Mineral structures
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Mineral Structures. the [SiO 4 ] 4- tetrahedron. Silicates are classified on the basis of Si-O polymerism . Mineral Structures. [SiO 4 ] 4- Independent tetrahedra Nesosilicates Examples: olivine garnet [Si 2 O 7 ] 6- Double tetrahedra Sorosilicates

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Mineral Structures

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Mineral structures

Mineral Structures

the [SiO4]4- tetrahedron

Silicates are classified on the basis of Si-O polymerism


Mineral structures1

Mineral Structures

[SiO4]4- Independent tetrahedra Nesosilicates

Examples: olivine garnet

[Si2O7]6- Double tetrahedra Sorosilicates

Examples: lawsonite epidote

n[SiO3]2- n = 3, 4, 6 Cyclosilicates

Examples: benitoite BaTi[Si3O9]

beryl Be3Al2[Si6O18]

Silicates are classified on the basis of Si-O polymerism


Mineral structures2

Mineral Structures

Inosilicates

[SiO3]2- single chains Inosilicates [Si4O11]4- Double chains

pryoxenes pyroxenoids amphiboles


Mineral structures3

Mineral Structures

Phyllosilicates

[Si2O5]2- Sheets of tetrahedra Phyllosilicates

micas talc clay minerals serpentine


Mineral structures4

Mineral Structures

Tectosilcates

low-quartz

[SiO2] 3-D frameworks of tetrahedra: fully polymerized Tectosilicates

quartz and the silica minerals feldspars feldspathoids zeolites


Nesosilicates independent sio 4 tetrahedra

b

Nesosilicates: independent SiO4 tetrahedra

c

M1 and M2 as polyhedra

Olivine (100) view blue = M1 yellow = M2


Mineral structures

Nesosilicates: Olivine (Mg,Fe)2SiO4

Olivine Occurrences:

Principally in mafic and ultramafic igneous rocks- Typically ~60+% of mantle source for basalts-

Fayalite in meta-ironstones and in some alkalic granitoids

Forsterite in some siliceous dolomitic marbles


Nesosilicates garnet

  • Garnet: A2+3 B3+2 [SiO4]3

  • “Pyralspites” - B = Al

    • Pyrope: Mg3 Al2 [SiO4]3

    • Almandine: Fe3 Al2 [SiO4]3

    • Spessartine: Mn3 Al2 [SiO4]3

  • “Ugrandites” - A = Ca

    • Uvarovite: Ca3 Cr2 [SiO4]3

    • Grossularite: Ca3 Al2 [SiO4]3

    • Andradite: Ca3 Fe2 [SiO4]3

  • Occurrence:

    • Mostly metamorphic

    • Some high-Al igneous

    • Also in some mantle peridotites

Nesosilicates: Garnet

Garnet (001) view blue = Si purple = A turquoise = B


Inosilicates single chains pyroxenes

b

Diopside: CaMg [Si2O6]

a sin

Where are the Si-O-Si-O chains??

Inosilicates: single chains- pyroxenes

Diopside (001) view blue = Si purple = M1 (Mg) yellow = M2 (Ca)


Inosilicates single chains pyroxenes1

b

a sin

Inosilicates: single chains- pyroxenes

Diopside (001) view blue = Si purple = M1 (Mg) yellow = M2 (Ca)


Mineral structures

Inosilicates: single chains- pyroxenes

The tetrahedral chain above the M1s is offset from that below

The result is a monoclinic unit cell, hence clinopyroxenes

e.g. Diopside, Augite

(+) M2

c

a

(+) M1

(+) M2


Mineral structures

Inosilicates: single chains- pyroxenes

Orthopyroxene

an orthorhombic unit cell

Enstatite (Mg2Si2O6)

c

(-) M1

(+) M2

a

(+) M1

(-) M2


Pyroxene chemistry

Pyroxene Chemistry

The general pyroxene formula:

W1-P (X,Y)1+P Z2O6

Where

  • W = Ca Na

  • X = Mg Fe2+ Mn Ni Li

  • Y = Al Fe3+ Cr Ti

  • Z = Si Al

    Anhydrous so high-temperature or dry conditions favor pyroxenes over amphiboles


Pyroxene chemistry1

Pyroxene Chemistry

The pyroxene quadrilateral and opx-cpx solvus

Coexisting opx + cpx in many rocks (pigeonite only in volcanics)

Wollastonite

pigeonite

1200oC

orthopyroxenes

clinopyroxenes

1000oC

Diopside

Hedenbergite

clinopyroxenes

Solvus

800oC

pigeonite

(Mg,Fe)2Si2O6

Ca(Mg,Fe)Si2O6

orthopyroxenes

Ferrosilite

Enstatite


Pyroxene chemistry2

Pyroxene Chemistry

Jadeite

Aegirine

NaAlSi2O6

“Non-quad” pyroxenes

NaFe3+Si2O6

0.8

Omphacite

aegirine- augite

Ca / (Ca + Na)

Ca-Tschermack’s molecule

0.2

CaAl2SiO6

Augite

Diopside-Hedenbergite

Ca(Mg,Fe)Si2O6


Inosilicates double chains amphiboles

b

Tremolite:

Ca2Mg5 [Si8O22] (OH)2

a sin

Inosilicates: double chains- amphiboles

Tremolite (001) view blue = Si purple = M1 rose = M2 gray = M3 (all Mg)

yellow = M4 (Ca)


Inosilicates double chains amphiboles1

b

Hornblende:

(Ca, Na)2-3 (Mg, Fe, Al)5 [(Si,Al)8O22] (OH)2

a sin

Inosilicates: double chains- amphiboles

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)

little turquoise ball = H


Mineral structures

Amphibole Chemistry

  • General formula:

    • W0-1 X2 Y5 [Z8O22] (OH, F, Cl)2

    • W = Na K

    • X = Ca Na Mg Fe2+ (Mn Li)

    • Y = Mg Fe2+ Mn Al Fe3+ Ti

    • Z = Si Al

  • Again, the great variety of sites and sizes  a great chemical range, and hence a broad stability range

  • The hydrous nature implies an upper temperature stability limit


Mineral structures

Amphibole Chemistry

Ca-Mg-Fe Amphibole “quadrilateral” (good analogy with pyroxenes)

Tremolite

Ferroactinolite

Actinolite

Ca2Mg5Si8O22(OH)2

Ca2Fe5Si8O22(OH)2

Clinoamphiboles

Cummingtonite-grunerite

Anthophyllite

Fe7Si8O22(OH)2

Mg7Si8O22(OH)2

Orthoamphiboles


Mineral structures

Amphibole Chemistry

  • Hornblende has Al in the tetrahedral site

    • 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 casting “hornblende” into end-member compositions and naming amphiboles after a well-represented end-member.

  • Sodic amphiboles

    • Glaucophane: Na2 Mg3 Al2 [Si8O22] (OH)2

    • Riebeckite: Na2 Fe2+3 Fe3+2 [Si8O22] (OH)2

    • Sodic amphiboles are commonly blue, and often called “blue amphiboles”


Mineral structures

Amphibole Occurrences

Tremolite (Ca-Mg) occurs in meta-carbonates

Actinolite occurs in low-grade metamorphosed basic igneous rocks

The complex solid solution called hornblende occurs in a broad variety of both igneous and metamorphic rocks

Sodic amphiboles are predominantly metamorphic where they are characteristic of high P/T subduction-zone metamorphism (commonly called “blueschist” in reference to the predominant blue sodic amphiboles


Inosilicates

pyroxene

amphibole

Inosilicates

b

a

Cleavage angles can be interpreted in terms of weak bonds in M2 sites

Narrow single-chain I-beams  90o cleavages in pyroxenes while wider double-chain I-beams  60-120o cleavages in amphiboles


Mineral structures

Phyllosilicates

SiO4 tetrahedra polymerized into 2-D sheets: [Si2O5]

Apical O’s are unpolymerized and are bonded to other constituents


Mineral structures

Phyllosilicates

Tetrahedral layers are bonded to octahedral layers

(OH) pairs are located in center of T rings where no apical O


Mineral structures

Phyllosilicates

a2

a1

Gibbsite: Al(OH)3

Layers of octahedral Al in coordination with (OH)

Al3+ means that only 2/3 of the VI sites may be occupied for charge-balance reasons

Brucite-type layers may be called trioctahedral and gibbsite-type dioctahedral


Mineral structures

Phyllosilicates

T O T K T O T K T O T

Muscovite:K Al2 [Si3AlO10] (OH)2 (coupled K - AlIV)

T-layer - diocathedral (Al3+) layer - T-layer - K

K between T - O - T groups is stronger than vdw


Mineral structures

Phyllosilicates

T O T K T O T K T O T

Phlogopite: K Mg3 [Si3AlO10] (OH)2

T-layer - triocathedral (Mg2+) layer - T-layer - K

K between T - O - T groups is stronger than vdw


Mineral structures

Phyllosilicates

  • Chlorite: (Mg, Fe)3 [(Si, Al)4O10] (OH)2 (Mg, Fe)3 (OH)6

  • = T - O - T - (brucite) - T - O - T - (brucite) - T - O - T -

    • Very hydrated (OH)8, so low-temperature stability (low-T metamorphism and alteration of mafics as cool)


Tectosilicates

Tectosilicates

After Swamy and Saxena (1994)J. Geophys. Res., 99, 11,787-11,794.


Tectosilicates1

Tectosilicates

Low Quartz Stishovite

SiIVSiVI


Tectosilicates2

Tectosilicates

Feldspars

Substitute Al3+ for Si4+ allows Na+ or K+ to be added

Substitute two Al3+ for Si4+ allows Ca2+ to be added

Albite: NaAlSi3O8


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