and the silica minerals feldspars feldspathoids zeolites
low-quartzlow-quartz
Nesosilicates: independent SiONesosilicates: independent SiO44 tetrahedra tetrahedra
Olivine (100) view blue = M1 yellow = M2Olivine (100) view blue = M1 yellow = M2
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M1 and M2 as polyhedraM1 and M2 as polyhedra
Nesosilicates: Olivine (Mg,Fe)Nesosilicates: Olivine (Mg,Fe)22SiOSiO44
Olivine Occurrences:Olivine Occurrences: Principally in mafic and ultramafic igneous rocks- Principally in mafic and ultramafic igneous rocks-
Typically ~60+% of mantle source for basaltsTypically ~60+% of mantle source for basalts-- Fayalite in meta-ironstones and in some alkalic Fayalite in meta-ironstones and in some alkalic
granitoidsgranitoids Forsterite in some siliceous dolomitic marblesForsterite in some siliceous dolomitic marbles
Nesosilicates: Garnet Nesosilicates: Garnet
Garnet (001) view blue = Si purple = A turquoise = BGarnet (001) view blue = Si purple = A turquoise = B
Garnet: AGarnet: A2+2+33 B B3+3+
22 [SiO [SiO44]]3 3
““Pyralspites”Pyralspites” - B = Al - B = AlPyPyrope: Mgrope: Mg33 Al Al22 [SiO [SiO44]]3 3
AlAlmandine: Femandine: Fe33 Al Al22 [SiO [SiO44]]33
SpSpessartine: Mnessartine: Mn33 Al Al22 [SiO [SiO44]]33
““Ugrandites”Ugrandites” - A = Ca - A = CaUUvarovite: Cavarovite: Ca33 Cr Cr22 [SiO [SiO44]]33
GrGrossularite: Caossularite: Ca33 Al Al22 [SiO [SiO44]]33
AndAndradite: Caradite: Ca33 Fe Fe22 [SiO [SiO44]]33
Occurrence:Occurrence:Mostly metamorphicMostly metamorphicSome high-Al igneousSome high-Al igneousAlso in some mantle peridotitesAlso in some mantle peridotites
Inosilicates: single chains- Inosilicates: single chains- pyroxenespyroxenes
Diopside (001) view blue = Si purple = M1 (Mg) yellow = M2 (Ca)Diopside (001) view blue = Si purple = M1 (Mg) yellow = M2 (Ca)
Diopside: CaMg [SiDiopside: CaMg [Si22OO66]]
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a si
na
sin
Where are the Si-O-Si-O chains??Where are the Si-O-Si-O chains??
Inosilicates: single chains- pyroxenes Inosilicates: single chains- pyroxenes
Diopside (001) view blue = Si purple = M1 (Mg) yellow = M2 (Ca)Diopside (001) view blue = Si purple = M1 (Mg) yellow = M2 (Ca)
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a si
na
sin
The tetrahedral chain The tetrahedral chain above the M1s is above the M1s is
offset from that below offset from that below
The result is a The result is a monoclinicmonoclinic unit cell, unit cell, hence hence clinopyroxenesclinopyroxenes
e.g. Diopside, Augitee.g. Diopside, Augite
Inosilicates: single chains- pyroxenes Inosilicates: single chains- pyroxenes
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(+) M1(+) M1
(+) M2(+) M2
(+) M2(+) M2
OrthopyroxeneOrthopyroxene
an an orthorhombicorthorhombic unit unit cellcell
Enstatite (MgEnstatite (Mg22SiSi22OO66))
Inosilicates: single chains- pyroxenes Inosilicates: single chains- pyroxenes
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(+) M1(+) M1
(-) M1(-) M1
(-) M2(-) M2
(+) M2(+) M2
Pyroxene ChemistryPyroxene Chemistry
The general pyroxene formula: The general pyroxene formula:
WW1-P1-P (X,Y) (X,Y)1+P1+P Z Z22OO66
WhereWhere W = W = CaCa Na Na X = X = Mg FeMg Fe2+2+ Mn Ni Li Mn Ni Li Y = Al FeY = Al Fe3+3+ Cr Ti Cr Ti Z = Z = SiSi Al Al
Anhydrous Anhydrous so high-temperature or dry conditions so high-temperature or dry conditions favor pyroxenes over amphibolesfavor pyroxenes over amphiboles
Pyroxene ChemistryPyroxene Chemistry
The pyroxene quadrilateral and opx-cpx solvusThe pyroxene quadrilateral and opx-cpx solvusCoexisting opx + cpx in many rocks (pigeonite only in volcanics)Coexisting opx + cpx in many rocks (pigeonite only in volcanics)
DiopsideDiopside HedenbergiteHedenbergite
WollastoniteWollastonite
EnstatiteEnstatite FerrosiliteFerrosiliteorthopyroxenes
clinopyroxenes
pigeonite (Mg,Fe)(Mg,Fe)22SiSi22OO66 Ca(Mg,Fe)SiCa(Mg,Fe)Si22OO66
pigeonite clinopyroxenes
orthopyroxenes
SolvusSolvus
12001200ooCC
10001000ooCC
800800ooCC
Pyroxene ChemistryPyroxene Chemistry
““Non-quad” pyroxenesNon-quad” pyroxenesJadeiteJadeite
NaAlSiNaAlSi22OO66
Ca(Mg,Fe)SiCa(Mg,Fe)Si22OO66
AegirineAegirine
NaFeNaFe3+3+SiSi22OO66
Diopside-HedenbergiteDiopside-Hedenbergite
Ca-Tschermack’s Ca-Tschermack’s moleculemolecule CaAlCaAl22SiOSiO66
Ca / (Ca + Na)Ca / (Ca + Na)
0.20.2
0.80.8
Omphaciteaegirine- augite
AugiteAugite
Inosilicates: double chains- Inosilicates: double chains- amphibolesamphiboles
Tremolite (001) view blue = Si purple = M1 rose = M2 gray = M3 (all Mg)Tremolite (001) view blue = Si purple = M1 rose = M2 gray = M3 (all Mg)yellow = M4 (Ca)yellow = M4 (Ca)
Tremolite:Tremolite:CaCa22MgMg55 [Si [Si88OO2222] (OH)] (OH)22
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a si
na
sin
Inosilicates: double chains- Inosilicates: double chains- amphibolesamphiboles
Hornblende:Hornblende:(Ca, Na)(Ca, Na)2-3 2-3 (Mg, Fe, Al) (Mg, Fe, Al)55
[(Si,Al)[(Si,Al)88OO2222] (OH)] (OH)22
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a si
na
sin
Hornblende (001) view dark blue = Si, Al purple = M1 rose = M2 Hornblende (001) view dark blue = Si, Al purple = M1 rose = M2 light blue = M3 (all Mg, Fe) yellow ball = M4 (Ca) purple ball = A (Na)light blue = M3 (all Mg, Fe) yellow ball = M4 (Ca) purple ball = A (Na)
little turquoise ball = Hlittle turquoise ball = H
General formula:General formula:
WW0-10-1 X X22 Y Y55 [Z [Z88OO2222] (OH, F, Cl)] (OH, F, Cl)22
W = Na KW = Na K
X = Ca Na Mg FeX = Ca Na Mg Fe2+2+ (Mn Li) (Mn Li)
Y = Mg FeY = Mg Fe2+2+ Mn Al Fe Mn Al Fe3+3+ Ti Ti
Z = Si AlZ = Si Al
Again, the great variety of sites and sizes Again, the great variety of sites and sizes a great chemical range, and a great chemical range, and hence a broad stability rangehence a broad stability range
The The hydroushydrous nature implies an upper temperature stability limit nature implies an upper temperature stability limit
Amphibole ChemistryAmphibole Chemistry
Ca-Mg-Fe Amphibole “quadrilateral” (good analogy with pyroxenes)Ca-Mg-Fe Amphibole “quadrilateral” (good analogy with pyroxenes)
Amphibole ChemistryAmphibole Chemistry
TremoliteTremoliteCaCa22MgMg55SiSi88OO2222(OH)(OH)22
FerroactinoliteFerroactinoliteCaCa22FeFe55SiSi88OO2222(OH)(OH)22
AnthophylliteAnthophyllite
MgMg77SiSi88OO2222(OH)(OH)22FeFe77SiSi88OO2222(OH)(OH)22
Actinolite
Cummingtonite-grunerite
OrthoamphibolesOrthoamphiboles
ClinoamphibolesClinoamphiboles
Hornblende has Al in the tetrahedral siteHornblende has Al in the tetrahedral site
Geologists traditionally use the term “hornblende” as a catch-all term for practically Geologists traditionally use the term “hornblende” as a catch-all term for practically any dark amphibole. Now the common use of the microprobe has petrologists any dark amphibole. Now the common use of the microprobe has petrologists casting “hornblende” into end-member compositions and naming amphiboles casting “hornblende” into end-member compositions and naming amphiboles after a well-represented end-member.after a well-represented end-member.
Sodic amphiboles Sodic amphiboles
Glaucophane: NaGlaucophane: Na2 2 MgMg3 3 AlAl2 2 [Si[Si88OO2222] (OH)] (OH)22
Riebeckite: NaRiebeckite: Na2 2 FeFe2+2+3 3 FeFe3+3+
2 2 [Si[Si88OO2222] (OH)] (OH)22
Sodic amphiboles are commonly blue, and often called “blue amphiboles”Sodic amphiboles are commonly blue, and often called “blue amphiboles”
Amphibole ChemistryAmphibole Chemistry
Tremolite (Ca-Mg) occurs in meta-carbonatesTremolite (Ca-Mg) occurs in meta-carbonates
Actinolite occurs in low-grade metamorphosed basic igneous rocksActinolite occurs in low-grade metamorphosed basic igneous rocks
The complex solid solution called hornblende occurs in a broad variety of both The complex solid solution called hornblende occurs in a broad variety of both igneous and metamorphic rocksigneous and metamorphic rocks
Sodic amphiboles are predominantly metamorphic where they are Sodic amphiboles are predominantly metamorphic where they are characteristic of high P/T subduction-zone metamorphism (commonly called characteristic of high P/T subduction-zone metamorphism (commonly called “blueschist” in reference to the predominant blue sodic amphiboles “blueschist” in reference to the predominant blue sodic amphiboles
Amphibole OccurrencesAmphibole Occurrences
InosilicatesInosilicates
Cleavage angles can be interpreted in terms of weak bonds in M2 sitesCleavage angles can be interpreted in terms of weak bonds in M2 sites
Narrow single-chain I-beams Narrow single-chain I-beams 90 90oo cleavages in pyroxenes while wider double- cleavages in pyroxenes while wider double-chain I-beams chain I-beams 60-120 60-120oo cleavages in amphiboles cleavages in amphiboles
pyroxenepyroxene amphiboleamphibole
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SiOSiO44 tetrahedra polymerized into 2-D sheets: [Si tetrahedra polymerized into 2-D sheets: [Si22OO55]]
Apical O’s are unpolymerized and are bonded to other constituentsApical O’s are unpolymerized and are bonded to other constituents
PhyllosilicatesPhyllosilicates
Tetrahedral layers are bonded to octahedral layers Tetrahedral layers are bonded to octahedral layers
(OH) pairs are located in center of T rings where no apical O(OH) pairs are located in center of T rings where no apical O
PhyllosilicatesPhyllosilicates
PhyllosilicatesPhyllosilicates
Gibbsite: Al(OH)Gibbsite: Al(OH)33
Layers of octahedral Al in coordination with (OH)Layers of octahedral Al in coordination with (OH)
AlAl3+3+ means that means that only 2/3 of the VI sites may be occupiedonly 2/3 of the VI sites may be occupied for charge-balance reasons for charge-balance reasons
Brucite-type layers may be called Brucite-type layers may be called trioctahedraltrioctahedral and gibbsite-type and gibbsite-type dioctahedraldioctahedral
aa11
aa22
PhyllosilicatesPhyllosilicates
Muscovite:Muscovite: KK Al Al22 [Si [Si33AlAlOO1010] (OH)] (OH)2 2 (coupled K - Al(coupled K - AlIVIV))
T-layer - T-layer - didiocathedral (Alocathedral (Al3+3+) layer - T-layer - ) layer - T-layer - KK
T T O O T T KK T T O O T T KK T T O O TT
K between T - O - T groups is stronger than vdwK between T - O - T groups is stronger than vdw
PhyllosilicatesPhyllosilicates
Phlogopite:Phlogopite: K Mg K Mg33 [Si [Si33AlOAlO1010] (OH)] (OH)22
T-layer - T-layer - tritriocathedral (Mgocathedral (Mg2+2+) layer - T-layer - ) layer - T-layer - KK
T T O O T T KK T T O O T T KK T T O O TT
K between T - O - T groups is stronger than vdwK between T - O - T groups is stronger than vdw
Chlorite: (Mg, Fe)Chlorite: (Mg, Fe)33 [(Si, Al) [(Si, Al)44OO1010] (OH)] (OH)22 (Mg, Fe) (Mg, Fe)33 (OH) (OH)66
= T - O - T - (brucite) - T - O - T - (brucite) - T - O - T -= T - O - T - (brucite) - T - O - T - (brucite) - T - O - T -
Very hydrated (OH)Very hydrated (OH)88, so low-temperature stability (low-T metamorphism , so low-temperature stability (low-T metamorphism
and alteration of mafics as cool)and alteration of mafics as cool)
PhyllosilicatesPhyllosilicates
TectosilicatesTectosilicates
After Swamy and Saxena (1994) J. Geophys. Res., 99, 11,787-11,794.
TectosilicatesTectosilicates
Low Quartz StishoviteLow Quartz Stishovite
SiSiIVIV Si SiVIVI
TectosilicatesTectosilicates
FeldsparsFeldspars
Albite: Albite: NaNaAlAlSiSi33OO88
Substitute two Substitute two AlAl3+3+ for Si for Si4+4+ allows Caallows Ca2+2+ to to be addedbe added
Substitute AlSubstitute Al3+3+ for Sifor Si4+4+ allows allows NaNa++ or K or K++ to be to be addedadded