Back to silicate Back to silicate structures: structures: nesosilicates nesosilicates inosilicates inosilicates tectosilicates tectosilicates phyllosilicates phyllosilicates cyclosilictaes cyclosilictaes sorosilicates sorosilicates
Feb 07, 2016
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
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projectionprojection
Olivine (001) view blue = M1 yellow = M2Olivine (001) view blue = M1 yellow = M2
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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Nesosilicates: independent SiONesosilicates: independent SiO44 tetrahedra tetrahedra
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
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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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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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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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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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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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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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)
Perspective viewPerspective view
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)
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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Inosilicates: single chains- pyroxenes Inosilicates: single chains- pyroxenes
M1 octahedronM1 octahedron
Inosilicates: single chains- pyroxenes Inosilicates: single chains- pyroxenes
M1 octahedronM1 octahedron
Inosilicates: single chains- pyroxenes Inosilicates: single chains- pyroxenes
M1 octahedronM1 octahedron
(+) type by convention(+) type by convention
(+)
Inosilicates: single chains- pyroxenes Inosilicates: single chains- pyroxenes
M1 octahedronM1 octahedron
This is a (-) typeThis is a (-) type
(-)
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
structure.structure.
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
(+)(+)
(+)(+)(+)(+)
(+)(+)(+)(+)
Inosilicates: single chains- pyroxenes Inosilicates: single chains- pyroxenes
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
(+)(+)
(+)(+)(+)(+)
Inosilicates: single chains- pyroxenes Inosilicates: single chains- pyroxenes
(+)(+)(+)(+)
Tetrehedra and M1 Tetrehedra and M1 octahedra share octahedra share
tetrahedral apical tetrahedral apical oxygen atoms oxygen atoms
Inosilicates: single chains- pyroxenes Inosilicates: single chains- pyroxenes
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
Inosilicates: single chains- pyroxenes Inosilicates: single chains- pyroxenes
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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
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 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