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Inosilicates (chain) Common Fe/Mg – bearing silicates Common Fe/Mg – bearing silicates Two common groups Two common groups Pyroxenes: single chains Pyroxenes:

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Presentation on theme: "Inosilicates (chain) Common Fe/Mg – bearing silicates Common Fe/Mg – bearing silicates Two common groups Two common groups Pyroxenes: single chains Pyroxenes:"— Presentation transcript:

1 Inosilicates (chain) Common Fe/Mg – bearing silicates Common Fe/Mg – bearing silicates Two common groups Two common groups Pyroxenes: single chains Pyroxenes: single chains Amphiboles: double chains Amphiboles: double chains Pyroxenes are common in MORB Pyroxenes are common in MORB Amphiboles more common on continents because of weathering Amphiboles more common on continents because of weathering

2 Pyroxene group General formula: XYZ 2 O 6 General formula: XYZ 2 O 6 Z/O ratio = 1/3 Z/O ratio = 1/3 Z cations usually Si, occasionally Al Z cations usually Si, occasionally Al Single chain extend along c axis Single chain extend along c axis Chains are stacked along a axis, alternating: Chains are stacked along a axis, alternating: Base faces base Base faces base Apex faces apex Apex faces apex

3 Fig Base facing base Apex facing Apex View down a axis View down c axis Two distinct sites, depending on location relative to chains M1 and M2

4 X cations in M2 sites X cations in M2 sites Between bases of tetrahedrons Between bases of tetrahedrons Distorted 6- and 8- fold coordination Distorted 6- and 8- fold coordination Depends on stacking and the size of the cations Depends on stacking and the size of the cations Y cations in M1 sites Y cations in M1 sites 6-fold coordination between apical oxygen 6-fold coordination between apical oxygen

5 “I-beams” “I-beams” Consist of two chains connected by Y cations Consist of two chains connected by Y cations Located in M1 sites Located in M1 sites Closeness of apical oxygen and 6-fold coordination make bonds strong Closeness of apical oxygen and 6-fold coordination make bonds strong I-beam Apex pointed at apex

6 I-beams held together by X cations in M2 site I-beams held together by X cations in M2 site Coordination number depends on how chains line up Coordination number depends on how chains line up 6-fold coordination gives orthorhombic symmetry - OPX 6-fold coordination gives orthorhombic symmetry - OPX 8-fold coordination gives monoclinic symmetry - CPX 8-fold coordination gives monoclinic symmetry - CPX

7 OPX - Orthorhombic Pigeonite – CPX - Monoclinic Crystallographic and optical axes align C crystallographic axis at 32 to 42º angle to the Z optical axis

8 Crystal shapes Crystal shapes Blocky prisms, nearly square Blocky prisms, nearly square Elongate along c axis Elongate along c axis Cleavage controlled by I-beams Cleavage controlled by I-beams Cleavage typically between 87º and 93º Cleavage typically between 87º and 93º Only when viewed down the c axis Only when viewed down the c axis Mineral grain must be cut parallel to (001) Mineral grain must be cut parallel to (001)

9 Fig Weak planes between “I beams” = cleavage Cleavage angles are 87º and 93º I beams – tightly bonded Weak zones between faces of I beams

10 Classification Based on two linked things Based on two linked things Which cations occurs in M2 sites (facing bases of tetrahedron) Which cations occurs in M2 sites (facing bases of tetrahedron) Cation determines symmetry Cation determines symmetry Most plot on ternary diagram with apices: Most plot on ternary diagram with apices: Wollastonite, Wo Wollastonite, Wo Enstatite, En Enstatite, En Ferrosilite, Fe Ferrosilite, Fe

11 Three major groups Three major groups Orthopyroxenes (opx) – orthorhombic Orthopyroxenes (opx) – orthorhombic Low-Ca clinopyroxenes (cpx) – monoclinic Low-Ca clinopyroxenes (cpx) – monoclinic Ca-rich clinopyroxenes (cpx) – monoclinic Ca-rich clinopyroxenes (cpx) – monoclinic The amount of Ca in the mineral controls the extinction angle The amount of Ca in the mineral controls the extinction angle

12 Orthopyroxenes: Fe and Mg, but little Ca Orthopyroxenes: Fe and Mg, but little Ca Both M1 and M2 are octahedral Both M1 and M2 are octahedral Larger Fe ion more concentrated in M2 site (larger) Larger Fe ion more concentrated in M2 site (larger)

13 Low-Ca clinopyroxene: more Ca, but no solid solution with Hi-Ca clinopyroxene Low-Ca clinopyroxene: more Ca, but no solid solution with Hi-Ca clinopyroxene Mineral species is Pigeonite Mineral species is Pigeonite Ca restricted to M2 sites, these still mostly Fe and Mg Ca restricted to M2 sites, these still mostly Fe and Mg M1 sites all Mg and Fe M1 sites all Mg and Fe

14 Ca- clinopyroxene Ca- clinopyroxene Diopside Mg(+Ca) to Hedenbergite Fe (+Ca) Diopside Mg(+Ca) to Hedenbergite Fe (+Ca) M2 site contains mostly Ca M2 site contains mostly Ca M1 site contains mostly Fe and Mg M1 site contains mostly Fe and Mg Most common specie is augite Most common specie is augite Al substitutes in M1 site, and for Si in tetrahedral site Al substitutes in M1 site, and for Si in tetrahedral site Na, Fe or Mg substitutes for Ca in M2 site Na, Fe or Mg substitutes for Ca in M2 site

15 Other common pyroxenes Other common pyroxenes Jadeite NaAlSi 2 O 6 Jadeite NaAlSi 2 O 6 Spodumene LiAlSi 2 O 6 Spodumene LiAlSi 2 O 6

16 Fig “Augite” Clinopyroxene Orthopyroxenes Na – bearing pyroxenes Possible ranges of solid solutions

17 Identification in hand-sample difficult Identification in hand-sample difficult Mostly based on occurrence Mostly based on occurrence Also color can be indicative Also color can be indicative Optical properties distinguish clino- from ortho-pyroxenes Optical properties distinguish clino- from ortho-pyroxenes If composition is important, need chemical analysis If composition is important, need chemical analysis

18 Geology of pyroxenes Igneous Igneous Common igneous pyroxenes: augite, pigeonite, and opx Common igneous pyroxenes: augite, pigeonite, and opx Augite most common Augite most common Usually in mafic and intermediate volcanics Usually in mafic and intermediate volcanics Both intrusive and extrusive Both intrusive and extrusive Zoning common: magma becomes enriched in Fe because of partition of Mg into crystals Zoning common: magma becomes enriched in Fe because of partition of Mg into crystals Requires 3 component phase diagram Requires 3 component phase diagram Exsolution common – cooling allows rearrangement of Ca Exsolution common – cooling allows rearrangement of Ca

19 Exsolution mechanisms Augite original crystallization Augite original crystallization Ca substitution in M2 sites restricted Ca substitution in M2 sites restricted As cools, Ca reorganizes As cools, Ca reorganizes Generally find exsolution lamellae of pigeonite (low Ca cpx) within host augite parallel to (001) or opx parallel to (100) Generally find exsolution lamellae of pigeonite (low Ca cpx) within host augite parallel to (001) or opx parallel to (100) Augite Matrix

20 Opx crystallize at high T with excess Ca – up to 10% Opx crystallize at high T with excess Ca – up to 10% Slow cooling allows Ca expelled to form exsolution of augite (hi- Ca cpx) Slow cooling allows Ca expelled to form exsolution of augite (hi- Ca cpx) Single lamellae of augite parallel to (100) Single lamellae of augite parallel to (100) Bushveld variety – S. Africa type location Bushveld variety – S. Africa type location Opx Matrix

21 Pigeonite grows in mafic magma Pigeonite grows in mafic magma Up to 10% Ca in M2 site Up to 10% Ca in M2 site Cooling causes Ca to expel and form augite (hi-Ca cpx) lamellae Cooling causes Ca to expel and form augite (hi-Ca cpx) lamellae Single lamellae parallel to (001) Single lamellae parallel to (001) Pigeonite Matrix

22 If slow enough pigeonite converts to opx If slow enough pigeonite converts to opx Pigeonite only preserved where cooling fast (volcanic) Pigeonite only preserved where cooling fast (volcanic) Slow cooling creates second set augite (hi-Ca cpx) parallel to (100) Slow cooling creates second set augite (hi-Ca cpx) parallel to (100) “Stillwater type” “Stillwater type” Opx Matrix

23 Metamorphic Carbonate rocks, typically diopside because of Ca and Mg from calcite and dolomite Carbonate rocks, typically diopside because of Ca and Mg from calcite and dolomite Amphibolite common association (water) Amphibolite common association (water) Na and Ca clinopyroxenes Na and Ca clinopyroxenes Typically restricted to high T and low P conditions Typically restricted to high T and low P conditions Found at subduction zones (blue schist facies) Found at subduction zones (blue schist facies)

24 Opx also in granulite facies rocks Opx also in granulite facies rocks Hot enough to remove water Hot enough to remove water Derived from amphiboles Derived from amphiboles

25 Sedimentary Not stable (anhydrous) Not stable (anhydrous) Converts to clay minerals Converts to clay minerals

26 Amphibole Group Structure, composition, and classification similar to pyroxenes Structure, composition, and classification similar to pyroxenes Primary difference is they are double chains Primary difference is they are double chains Z/O ratio is 4/11 Z/O ratio is 4/11

27 Structure Chains extend parallel to c axis Chains extend parallel to c axis Stacked in alternating fashion like pyroxenes Stacked in alternating fashion like pyroxenes Points face points and bases face bases Points face points and bases face bases

28 Fig Chains are linked by sheets of octahedral sites Chains are linked by sheets of octahedral sites Three unique sites: M1, M2, and M3 Three unique sites: M1, M2, and M3 Depend on location relative to Si tetrahedron Depend on location relative to Si tetrahedron OH Not shared O Shared O

29 TOT layers TOT layers Two T layers (tetrahedral layers with Z ions) Two T layers (tetrahedral layers with Z ions) Intervening O layer (octahedron) with M1, M2, and M3 sites Intervening O layer (octahedron) with M1, M2, and M3 sites Form “I-beams” similar to pyroxenes Form “I-beams” similar to pyroxenes

30 Geometry produces five different structure sites Geometry produces five different structure sites M1, M2, and M3 between points of chains M1, M2, and M3 between points of chains M4 and A sites between bases of chains M4 and A sites between bases of chains

31 Bonds at M4 and A sites weaker than bonds within “I-beams” Bonds at M4 and A sites weaker than bonds within “I-beams” Cleavage forms along the weak bonds Cleavage forms along the weak bonds “I-beams” wider than pyroxenes “I-beams” wider than pyroxenes Cleavage angles around 56º and 124º Cleavage angles around 56º and 124º Weak planes between “I beams” = cleavage

32 Composition W 0-1 X 2 Y 5 Z 8 O 22 (OH) 2 Each cation fits a particular site Each cation fits a particular site W cation W cation Occurs in A site Occurs in A site Has ~10 fold coordination Has ~10 fold coordination Generally large, usually Na + Generally large, usually Na +

33 W 0-1 X 2 Y 5 Z 8 O 22 (OH) 2 X cations X cations Located in M4 sites Located in M4 sites Analogous to M2 sites in pyroxenes Analogous to M2 sites in pyroxenes Have 6 or 8 fold coordination depending on arrangement of chains Have 6 or 8 fold coordination depending on arrangement of chains If 8-fold, X usually Ca If 8-fold, X usually Ca If 6-fold, X usually Fe or Mg If 6-fold, X usually Fe or Mg

34 W 0-1 X 2 Y 5 Z 8 O 22 (OH) 2 Y cations Y cations Located in M1, M2, and M3 sites; Octahedral cations in TOT strips Located in M1, M2, and M3 sites; Octahedral cations in TOT strips Usually Mg, Fe 2+, Fe 3+, Al Usually Mg, Fe 2+, Fe 3+, Al Z cations Z cations Usually Si and Al Usually Si and Al

35 Composition Composition Most common amphiboles shown on ternary diagram Most common amphiboles shown on ternary diagram Wide variety of substitution, simple and coupled Wide variety of substitution, simple and coupled Divided into ortho and clino amphiboles Divided into ortho and clino amphiboles Depends on X cations in M4 site (largely amount of Ca), distorts structure Depends on X cations in M4 site (largely amount of Ca), distorts structure Reduces symmetry from orthorhombic to monoclinic Reduces symmetry from orthorhombic to monoclinic

36 Fig ~30% Ca exactly 2/7 of sites available for Ca Anthophylite Orthorhomic Grunerite Monoclinic TremoliteFerroactinolite W 0-1 X 2 Y 5 Z 8 O 22 (OH) 2

37 Pyroxenes and Amphiboles

38 Identification Identification Hand sample and thin section difficult Hand sample and thin section difficult Best method is association Best method is association Ca and Na amphiboles commonly dark green to black, pleochroic: usually Hornblende Ca and Na amphiboles commonly dark green to black, pleochroic: usually Hornblende White or pale green amphiboles usually called tremolite White or pale green amphiboles usually called tremolite

39 Geology of amphiboles Several important aspects Several important aspects Hydrous – water part of their structure Hydrous – water part of their structure Not stable in anhydrous environments Not stable in anhydrous environments Dehydrate at high temperature Dehydrate at high temperature High Z/O ratio (4/11) mean they should occur in Si-rich rocks High Z/O ratio (4/11) mean they should occur in Si-rich rocks

40 Generalization Generalization 1. Not common in mafic and ultramafic rocks 1. Crystallize late in magmatic history; melt rich in Si and H 2 O 2. Overgrowths of amphibole on pyroxenes common 2. Common in felsic to intermediate rocks 1. Fe and Mg minerals either amphibole or biotite 2. Depends on abundance of K (biotite) and Ca/Na (amphiboles) Generally amphibole tends toward intermediate rocks; biotite toward felsic Generally amphibole tends toward intermediate rocks; biotite toward felsic

41 1) Amphiboles common in regional metamorphism of intermediate to mafic rocks 1) Usually water rich from breakdown of clay and micas 2) Metamorphic rock with abundant amphiboles called amphibolite facies 3) At high T, amphiboles break down to pyroxenes Note – these generalities are likely to be wrong

42 Pyroxenoid Group Similar to pyroxenes Similar to pyroxenes Single chains Single chains Z/O ratio 1/3 Z/O ratio 1/3 Differ in repeat distance along c axis Differ in repeat distance along c axis Pyroxene – 2 tetrahedron repeat (5.2 Å) Pyroxene – 2 tetrahedron repeat (5.2 Å) Pyroxenoid – 3 or more repeat (more than 7.3 Å) Pyroxenoid – 3 or more repeat (more than 7.3 Å) Difference is the pyroxenes are straight pyroxenoids are kinked Difference is the pyroxenes are straight pyroxenoids are kinked Cased by larger linking cations Cased by larger linking cations

43 Pyroxenes Wollastonite - Ca Rhodenite - Mn

44 Only a few minerals Only a few minerals Most common Wollastonite – Ca Most common Wollastonite – Ca Others are Rhodonite – Mn Others are Rhodonite – Mn Pectolite – Ca and Na Pectolite – Ca and Na

45 Wollastonite Wollastonite Composition: Ca with some Mn and Fe substitution Composition: Ca with some Mn and Fe substitution Common in altered carbonate rocks, particularly with reaction with qtz Common in altered carbonate rocks, particularly with reaction with qtz Useful industrial mineral, replacing asbestose, also used in paints and plastics Useful industrial mineral, replacing asbestose, also used in paints and plastics


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