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Back to silicate structures:Back to silicate structures:

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

bb

cc

projectionprojection

Inosilicates: single chains- pyroxenes Inosilicates: single chains- pyroxenes

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


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

(+)(+)

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


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


composed of composed of alternation I-beamsalternation I-beams

Orthoopyroxenes Orthoopyroxenes have I-beams oriented have I-beams oriented in alternate direction in alternate direction

in different layersin different layers

(-)(-)

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


(-)(-)(-)(-)

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


cc

aa

(+) 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


cc

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


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)

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


““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 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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nesosilicatesnesosilicates

inosilicatesinosilicates

tectosilicatestectosilicates

phyllosilicatesphyllosilicates

cyclosilictaescyclosilictaes

sorosilicatessorosilicates

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

bb

a si

na

sin


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

bb

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


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)light blue = M3 (all Mg, Fe)

Hornblende:Hornblende:(Ca, Na)(Ca, Na)2-3 2-3 (Mg, Fe, (Mg, Fe,

Al)Al)55 [(Si,Al) [(Si,Al)88OO2222] (OH)] (OH)22

Same I-beam Same I-beam architecture, but architecture, but the I-beams are the I-beams are fatter (double fatter (double

chains)chains)


bb

a si

na

sin

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

(+)(+)

(+)(+)

(+)(+)

Same I-beam Same I-beam architecture, but architecture, but the I-beams are the I-beams are fatter (double fatter (double

chains)chains)

All are (+) on All are (+) on clinoamphiboles clinoamphiboles and alternate in and alternate in

orthoamphibolesorthoamphiboles



Hornblende:Hornblende:(Ca, Na)(Ca, Na)2-3 2-3 (Mg, Fe, (Mg, Fe,

Al)Al)55 [(Si,Al) [(Si,Al)88OO2222] (OH)] (OH)22





[(Si,Al)[(Si,Al)88OO2222] (OH)] (OH)22

M1-M3 are small sitesM1-M3 are small sites

M4 is larger (Ca)M4 is larger (Ca)

A-site is really bigA-site is really big

Variety of sites Variety of sites great chemical rangegreat chemical range





[(Si,Al)[(Si,Al)88OO2222] (OH)] (OH)22

(OH) is in center of (OH) is in center of tetrahedral ring where O tetrahedral ring where O is a part of M1 and M3 is a part of M1 and M3

octahedraoctahedra

(OH)(OH)

See handout for more informationSee handout for more information

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)


Al and Na tend to stabilize the orthorhombic form in low-Ca amphiboles, so anthophyllite Al and Na tend to stabilize the orthorhombic form in low-Ca amphiboles, so anthophyllite gedrite orthorhombic series extends to Fe-rich gedrite in more Na-Al-rich compositions gedrite orthorhombic series extends to Fe-rich gedrite in more Na-Al-rich compositions

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”


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

Orthoamphiboles and cummingtonite-grunerite (all Ca-free, Mg-Fe-rich Orthoamphiboles and cummingtonite-grunerite (all Ca-free, Mg-Fe-rich amphiboles) are metamorphic and occur in meta-ultrabasic rocks and some amphiboles) are metamorphic and occur in meta-ultrabasic rocks and some meta-sediments. The Fe-rich grunerite occurs in meta-ironstonesmeta-sediments. The Fe-rich grunerite occurs in meta-ironstones

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

Riebeckite occurs commonly in sodic granitoid rocksRiebeckite occurs commonly in sodic granitoid rocks

Amphibole OccurrencesAmphibole Occurrences

InosilicatesInosilicates

Pyroxenes and amphiboles are very similar:Pyroxenes and amphiboles are very similar: Both have chains of SiOBoth have chains of SiO44 tetrahedra tetrahedra The chains are connected into stylized I-beams by M octahedraThe chains are connected into stylized I-beams by M octahedra High-Ca monoclinic forms have all the T-O-T offsets in the same directionHigh-Ca monoclinic forms have all the T-O-T offsets in the same direction Low-Ca orthorhombic forms have alternating (+) and (-) offsetsLow-Ca orthorhombic forms have alternating (+) and (-) offsets

++++ ++

++

++++++

++++ ---- --

----

--

++

++++

aa

aa

++++ ++

++++ ++

++++ ++

++++ ++

----

--

----

--

ClinopyroxeneClinopyroxene

OrthopyroxeneOrthopyroxene OrthoamphiboleOrthoamphibole

ClinoamphiboleClinoamphibole

InosilicatesInosilicates

Cleavage angles can be interpreted in terms of weak bonds in M2 sites Cleavage angles can be interpreted in terms of weak bonds in M2 sites (around I-beams instead of through them)(around I-beams instead of through them)

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

aa

bb

TectosilicatesTectosilicates

Stishovite

Coesite

- quartz

- quartz

Liquid

TridymiteCristobalite

600 1000 1400 1800 2200 2600

2

4

6

8

10P

ress

ure

(GP

a)

Temperature oC

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


Low QuartzLow Quartz

001 Projection Crystal Class 32001 Projection Crystal Class 32

Stishovite

Coesite

- quartz

- quartz

Liquid



High Quartz at 581High Quartz at 581ooCC

001 Projection Crystal Class 622001 Projection Crystal Class 622

Stishovite

Coesite

- quartz

- quartz

Liquid



CristobaliteCristobalite

001 Projection Cubic Structure001 Projection Cubic Structure

Stishovite

Coesite

- quartz

- quartz

Liquid



StishoviteStishovite

High pressure High pressure Si SiVIVI

Stishovite

Coesite

- quartz

- quartz

Liquid



Low Quartz StishoviteLow Quartz Stishovite

SiSiIVIV Si SiVIVI

Igneous MineralsIgneous Minerals

Quartz, Feldspars (plagioclase and alkaline), Quartz, Feldspars (plagioclase and alkaline), Olivines, Pyroxenes, AmphibolesOlivines, Pyroxenes, Amphiboles

Accessory Minerals – mostly in small quantities Accessory Minerals – mostly in small quantities or in ‘special’ rocksor in ‘special’ rocks Magnetite (FeMagnetite (Fe33OO44))

Ilmenite (FeTiOIlmenite (FeTiO33))

Apatite (CaApatite (Ca55(PO(PO44))33(OH,F,Cl)(OH,F,Cl)

Zircon (ZrSiOZircon (ZrSiO44))

Titanite (CaTiSiOTitanite (CaTiSiO55))

Pyrite (FeSPyrite (FeS22))

Fluorite (CaFFluorite (CaF22))

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