Groups Considered Properties of Common • Framework silicates Minerals • Sheet silicates • Pyroxenes Figures from Winter’s web page (2002) • Amphiboles • Other Silicates • Non Silicates Framework Silicates Feldspars • Simple chemistry •Feldspars • Substitution of Na for K or NaAl for CaAl • Feldspathoids • Crystal systems • Silica polymorphs – Monoclinic (orthoclase, sanidine) – Triclinic (plagioclase, microcline) • Tabular habit Feldspar Twinning K-spars • Simple twins (two parts) • Orthoclase, Sanidine, Microcline – Carlsbad • All optically negative – Common in monoclinic feldspars • All have n ~ 1.53 • Polysynthetic twins • Distinguished by 2V – Albite and others – Sanidine 2V = 0-30º – Common in triclinic feldspars – Orthoclase 2V = 30-70º – Microcline 2V = 70-90º • Microcline has Scotch plaid twins 1 Plagioclase Common Sheet Silicates • Refractive index increases with Ca content •Muscovite – Varies between 1.53 (Ab) to 1.57 (An) • Phlogopite • 2V is large and varies with composition • Biotite • Optic sign depends on composition (+/-) • Chlorite Sheet Structures Phyllosilicates Classified on the basis of Si-O polymerism SiO4 tetrahedra polymerized into 2-D sheets: [Si2O5] Apical O’s are unpolymerized and are bonded to other constituents 2- [Si2O5] Sheets of tetrahedra micas talc clay minerals serpentine Building Blocks Common Sheet Properties • Tetrahedral layers • Crystals are platy parallel to (001) • Perfect cleavage follows (001) • Octahedral layers • 2V is small (0-40º) • Large Cation layers • Extinction is parallel to (001) • BxA perpendicular to (001) • Week bonds along (001) • Optic sign can be determine by 1st order plate 2 Brucite: Mg(OH)2 Gibbsite Al(OH)3 Octahedral layers can be understood by analogy with hydroxides a2 Layers of octahedral Mg in coordination with (OH) c Large spacing along c due a to weak van der waals a1 bonds • Layers of octahedral Al in coordination with (OH) • Al3+ means that only 2/3 of the VI sites may be occupied for charge-balance • Brucite-type layers may be called trioctahedral and gibbsite-type dioctahedral Kaolinite: Al2 [Si2O5] (OH)4 Serpentine: Mg3 [Si2O5] (OH)4 T T Yellow = (OH) O Yellow = (OH) O - vdw - vdw T-layers and dioctahedral (Al3+) layers T T-layers and trioctahedral (Mg2+) layers T O O → → (OH) at center of T-rings and fill base of VI layer - vdw (OH) at center of T-rings and fill base of VI layer - vdw T T weak van der Waals bonds between T-O groups O weak van der Waals bonds between T-O groups O Chlorite Mica Properties •(Mg, Fe)3 [(Si, Al)4O10] (OH)2 (Mg, Fe)3 (OH)6 • All micas are optically negative • T - O - T - (brucite) - T - O - T - (brucite) - T - O - T • 2V is small (0-40°) • Very hydrated (OH)8 • (001) sheets give BxA figures • Low-temperature stability • Birefringence is large (0.035-0.045) • Low-T metamorphism and alteration product of mafics • “Birds eye” effect is obvious due to bent cleavages 3 Chlorite Optics Other Sheet Silicates • 2V is small (0-20°) •Talc • Birefringence is low (0.001-0.010) – Pale green, high birefringence • Some optically positive, some • Stilpnomelane negative – Pleochroic (yellow to brown or green) • Extinction parallel to (001) • Some types have strong dispersion • Chloritoid • Some types weakly pleochroic – Hour glass inclusions, polysynthetic twins, cross fractures, inclined extinction Mineral Structures Silicates are classified on the basis of Si-O polymerism Amphiboles • Two cleavages at 120º • Crystals elongate parallel to c • Extinction Z^c small (10-20º) • Color and pleochroism generally strong • Optic sign negative 2- 4- [SiO3] single chains Inosilicates [Si4O11] Double tetrahedra pryoxenes pyroxenoids amphiboles • 2V is large (70-90º) Amphibole Chemistry Amphibole Groups See handout for more information • Non calcic amphiboles General formula: – Anthophyllite (orthorhombic) – Cummingtonite (monoclinic) W0-1 X2 Y5 [Z8O22] (OH, F, Cl)2 W = Na K • Calcic amphiboles X = Ca Na Mg Fe2+ (Mn Li) – Tremolite-actinolite Y = Mg Fe2+ Mn Al Fe3+ Ti – Hornblende-oxyhornblende Z = Si Al Again, the great variety of sites and sizes → a great chemical range, and • Sodic amphiboles hence a broad stability range The hydrous nature implies an upper temperature stability limit – Glaucophane, Riebeckite, Arfvedsonite 4 Amphibole Chemistry Hornblende Ca-Mg-Fe Amphibole “quadrilateral” (good analogy with pyroxenes) • Strongly pleochroic • Z^c = 20º Tremolite Ferroactinolite Ca Mg Si O (OH) Actinolite • Birefringence = 0.020 Ca2Mg5Si8O22(OH)2 Ca2Fe5Si8O22(OH)2 •2Vx = 70º Clinoamphiboles Cummingtonite-grunerite Anthophyllite • Can be named green or brown hornblende Mg Si O (OH) Fe7Si8O22(OH)2 7 8 22 2 Orthoamphiboles • Reddish varieties are oxyhornblende and kaersutite (Ti-rich hornblende) 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 General Pyroxene Hornblende has Al in the tetrahedral site Characteristics 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 • Two cleavages at 87º casting “hornblende” into end-member compositions and naming amphiboles after a well-represented end-member. • Stumpy crystals Sodic amphiboles • Z^c large for monoclinic minerals • Colorless or weakly pleochroic Glaucophane: Na2 Mg3 Al2 [Si8O22] (OH)2 2+ 3+ Riebeckite: Na2 Fe 3 Fe 2 [Si8O22] (OH)2 • Optic sign positive • 2V is moderate (40-60º) Sodic amphiboles are commonly blue, and often called “blue amphiboles” Single Chains- Pyroxenes Pyroxene Chemistry b The general pyroxene formula: W1-P (X,Y)1+P Z2O6 Where β β –W = Ca Na 2+ a sin a sin – X = Mg Fe Mn Ni Li – Y = Al Fe3+ Cr Ti –Z = Si Al Anhydrous so high-temperature or dry Diopside (001) view blue = Si purple = M1 (Mg) yellow = M2 (Ca) conditions favor pyroxenes over amphiboles 5 Pyroxene Chemistry The pyroxene quadrilateral and opx-cpx solvus Enstatite and Hypersthene Coexisting opx + cpx in many rocks (pigeonite only in volcanics) Wollastonite • Non-calcic Pyroxenes c pigeonite li n 1200oC orthopyroxenes op • Enstatite yr ox en –2V = 60-90º es z – Birefringence = 0.008 1000oC Diopside Hedenbergite clinopyroxenes Solvus •Hypersthene o 800 C –2Vx = 50-90º pigeonite (Mg,Fe) Si O Ca(Mg,Fe)Si O – Birefringence – 0.014 orthopyroxenes 2 2 6 2 6 Enstatite Ferrosilite – Weakly pleochroic pink to pale green Diopside and Augite Sodic Pyroxenes • Colorless • Aegerine (NaFeSi2O6) • Birefringence of 0.020-0.030 – Strongly pleochroic in green – X^c = 2-6º •2Vz = 60º • Z^c = 40º – High birefringence (~0.050) • Pigeonite has 2Vz = 0-30º • Jadeite (NaAlSi2O6) – Moderate birefringence (~0.020) – Z^c = 30º Independent SiO4 Tetrahedra • 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 6.