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nesosilicatesnesosilicates

inosilicatesinosilicates

tectosilicatestectosilicates

phyllosilicatesphyllosilicates

cyclosilictaescyclosilictaes

sorosilicatessorosilicates

Nesosilicates: independent SiONesosilicates: independent SiO44 tetrahedra tetrahedra

Olivine (100) view blue = M1 yellow = M2Olivine (100) view blue = M1 yellow = M2

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projectionprojection


M1 in rows M1 in rows and share and share edgesedges

M2 form M2 form layers in a-c layers in a-c that share that share corners corners

Some M2 and Some M2 and M1 share M1 share edgesedges

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M1 and M2 as polyhedraM1 and M2 as polyhedra

Olivine – complete solid solution Olivine – complete solid solution Forsterite-Fayalite Forsterite-Fayalite Fo FoxxFaFayy

Fayalite – Fe end-member Fayalite – Fe end-member Forsterite – Mg end-memberForsterite – Mg end-member

Olivine Occurrences:Olivine Occurrences: Principally in mafic and ultramafic igneous and meta-igneous Principally in mafic and ultramafic igneous and meta-igneous

rocksrocks Fayalite in meta-ironstones and in some alkalic granitoidsFayalite in meta-ironstones and in some alkalic granitoids Forsterite in some siliceous dolomitic marblesForsterite in some siliceous dolomitic marbles

Monticellite CaMgSiOMonticellite CaMgSiO44 Ca Ca M2 (larger ion, larger site) M2 (larger ion, larger site)High grade metamorphic siliceous carbonatesHigh grade metamorphic siliceous carbonates

Olivine mineralsOlivine minerals Solid solution forsterite-fayalite, tephroite-Solid solution forsterite-fayalite, tephroite-

glaucochroite, monticellite-kirschsteiniteglaucochroite, monticellite-kirschsteinite Not in between Not in between no forsterite-tephroite series no forsterite-tephroite series

Larnite – CaLarnite – Ca22SiOSiO44

Distinguishing Forsterite-FayaliteDistinguishing Forsterite-Fayalite

Petrographic MicroscopePetrographic Microscope Index of refraction Index of refraction careful of zoning!! careful of zoning!! 2V different in different composition ranges2V different in different composition ranges Pleochroism/ color slightly differentPleochroism/ color slightly different

Spectroscopic techniques – many ways to Spectroscopic techniques – many ways to determine Fe vs. Mgdetermine Fe vs. Mg

Same space group (Pbnm), Orthorhombic, slight Same space group (Pbnm), Orthorhombic, slight differences in unit cell dimensions onlydifferences in unit cell dimensions only

Back to silicate structures:Back to silicate structures:

nesosilicatesnesosilicates

inosilicatesinosilicates

tectosilicatestectosilicates

phyllosilicatesphyllosilicates

cyclosilictaescyclosilictaes

sorosilicatessorosilicates

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


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a si

na

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Perspective viewPerspective view



SiOSiO44 as polygons as polygons

(and larger area)(and larger area)IV slabIV slab

IV slabIV slab

IV slabIV slab

IV slabIV slab

VI slabVI slab

VI slabVI slab

VI slabVI slab

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M1 octahedronM1 octahedron





(+) type by convention(+) type by convention

(+)



This is a (-) typeThis is a (-) type

(-)


TT

M1M1

TT

Creates an “I-beam” Creates an “I-beam” like unit in the like unit in the

structure.structure.


TT

M1M1

TT

Creates an “I-beam” Creates an “I-beam” like unit in the like unit in the

structurestructure

(+)(+)

The pyroxene The pyroxene structure is then structure is then

composed of composed of alternating I-beamsalternating I-beams

Clinopyroxenes have Clinopyroxenes have all I-beams oriented all I-beams oriented the same: all are (+) the same: all are (+) in this orientation in this orientation

(+)(+)

(+)(+)(+)(+)

(+)(+)(+)(+)


Note that M1 sites are Note that M1 sites are smaller than M2 sites, since smaller than M2 sites, since they are at the apices of the they are at the apices of the

tetrahedral chainstetrahedral chains

The pyroxene The pyroxene structure is then structure is then

composed of composed of alternation I-beamsalternation I-beams

Clinopyroxenes have Clinopyroxenes have all I-beams oriented all I-beams oriented the same: all are (+) the same: all are (+) in this orientation in this orientation

(+)(+)

(+)(+)(+)(+)


(+)(+)(+)(+)

Tetrehedra and M1 Tetrehedra and M1 octahedra share octahedra share

tetrahedral apical tetrahedral apical oxygen atoms oxygen atoms


The tetrahedral chain The tetrahedral chain above the M1s is thus above the M1s is thus offset from that below offset from that below

The M2 slabs have a The M2 slabs have a similar effectsimilar effect

The result is a The result is a monoclinicmonoclinic unit cell, unit cell, hence hence clinopyroxenesclinopyroxenes


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(+) M1(+) M1

(+) M2(+) M2

(+) M2(+) M2

OrthopyroxenesOrthopyroxenes have have alternating (+) and (-) alternating (+) and (-)

I-beams I-beams

the offsets thus the offsets thus compensate and result compensate and result in an in an orthorhombicorthorhombic

unit cellunit cell


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aa

(+) 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 ChemistryThe pyroxene quadrilateral and opx-cpx solvusThe pyroxene quadrilateral and opx-cpx solvus

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

DiopsideDiopsideCaMgSiCaMgSi22OO66

HedenbergiteHedenbergite CaFeSiCaFeSi22OO66

Wollastonite CaWollastonite Ca22SiSi22OO66

EnstatiteEnstatiteMgMg22SiSi22OO66

FerrosiliteFerrosiliteFeFe22SiSi22OO66

orthopyroxenes

clinopyroxenes

pigeonite

•OrthopyroxenesOrthopyroxenes – solid soln – solid soln between Enstatite-Ferrosilitebetween Enstatite-Ferrosilite•Clinopyroxenes – solid soln – solid soln between Diopside-Hedenbergitebetween Diopside-Hedenbergite

Joins – lines between end Joins – lines between end members – limited mixing members – limited mixing away from joinaway from join

Orthopyroxene - ClinopyroxeneOrthopyroxene - ClinopyroxeneOPX and CPX have different crystal structures OPX and CPX have different crystal structures

– results in a complex solvus between them– results in a complex solvus between themCoexisting opx + cpx in many rocks (pigeonite only in volcanics)Coexisting opx + cpx in many rocks (pigeonite only in volcanics)

DiopsideDiopsideCaMgSiCaMgSi22OO66

HedenbergiteHedenbergite CaFeSiCaFeSi22OO66

Wollastonite CaWollastonite Ca22SiSi22OO66

EnstatiteEnstatiteMgMg22SiSi22OO66

FerrosiliteFerrosiliteFeFe22SiSi22OO66

orthopyroxenes

clinopyroxenes

pigeonite

(Mg,Fe)(Mg,Fe)22SiSi22OO66 Ca(Mg,Fe)SiCa(Mg,Fe)Si22OO66

pigeonite clinopyroxenes

orthopyroxenes

SolvusSolvus

12001200ooCC

10001000ooCC

800800ooCC

OPXOPX CPXCPX

CPXCPX

OPXOPX

Orthopyroxene – ClinopyroxeneOrthopyroxene – Clinopyroxenesolvus T dependencesolvus T dependence

Complex solvus – the ‘stability’ of a particular mineral changes Complex solvus – the ‘stability’ of a particular mineral changes with T. A different mineral’s ‘stability’ may change with T with T. A different mineral’s ‘stability’ may change with T differently…differently…

OPX-CPX exsolution lamellae OPX-CPX exsolution lamellae Geothermometer… Geothermometer…

MiscibilityGap

FsFsEnEn

DiDi HdHd

FsFsEnEn

DiDi HdHd

OPXOPXOPXOPX

CPXCPX CPXCPX

pigeonite

augite

orthopyroxene

Pigeonite + orthopyroxene

orthopyroxene

Subcalcic augite

pigeonite

augite

MiscibilityGap

800800ºCºC 12001200ºCºC

Pyroxene ChemistryPyroxene Chemistry““Non-quad” pyroxenesNon-quad” pyroxenesJadeiteJadeite

NaAlSiNaAlSi22OO66

Ca(Mg,Fe)SiCa(Mg,Fe)Si22OO66

AegirineAegirineNaFeNaFe3+3+SiSi22OO66

Diopside-HedenbergiteDiopside-Hedenbergite

Ca-Tschermack’s Ca-Tschermack’s moleculemolecule CaAl2SiOCaAl2SiO66

Ca / (Ca + Na)Ca / (Ca + Na)

0.20.2

0.80.8

Omphaciteaegirine- augite

AugiteAugite

Spodumene: Spodumene: LiAlSiLiAlSi22OO66

PyroxenoidsPyroxenoids““Ideal” pyroxene chains with Ideal” pyroxene chains with

5.2 A repeat (2 tetrahedra) 5.2 A repeat (2 tetrahedra) become distorted as other become distorted as other cations occupy VI sitescations occupy VI sites

WollastoniteWollastonite (Ca (Ca M1) M1) 3-tet repeat3-tet repeat

RhodoniteRhodoniteMnSiOMnSiO33

5-tet repeat5-tet repeat

PyroxmangitePyroxmangite (Mn, Fe)SiO(Mn, Fe)SiO33

7-tet repeat7-tet repeat

PyroxenePyroxene2-tet repeat2-tet repeat

7.1 A12.5 A

17.4 A

5.2 A

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