Chapter 8: Major Elements - Whitman College

Chapter 8: Major Elements - Whitman College

Mineral Structures Silicates are classified on the basis of Si-O polymerism The culprit: the [SiO4]4- tetrahedron Mineral Structures Silicates are classified on the basis of Si-O polymerism [SiO4]4- Independent tetrahedra Nesosilicates Examples: olivine garnet [Si2O7]6- Double tetrahedra

Sorosilicates Examples: lawsonite n[SiO3]2- n = 3, 4, 6 Cyclosilicates Examples: benitoite BaTi[Si3O9] axinite Ca3Al2BO3[Si4O12]OH beryl Be3Al2[Si6O18] Mineral Structures Silicates are classified on the basis of Si-O polymerism [SiO3]2pryoxenes

single chains pyroxenoids Inosilicates [Si4O11]4- Double tetrahedra amphiboles Mineral Structures Silicates are classified on the basis of Si-O polymerism [Si2O5]2-

Sheets of tetrahedra micas talc clay minerals serpentine Phyllosilicates Mineral Structures Silicates are classified on the basis of Si-O polymerism low-quartz [SiO2] 3-D frameworks of tetrahedra: fully polymerized quartz and the silica minerals feldspars feldspathoids zeolites

Tectosilicates Mineral Structures Nesosilicates: independent SiO4 tetrahedra Nesosilicates: independent SiO4 tetrahedra b c projection Olivine (100) view blue = M1 yellow = M2 Nesosilicates: independent SiO4 tetrahedra

b c perspective Olivine (100) view blue = M1 yellow = M2 Nesosilicates: independent SiO4 tetrahedra b M1 in rows and share edges a

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

M1 and M2 as polyhedra Olivine (100) view blue = M1 yellow = M2 Nesosilicates: independent SiO4 tetrahedra Olivine Occurrences: Principally in mafic and ultramafic igneous and metaigneous 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 Nesosilicates: independent SiO4 tetrahedra Garnet: A2+3 B3+2 [SiO4]3 Pyralspites - B = Al Pyrope: Mg3 Al2 [SiO4]3 Almandine: Fe3 Al2 [SiO4]3 Spessartine: Mn3 Al2 [SiO4]3 Ugrandites - A = Ca Uvarovite: Ca3 Cr2 [SiO4]3 Grossularite: Ca3 Al2 [SiO4]3 Andradite: Ca3 Fe2 [SiO4]3

Occurrence: Mostly metamorphic Some high-Al igneous Also in some mantle peridotites Garnet (001) view blue = Si purple = A turquoise = B Nesosilicates: independent SiO4 tetrahedra Garnet: A2+3 B3+2 [SiO4]3 a2 a1 a3 Pyralspites - B = Al Pyrope: Mg3 Al2 [SiO4]3

Almandine: Fe3 Al2 [SiO4]3 Spessartine: Mn3 Al2 [SiO4]3 Ugrandites - A = Ca Uvarovite: Ca3 Cr2 [SiO4]3 Grossularite: Ca3 Al2 [SiO4]3 Andradite: Ca3 Fe2 [SiO4]3 Occurrence: Mostly metamorphic Pyralspites in meta-shales Ugrandites in meta-carbonates Some high-Al igneous Garnet (001) view blue = Si purple =Also A turquoise =B

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

b Diopside (001) view blue = Si purple = M1 (Mg) yellow = M2 (Ca) Inosilicates: single chains- pyroxenes a sin b Diopside (001) view blue = Si purple = M1 (Mg) yellow = M2 (Ca) Inosilicates: single chains- pyroxenes a sin

b Diopside (001) view blue = Si purple = M1 (Mg) yellow = M2 (Ca) Inosilicates: single chains- pyroxenes a sin b Diopside (001) view blue = Si purple = M1 (Mg) yellow = M2 (Ca) Inosilicates: single chains- pyroxenes a sin

b Diopside (001) view blue = Si purple = M1 (Mg) yellow = M2 (Ca) Inosilicates: single chains- pyroxenes Perspective view Diopside (001) view blue = Si purple = M1 (Mg) yellow = M2 (Ca) Inosilicates: single chains- pyroxenes IV slab SiO4 as polygons (and larger area)

VI slab a sin IV slab VI slab IV slab VI slab IV slab b Diopside (001) view blue = Si purple = M1 (Mg) yellow = M2 (Ca) Inosilicates: single chains- pyroxenes

M1 octahedron Inosilicates: single chains- pyroxenes M1 octahedron Inosilicates: single chains- pyroxenes (+) M1 octahedron (+) type by convention Inosilicates: single chains- pyroxenes (-)

M1 octahedron This is a (-) type Inosilicates: single chains- pyroxenes T M1 T Creates an I-beam like unit in the structure. Inosilicates: single chains- pyroxenes T

M1 (+) T Creates an I-beam like unit in the structure Inosilicates: single chains- pyroxenes (+) (+) Clinopyroxenes have

all I-beams oriented the same: all are (+) in this orientation (+) (+) The pyroxene structure is then composed of alternating I-beams (+) Note that M1 sites are

smaller than M2 sites, since they are at the apices of the tetrahedral chains Inosilicates: single chains- pyroxenes (+) (+) Clinopyroxenes have all I-beams oriented the same: all are (+) in this orientation (+)

(+) The pyroxene structure is then composed of alternation I-beams (+) Inosilicates: single chains- pyroxenes Tetrehedra and M1 octahedra share tetrahedral apical oxygen atoms

Inosilicates: single chains- pyroxenes (+) M2 The tetrahedral chain above the M1s is thus offset from that below c a (+) M1 (+) M2

The M2 slabs have a similar effect The result is a monoclinic unit cell, hence clinopyroxenes Inosilicates: single chains- pyroxenes Orthopyroxenes have alternating (+) and (-) I-beams c (-) M1 (+) M2

a the offsets thus compensate and result in an orthorhombic unit cell (+) M1 (-) M2 This also explains the double a cell dimension and why orthopyroxenes have {210} cleavages instead of {110) as in

clinopyroxenes (although both are at 90o) 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 pigeonite orthopyroxenes Diopside clinopyroxenes cli no py ro xe ne s

1200oC 1000oC Hedenbergite Solvus 800oC pigeonite (Mg,Fe)2Si2O6 orthopyroxenes Enstatite

Ferrosilite Ca(Mg,Fe)Si2O6 Pyroxene Chemistry Non-quad pyroxenes Jadeite NaAlSi2O6 Aegirine NaFe3+Si2O6 0.8 Omphacite

aegirineaugite Spodumene: LiAlSi2O6 Ca / (Ca + Na) 0.2 Ca-Tschermacks molecule CaAl2SiO6 Augite Diopside-Hedenbergite Ca(Mg,Fe)Si2O6

Ideal pyroxene chains with 5.2 A repeat (2 tetrahedra) become distorted as other cations occupy VI sites Pyroxenoids 17.4 A 7.1 A 12.5 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 b

a sin Tremolite: Ca2Mg5 [Si8O22] (OH)2 Tremolite (001) view blue = Si purple = M1 rose = M2 gray = M3 (all Mg) yellow = M4 (Ca) Inosilicates: double chains- amphiboles b a sin Hornblende: (Ca, Na)2-3 (Mg, Fe, Al)5

[(Si,Al)8O22] (OH)2 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 Inosilicates: double chains- amphiboles 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) Inosilicates: double chains- amphiboles b (+) a sin (+) (+) (+)

(+) 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) 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 Inosilicates: double chains- amphiboles 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 Inosilicates: double chains- amphiboles 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 Ca2Mg5Si8O22(OH)2 Anthophyllite Mg7Si8O22(OH)2 Actinolite Cummingtonite-grunerite Orthoamphiboles Ferroactinolite Ca2Fe5Si8O22(OH)2 Clinoamphiboles Fe7Si8O22(OH)2

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

Amphibole Occurrences Tremolite (Ca-Mg) occurs in meta-carbonates Actinolite occurs in low-grade metamorphosed basic igneous rocks Orthoamphiboles and cummingtonite-grunerite (all Ca-free, Mg-Fe-rich amphiboles) are metamorphic and occur in meta-ultrabasic rocks and some meta-sediments. The Fe-rich grunerite occurs in meta-ironstones The complex solid solution called hornblende occurs in a broad variety of both igenous and metamorphic rocks Sodic amphiboles are predominantly metamorphic where they are characteristic of high P/T subduction-zone metamorphism (commonly called blueschist in reference to the predominant blue sodic amphiboles Riebeckite occurs commonly in sodic granitoid rocks Inosilicates +

+ + a + + + + + +

+ + + + + + + + Clinopyroxene a

+ - + - - + Clinoamphibole -

+ + - - 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

pyroxene Inosilicates amphibole b a 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 doublechain I-beams 60-120o cleavages in amphiboles Phyllosilicates

SiO4 tetrahedra polymerized into 2-D sheets: [Si2O5] Apical Os 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 c Layers of octahedral Mg in coordination with (OH) Large spacing along c due to weak van der waals

bonds 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 Yellow = (OH)

Kaolinite: Al2 [Si2O5] (OH)4 T-layers and diocathedral (Al3+) layers (OH) at center of T-rings and fill base of VI layer weak van der Waals bonds between T-O groups T O T O T O vdw vdw

Phyllosilicates Yellow = (OH) Serpentine: Mg3 [Si2O5] (OH)4 T-layers and triocathedral (Mg2+) layers (OH) at center of T-rings and fill base of VI layer weak van der Waals bonds between T-O groups T O T O T O

vdw vdw Serpentine Antigorite maintains a sheet-like form by alternating segments of opposite curvature Chrysotile does not do this and tends to roll into tubes Octahedra are a bit larger than tetrahedral match, so they cause bending of the T-O layers (after Klein and Hurlbut, 1999).

Serpentine Nagby and Faust (1956) Am. Mineralogist 41, 817-836. Veblen and Busek, 1979, Science 206, 1398-1400. S = serpentine T = talc The rolled tubes in chrysotile resolves the apparent paradox of asbestosform sheet silicates Phyllosilicates

Yellow = (OH) Pyrophyllite: Al2 [Si4O10] (OH)2 T-layer - diocathedral (Al3+) layer - T-layer weak van der Waals bonds between T - O - T groups T O T T O T T O T

vdw vdw Phyllosilicates Yellow = (OH) Talc: Mg3 [Si4O10] (OH)2 T-layer - triocathedral (Mg2+) layer - T-layer weak van der Waals bonds between T - O - T groups T O T T

O T T O T vdw vdw Phyllosilicates 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

T O T K T O T K T O T Phyllosilicates Phlogopite: K Mg3 [Si3AlO10] (OH)2 T-layer - triocathedral (Mg2+) layer - T-layer - K

K between T - O - T groups is stronger than vdw T O T K T O T K T O T Phyllosilicates

A Summary of Phyllosilicate Structures Fig 13.84 Klein and Hurlbut Manual of Mineralogy, John Wiley & Sons Phyllosilicates Chlorite: (Mg, Fe)3 [(Si, Al)4O10] (OH)2 (Mg, Fe)3 (OH)6 = T - O - T - (brucite) - T - O - T - (brucite) - T - O - T Very hydrated (OH)8, so low-temperature stability (low-T metamorphism and alteration of mafics as cool) Biopyriboles Why are there single-chain-, double-chain-, and sheet-polymer types, and not triple chains, quadruple chains, etc??

Biopyriboles It turns out that there are some intermediate types, predicted by J.B. Thompson and discovered in 1977 Veblen, Buseck, and Burnham Cover of Science: anthophyllite (yellow) reacted to form chesterite (blue & green) and jimthompsonite (red) Streaked areas are highly disordered Cover of Science, October 28, 1977 AAAS Fig. 6, Veblen et al (1977) Science 198 AAAS

anthophyllite jimthompsonite chesterite HRTEM image of anthophyllite (left) with typical double-chain width Jimthompsonite (center) has triple-chains Chesterite is an ordered alternation of double- and triple-chains Biopyriboles Fig. 7, Veblen et al (1977) Science 198 AAAS

Disordered structures show 4-chain widths and even a 7-chain width Obscures the distinction between pyroxenes, amphiboles, and micas (hence the term biopyriboles: biotite-pyroxene-amphibole) Tectosilicates Stishovite Pressure (GPa) 10 8 6 Coesite

4 2 - quartz - quartz 600 Liquid Cristobalite Tridymite 1000

1400 1800 2200 Temperature oC 2600 After Swamy and Saxena (1994) J. Geophys. Res., 99, 11,787-11,794. Tectosilicates

Stishovite Coesite Low Quartz - quartz - quartz Cristobalite Tridymite 001 Projection Crystal Class 32 Liquid

Tectosilicates Stishovite Coesite High Quartz at 581oC - quartz - quartz Cristobalite Tridymite 001 Projection Crystal Class 622

Liquid Tectosilicates Stishovite Coesite Cristobalite - quartz - quartz Cristobalite Tridymite

001 Projection Cubic Structure Liquid Tectosilicates Stishovite Coesite Stishovite - quartz - quartz

Cristobalite Tridymite High pressure SiVI Liquid Tectosilicates Low Quartz SiIV Stishovite SiVI

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

Recently Viewed Presentations