Feldspar Group Most abundant mineral in the crust  6 of 7 most common elements Defined through 3 end-members  Albite (Na), Anorthite (Ca), Orthoclase.

Name: Feldspar Group Most abundant mineral in the crust  6 of 7 most common elements Defined through 3 end-members  Albite (Na), Anorthite (Ca), Orthoclase.
Uploaded: 2017-08-25T22:56:32+00:00
Duration: PTM26S53
Description: Feldspar Group Most abundant mineral in the crust  6 of 7 most common elements Defined through 3 end-members  Albite (Na), Anorthite (Ca), Orthoclase.

Feldspar Group Most abundant mineral in the crust  6 of 7 most common elements Defined through 3 end-members  Albite (Na), Anorthite (Ca), Orthoclase (K) Comprised of 2 series: Albite-anorthite (Na-Ca) Albite-orthoclase (Na-K)

Tectosilicates Feldspars
Substitute Al3+ for Si4+ allows Na+ or K+ to be added Albite-Orthoclase Substitute two Al3+ for Si4+ allows Ca2+ to be added Albite-Anorthite Albite: NaAlSi3O8

Feldspar Group – Albite-Anorthite series
Complete solid solution  Plagioclase Feldspars 6 minerals Albite (Na) Oligoclase Andesine Labradorite Bytownite Anorthite (Ca) Albite-Anorthite double duty End-members (Pure Na or Ca) Minerals % Na or Ca Notation: AnxAby  An20Ab80=Oligoclase

Feldspar Group – Albite-Anorthite series
Optical techniques to distinguish between plagioclase feldspars: Michel-Levy Method – uses extinction angles of twinned forms to determine An-Ab content Combined Carlsbad-Albite Method  uses Michel-Levy technique for both sides of a twin form

Feldspar Group – Albite-Orthoclase series
Several minerals – Alkali Feldspars High – T minerals Sanidine Anorthoclase Monalbite High Albite Low Temperature exsolution at solvus Chicken soup separation Forms 2 minerals, in igneous rocks these are typically intergrowths, or exsolution lamellae – perthitic texture Miscibility Gap microcline orthoclase sanidine anorthoclase monalbite high albite low albite intermediate albite Orthoclase KAlSi3O8 Albite NaAlSi3O8 % NaAlSi3O8 Temperature (ºC) 300 900 700 500 1100 10 90 70 50 30

Alkali Feldspar Exsolution
Miscibility Gap microcline orthoclase sanidine anorthoclase monalbite high albite low albite intermediate albite Orthoclase KAlSi3O8 Albite NaAlSi3O8 % NaAlSi3O8 Temperature (ºC) 300 900 700 500 1100 10 90 70 50 30 Liquid Melt cools past solvus (line defining miscibility gap) Anorthoclase, that had formed (through liquidus/solidus) separates (if cooling is slow enough) to form orthoclase and low albite In hand sample – schiller effect  play of colors caused by lamellae

Alkali Feldspar lamellae

Feldspathoid Group Very similar to feldspars and zeolites
Include Nepheline, Analcime, and Leucite Also framework silicates, but with another Al substitution for Si Only occur in undersaturated rocks (no free Quartz, Si-poor) because they react with SiO2 to form feldspars

Nesosilicates: independent SiO4 tetrahedra
b M1 in rows and share edges M2 form layers in a-c that share corners Some M2 and M1 share edges a Olivine (001) view blue = M1 yellow = M2

Olivine – complete solid solution
Forsterite-Fayalite  FoxFay Fayalite – Fe end-member Forsterite – Mg end-member Olivine Occurrences: Principally in mafic and ultramafic igneous and meta-igneous rocks Fayalite in meta-ironstones and in some alkalic granitoids Forsterite in some siliceous dolomitic marbles Monticellite CaMgSiO4 Ca  M2 (larger ion, larger site) High grade metamorphic siliceous carbonates

Distinguishing Forsterite-Fayalite
Petrographic Microscope Index of refraction  careful of zoning!! 2V different in different composition ranges Pleochroism/ color slightly different Spectroscopic techniques – many ways to determine Fe vs. Mg Same space group (Pbnm), Orthorhombic, slight differences in unit cell dimensions only

Inosilicates: single chains- pyroxenes
b Diopside: CaMg [Si2O6] a sin Where are the Si-O-Si-O chains?? Diopside (001) view blue = Si purple = M1 (Mg) yellow = M2 (Ca)

b a sin Diopside (001) view blue = Si purple = M1 (Mg) yellow = M2 (Ca)

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

SiO4 as polygons (and larger area) IV slab VI slab IV slab a sin VI slab IV slab VI slab IV slab b Diopside (001) view blue = Si purple = M1 (Mg) yellow = M2 (Ca)

M1 octahedron

(+) M1 octahedron (+) type by convention

M1 octahedron This is a (-) type (-)

M1 Creates an “I-beam” like unit in the structure.

(+) T M1 Creates an “I-beam” like unit in the structure

The pyroxene structure is then composed of alternating I-beams
Inosilicates: single chains- pyroxenes (+) (+) The pyroxene structure is then composed of alternating I-beams Clinopyroxenes have all I-beams oriented the same: all are (+) in this orientation (+) (+) (+) Note that M1 sites are smaller than M2 sites, since they are at the apices of the tetrahedral chains

(+) (+) The pyroxene structure is then composed of alternation I-beams Clinopyroxenes have all I-beams oriented the same: all are (+) in this orientation Orthopyroxenes have alternating (+) and (-) orientations (+) (+) (+)

Tetrehedra and M1 octahedra share tetrahedral apical oxygen atoms
Inosilicates: single chains- pyroxenes Tetrehedra and M1 octahedra share tetrahedral apical oxygen atoms

The tetrahedral chain above the M1s is thus offset from that below The M2 slabs have a similar effect The result is a monoclinic unit cell, hence clinopyroxenes (+) M2 c a (+) M1 (+) M2

Orthopyroxenes have alternating (+) and (-) I-beams the offsets thus compensate and result in an orthorhombic unit cell c (-) M1 (+) M2 a (+) M1 (-) M2

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 Chemistry The pyroxene quadrilateral and opx-cpx solvus
Coexisting opx + cpx in many rocks (pigeonite only in volcanics) Wollastonite Ca2Si2O6 Orthopyroxenes – solid soln between Enstatite-Ferrosilite Clinopyroxenes – solid soln between Diopside-Hedenbergite Diopside CaMgSi2O6 Hedenbergite CaFeSi2O6 clinopyroxenes Joins – lines between end members – limited mixing away from join pigeonite orthopyroxenes Enstatite Mg2Si2O6 Ferrosilite Fe2Si2O6

Orthopyroxene - Clinopyroxene
OPX and CPX have different crystal structures – results in a complex solvus between them Coexisting opx + cpx in many rocks (pigeonite only in volcanics) Diopside CaMgSi2O6 Hedenbergite CaFeSi2O6 Wollastonite Ca2Si2O6 Enstatite Mg2Si2O6 Ferrosilite Fe2Si2O6 orthopyroxenes clinopyroxenes pigeonite pigeonite 1200oC orthopyroxenes clinopyroxenes 1000oC CPX Solvus 800oC (Mg,Fe)2Si2O6 Ca(Mg,Fe)Si2O6 OPX OPX CPX

Orthopyroxene – Clinopyroxene solvus T dependence
Complex solvus – the ‘stability’ of a particular mineral changes with T. A different mineral’s ‘stability’ may change with T differently… OPX-CPX exsolution lamellae  Geothermometer… CPX CPX Di Hd Di Hd augite augite Miscibility Gap Miscibility Gap Subcalcic augite pigeonite pigeonite orthopyroxene orthopyroxene En Fs En Fs OPX OPX 800ºC 1200ºC Pigeonite + orthopyroxene

Pyroxenoids “Ideal” pyroxene chains with 5.2 A repeat (2 tetrahedra) become distorted as other cations occupy VI sites 7.1 A 12.5 A 17.4 A 5.2 A Pyroxene 2-tet repeat Wollastonite (Ca  M1)  3-tet repeat Rhodonite MnSiO3  5-tet repeat Pyroxmangite (Mn, Fe)SiO3  7-tet repeat

Inosilicates: double chains- amphiboles
Tremolite: Ca2Mg5 [Si8O22] (OH)2 a sin Tremolite (001) view blue = Si purple = M1 rose = M2 gray = M3 (all Mg) yellow = M4 (Ca)

Hornblende: (Ca, Na)2-3 (Mg, Fe, Al)5 [(Si,Al)8O22] (OH)2 a sin 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

Hornblende: (Ca, Na)2-3 (Mg, Fe, Al)5 [(Si,Al)8O22] (OH)2 Same I-beam architecture, but the I-beams are fatter (double chains) Hornblende (001) view dark blue = Si, Al purple = M1 rose = M2 light blue = M3 (all Mg, Fe)

Hornblende: (Ca, Na)2-3 (Mg, Fe, Al)5 [(Si,Al)8O22] (OH)2 (+) (+) (+) Same I-beam architecture, but the I-beams are fatter (double chains) a sin (+) (+) All are (+) on clinoamphiboles and alternate in orthoamphiboles 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

Hornblende: (Ca, Na)2-3 (Mg, Fe, Al)5 [(Si,Al)8O22] (OH)2 M1-M3 are small sites M4 is larger (Ca) A-site is really big Variety of sites  great chemical range 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

Hornblende: (Ca, Na)2-3 (Mg, Fe, Al)5 [(Si,Al)8O22] (OH)2 (OH) is in center of tetrahedral ring where O is a part of M1 and M3 octahedra (OH) 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

Amphibole Chemistry See handout for more information 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

Amphibole Chemistry Ca-Mg-Fe Amphibole “quadrilateral” (good analogy with pyroxenes) Tremolite Ferroactinolite Ca2Mg5Si8O22(OH)2 Actinolite Ca2Fe5Si8O22(OH)2 Clinoamphiboles Cummingtonite-grunerite Anthophyllite Mg7Si8O22(OH)2 Fe7Si8O22(OH)2 Orthoamphiboles 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

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”

Inosilicates - - - - - - - - - - - -
+ + + + + + a + + + + + + + + + + + + - - - - - Clinopyroxene - Clinoamphibole + + a + + + + - - - - - - Orthopyroxene Orthoamphibole Pyroxenes and amphiboles are very similar: Both have chains of SiO4 tetrahedra The chains are connected into stylized I-beams by M octahedra High-Ca monoclinic forms have all the T-O-T offsets in the same direction Low-Ca orthorhombic forms have alternating (+) and (-) offsets

Inosilicates pyroxene amphibole
Cleavage angles can be interpreted in terms of weak bonds in M2 sites (around I-beams instead of through them) Narrow single-chain I-beams  90o cleavages in pyroxenes while wider double-chain I-beams  o cleavages in amphiboles

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

Tectosilicates Low Quartz 001 Projection Crystal Class 32

Tectosilicates High Quartz at 581oC 001 Projection Crystal Class 622

Tectosilicates Cristobalite 001 Projection Cubic Structure

Tectosilicates Stishovite High pressure  SiVI

Tectosilicates Low Quartz Stishovite SiIV SiVI

Phyllosilicates SiO4 tetrahedra polymerized into 2-D sheets: [Si2O5]
Apical O’s are unpolymerized and are bonded to other constituents

Phyllosilicates Tetrahedral layers are bonded to octahedral layers
(OH) pairs are located in center of T rings where no apical O

Phyllosilicates Octahedral layers can be understood by analogy with hydroxides Brucite: Mg(OH)2 Layers of octahedral Mg in coordination with (OH) Large spacing along c due to weak van der waals bonds c

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

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

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

Igneous Minerals Quartz, Feldspars (plagioclase and alkaline), Olivines, Pyroxenes, Amphiboles Accessory Minerals – mostly in small quantities or in ‘special’ rocks Magnetite (Fe3O4) Ilmenite (FeTiO3) Apatite (Ca5(PO4)3(OH,F,Cl) Zircon (ZrSiO4) Sphene (a.k.a. Titanite) (CaTiSiO5) Pyrite (FeS2) Fluorite (CaF2)

Feldspar Group Most abundant mineral in the crust  6 of 7 most common elements Defined through 3 end-members  Albite (Na), Anorthite (Ca), Orthoclase.

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Feldspar Group Most abundant mineral in the crust  6 of 7 most common elements Defined through 3 end-members  Albite (Na), Anorthite (Ca), Orthoclase.

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Presentation on theme: "Feldspar Group Most abundant mineral in the crust  6 of 7 most common elements Defined through 3 end-members  Albite (Na), Anorthite (Ca), Orthoclase."— Presentation transcript:

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