ReferencesOct. 24, 2003Mineralogy 231Lecture 20: oxides and hydroxides: from weathering to lower mantlereadings. Monday: brines and evaporites: sulfates, halides, boratesN 340-346, 353-354, 374-377; K 377-378, 398-402, 408-409, 423-430; DHZ, 623-662oxide associationscommon accessory minerals and in weathering products in many rocksoccasionally concentrated enough to be mined, e.g.sediments: Fe ore from magnetite, hematite in BIF, Ti ore from rutile, ilmenite in sandmetamorphic rocks: Fe ore in metamorphosed BIFs; Zn, Pb sulfides from marblesigneous rx: Nb ore from CaNb2O6 (pyrochlore) in carbonatites, U ore from betafite in pegmatitesweathering processes producing oxides, hydroxidesFe silicates replaced by oxides, hydroxides and/or oxyhydroxides during weathering at low Te.g., almandine: 4Fe3Al2Si3O12 + 12H2O + 3O2 -> 6Fe2O3 + 8Al(OH)3 + 12SiO2alkali silicates leached of alkali and alkaline earth elements during their decompositione.g., K-feldspar: KAlSi3O8 + H+ -> AlO(OH) + 3SiO2 + K+ oxides hydrated and/or oxidized to hydroxides or oxyhydroxides during weathering at low Thematite: Fe2O3 + H2O -> 2FeO(OH)corundum: Al2O3 + 3H2O -> 2Al(OH)3magnetite: 4Fe3O4 + 6H2O + O2 -> 12FeO(OH)these kinds of reactions occur especially in lateritic soils, bauxites where are they forming today?oxides also common in lower mantle because silicates tend to react to oxides: MgSiO3 -> MgO + SiO2oxides and hydroxides -- modification of Nesse’s Table 18.1group formula name commentR2O H2O ice common surficial mineral, not seen in ordinary thin sections!Cu2O cuprite uncommon, forms in weathering zone of ore depositsRO MgO periclase in some high-temperature contact aureolesFe1-xO wüstite x = 0 - 0.2, rare in ultramafic rocksR2O3Fe2O3hematite (maghemite) iron formations, quartzites, some granitesAl2O3corundum rare mineral in metabauxites, marblesABO3FeTiO3ilmenite common accessory in basalts, amphibolites, metapelitesCaTiO3perovskite common accessory in alkaline basalts, carbonatites, kimberlitesR3O4Fe3O4magnetite common accessory in many metamorphic and igneous rocksAB2O4MgAl2O4spinel common accessory in marbles, ultramafic rocksFeAl2O4hercynite common accessory in metapelites, gabbros, metabauxitesFeCr2O4chromite common accessory in ultramafic rocks, solid solution w/ magnetiteZnAl2O4gahnite metapelites, metam. ore deposits: ss w/ spinel, hercyniteFe2TiO4ulvöspinel accessory in ocean floor basalts, solid solution w/ magnetiteRO2TiO2rutile accessory in high-grade metamorphic rocksSiO2stishovite in impact (shock metamorphosed) rocks (e.g., Meteor Crater)SnO2cassiterite accessory in granites, metamorphosed ore depositsMnO2pyrolusite common as weathering product in sedimentsUO2uraninite rare accessory in pegmatites, sandstones, major ore mineral of URO(OH) FeOOH goethite (lepidocrocite) common in weathered rocks with Fe mineralsAlOOH diaspore (boehmite) common in bauxitesR(OH)2Mg(OH)2brucite in some marbles, serpentinitesR(OH)3Al(OH)3gibbsite (several) common in weathered rocks that contain Al mineralssolid solutions and solvirock salt group, RO: continuous solid solution between MgO and Fe1-xOrhombohedral group, R2O3 and FeTiO3: continuous solid solution at high T, solvi at low Tspinel group, AB2O4: continuous solid solution at high T, solvi at low Trutile group, RO2: symmetrical solvus for rutile-cassiterite, unknown relationship for rutile-stishovitegoethite group, RO(OH): limited solid solution observed in natural goethite and diasporebonding of oxideslargely ionic, can be viewed as simple analogues to silicatesopacity of Fe and Mn oxidescause of opacity: electron transfer between cations, e- hopping strongly absorbs lightmost important oxide structure groupsrhombohedral oxides, R2O3 and ABO3, hematite and ilmenite groupsstructure of R octahedra with hcp oxygen packing Fig. 18.5face sharing, edge sharing and vertex sharing of oxygenssolid solution between hematite and ilmenite: 2Fe3+ = Fe2+Ti4+ exchangespinel group AB2O4structure with A tetrahedra and B octahedra with ccp packed oxygens Fig. 18.3includes ferrites (magnetite), aluminates (spinels), chromites, titanites (ulvöspinel)inverse spinel: magnetite, Fe3+(Fe2+Fe3+)O4; normal spinel: hercynite, Fe2+(Al3+)2O4common solid solutions: Mg = Fe2+ = Zn, Al = Cr = Fe3+, 2Fe3+ = Fe2+Ti4+rutile group RO2structure with edge sharing chains of octahedra Fig. 18.7complex oxides with large cation A and small cation Bgroup formula name commentsABO3CaTiO3perovskite accessory in alkalic igneous rocks, high grade marblesMgSiO3not named perovskite structure, SiVI, in lower mantle, impactitesAB8O16KFe2Ti6O16priderite B site is (R3+)2(R4+)6, accessory in alkalic igneous rocksAB12O19CaAl12O19hibonite high-T contact aureoles, chondritic meteoritesAB21O38BaFe10Ti11O19lindsleyite B site is (R3+)2(R4+)6, accessory in alkalic igneous rocksgenerally uncommon phases forming accessory minerals in uncommon rocksnot responsible for knowing these, included to show that complexity of some oxidesclassification odditiessome oxides classified under silicates even though are oxidesexamples: SiO2, structure similar to feldspars (discussed under framework silicates)chrysoberyl, BeAl2O4, structure same as olivine although formula is like spinel (AB2O4)some silicates classified with oxides although are silicateshigh-pressure polymorph of SiO2, stishovite, rutile structurehigh pressure polymorph of Mg2SiO4, ringwoodite, spinel structure, abundant in mantlehigh pressure polymorph of MgSiO3, “Mg perovskite”, perovskite structureReferencesDeer, W.A., Howie, R.A. and Zussman, J. (1991) An Introduction to The Rock-Forming Minerals, 2nd ed.,Longman, NY, 696 p.Lindsley, D.H., ed. (1991) Oxide Minerals: Petrologic and Magnetic Significance. Reviews in Mineralogy25, 509