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1Properties of Common Properties of Common MineralsMineralsFigures from Winter’s web page (2002)Groups ConsideredGroups Considered• Framework silicates• Sheet silicates• Pyroxenes• Amphiboles• Other Silicates• Non SilicatesFramework SilicatesFramework Silicates•Feldspars• Feldspathoids• Silica polymorphsFeldsparsFeldspars• Simple chemistry• Substitution of Na for K or NaAl for CaAl• Crystal systems– Monoclinic (orthoclase, sanidine)– Triclinic (plagioclase, microcline)• Tabular habitFeldspar TwinningFeldspar Twinning• Simple twins (two parts)– Carlsbad– Common in monoclinic feldspars• Polysynthetic twins– Albite and others– Common in triclinic feldsparsKK--sparsspars• Orthoclase, Sanidine, Microcline• All optically negative• All have n ~ 1.53• Distinguished by 2V– Sanidine 2V = 0-30º– Orthoclase 2V = 30-70º– Microcline 2V = 70-90º• Microcline has Scotch plaid twins2PlagioclasePlagioclase• Refractive index increases with Ca content– Varies between 1.53 (Ab) to 1.57 (An)• 2V is large and varies with composition• Optic sign depends on composition (+/-)Common Sheet SilicatesCommon Sheet Silicates•Muscovite• Phlogopite• Biotite• ChloriteSheet StructuresSheet StructuresClassified on the basis of Si-O polymerism [Si2O5]2-Sheets of tetrahedramicas talc clay minerals serpentineSiO4tetrahedra polymerized into 2-D sheets: [Si2O5]Apical O’s are unpolymerized and are bonded to other constituentsPhyllosilicatesPhyllosilicatesBuilding BlocksBuilding Blocks• Tetrahedral layers• Octahedral layers• Large Cation layers• Week bonds along (001)Common Sheet PropertiesCommon Sheet Properties• Crystals are platy parallel to (001)• Perfect cleavage follows (001)• 2V is small (0-40º)• Extinction is parallel to (001)• BxA perpendicular to (001)• Optic sign can be determine by 1storder plate3Octahedral layers can be understood by analogy with hydroxidesBrucite: Mg(OH)Brucite: Mg(OH)22Layers of octahedral Mg in Layers of octahedral Mg in coordination with (OH)coordination with (OH)Large spacing along Large spacing along ccdue due to weak van der waals to weak van der waals bondsbondsccGibbsite Al(OH)Gibbsite Al(OH)33••Layers of octahedral Al in coordination with (OH)Layers of octahedral Al in coordination with (OH)••AlAl3+3+means that only 2/3 of the VI sites may be occupied for chargemeans that only 2/3 of the VI sites may be occupied for charge--balancebalance••BruciteBrucite--type layers may be called trioctahedral and gibbsitetype layers may be called trioctahedral and gibbsite--type dioctahedraltype dioctahedralaa11aa22Kaolinite: AlKaolinite: Al22[Si[Si22OO55] (OH)] (OH)44TT--layers and dioctahedral (Allayers and dioctahedral (Al3+3+) layers ) layers (OH) at center of T(OH) at center of T--rings and fill base of VI layer rings and fill base of VI layer →→Yellow = (OH)Yellow = (OH)T T O O --T T O O --T T OOvdwvdwvdwvdwweak van der Waals bonds between Tweak van der Waals bonds between T--O groupsO groupsSerpentine: MgSerpentine: Mg33[Si[Si22OO55] (OH)] (OH)44TT--layers and trioctahedral (Mglayers and trioctahedral (Mg2+2+) layers ) layers (OH) at center of T(OH) at center of T--rings and fill base of VI layer rings and fill base of VI layer →→Yellow = (OH)Yellow = (OH)T T O O --T T O O --T T OOvdwvdwvdwvdwweak van der Waals bonds between Tweak van der Waals bonds between T--O groupsO groupsMica PropertiesMica Properties• All micas are optically negative• 2V is small (0-40°)• (001) sheets give BxA figures• Birefringence is large (0.035-0.045)• “Birds eye” effect is obvious due to bent cleavages•(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• Low-temperature stability • Low-T metamorphism and alteration product of maficsChloriteChlorite4Chlorite OpticsChlorite Optics• 2V is small (0-20°)• Birefringence is low (0.001-0.010)• Some optically positive, some negative• Extinction parallel to (001)• Some types have strong dispersion• Some types weakly pleochroicOther Sheet SilicatesOther Sheet Silicates•Talc– Pale green, high birefringence• Stilpnomelane– Pleochroic (yellow to brown or green)• Chloritoid– Hour glass inclusions, polysynthetic twins, cross fractures, inclined extinctionMineral StructuresMineral StructuresSilicates are classified on the basis of Si-O polymerism [SiO3]2-single chains Inosilicates [Si4O11]4-Double tetrahedrapryoxenes pyroxenoids amphibolesAmphibolesAmphiboles• Two cleavages at 120º• Crystals elongate parallel to c• Extinction Z^c small (10-20º)• Color and pleochroism generally strong• Optic sign negative• 2V is large (70-90º)Amphibole GroupsAmphibole Groups• Non calcic amphiboles– Anthophyllite (orthorhombic)– Cummingtonite (monoclinic)• Calcic amphiboles– Tremolite-actinolite– Hornblende-oxyhornblende• Sodic amphiboles– Glaucophane, Riebeckite, ArfvedsoniteSee handout for more informationGeneral formula:W0-1X2Y5[Z8O22] (OH, F, Cl)2W = Na KX = Ca Na Mg Fe2+(Mn Li)Y = Mg Fe2+Mn Al Fe3+TiZ = Si AlAgain, the great variety of sites and sizes → a great chemical range, and hence a broad stability rangeThe hydrous nature implies an upper temperature stability limitAmphibole ChemistryAmphibole Chemistry5Ca-Mg-Fe Amphibole “quadrilateral” (good analogy with pyroxenes)Amphibole ChemistryAmphibole ChemistryAl 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 compositionsTremoliteTremoliteCaCa22MgMg55SiSi88OO2222(OH)(OH)22FerroactinoliteFerroactinoliteCaCa22FeFe55SiSi88OO2222(OH)(OH)22AnthophylliteAnthophylliteMgMg77SiSi88OO2222(OH)(OH)22FeFe77SiSi88OO2222(OH)(OH)22ActinoliteCummingtonite-gruneriteOrthoamphibolesOrthoamphibolesClinoamphibolesClinoamphibolesHornblendeHornblende• Strongly pleochroic• Z^c = 20º• Birefringence = 0.020•2Vx= 70º• Can be named green or brown hornblende• Reddish varieties are oxyhornblende and kaersutite (Ti-rich hornblende)Hornblende has Al in the tetrahedral siteGeologists 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


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UB GLY 206 - Properties of Common Minerals

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