Back to silicate structures: nesosilicates phyllosilicates sorosilicates inosilicates cyclosilictaes tectosilicates

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

Inosilicates: single chains- pyroxenes (+) 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 Inosilicates: single chains- pyroxenes (+) (+) The pyroxene structure is then composed of alternation I-beams 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 (-) (-) The pyroxene structure is then composed of alternation I-beams Orthoopyroxenes have I-beams oriented in alternate direction in different layers (-) (+) (+)

Inosilicates: single chains- pyroxenes 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

Inosilicates: single chains- pyroxenes 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

Ca-Tschermack’s molecule Pyroxene Chemistry “Non-quad” pyroxenes Jadeite Aegirine NaAlSi2O6 NaFe3+Si2O6 0.8 Omphacite aegirine- augite Spodumene: LiAlSi2O6 Ca / (Ca + Na) Ca-Tschermack’s molecule 0.2 CaAl2SiO6 Augite Diopside-Hedenbergite Ca(Mg,Fe)Si2O6

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

Back to silicate structures: nesosilicates phyllosilicates sorosilicates inosilicates cyclosilictaes tectosilicates

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)

Inosilicates: double chains- amphiboles 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

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 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

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 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”

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 igneous 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 - 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  60-120o 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

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) Titanite (CaTiSiO5) Pyrite (FeS2) Fluorite (CaF2)