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WUSTL EPSC 352 - Lecture Overview for Part 6 Systematic Mineralogy

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D:\352\2007\LectureNotes\Part6SystematicMineralogy.wpdEPSc 352: Lecture Overview for Part 6, Systematic MineralogyK Systematic MineralogyHave looked at basic compositional and structural properties of minerals, as well as some waysto infer these properties (XRD, electron microprobe, Raman spectroscopy)We’ll examine examples from some of the major mineral groups, in order to understand thethemes of composition, structure, physical properties (hand-sample and optical), occurrence,and usages.Begin with non-silicates. Unfortunately, often lumped together as if they were a fairlyhomogeneous group (not!).Non-silicates include:Most metallic ores (sulfides, oxides, hydroxides, carbonates)Many “industrial [non-metallic] minerals” (sulfates, carbonates, phosphates, halides)Many environmentally important phasesbiominerals: phosphates, oxides, carbonatesacid-drainage products: sulfides, sulfates, oxides, hydroxidesremediation aids: carbonates, phosphates, silicatesNon-silicate minerals form in many cases from aqueous solutions, including hydrothermal (hotwater) solutions.Their compositions are a reminder of dissolved constituents in natural waters, which can reactand precipitate. E.g., introduce ions, change oxygen pressure or pH or temperature or somecombination of parameters, cause precipitation.Minerals become the dumping grounds of elementsMinerals become the source of our needed elementsCan organize such minerals by property or usage (e.g., classification for scarce metals)precious/noble metals (e.g., Ag, Au, Pt)base metals (e.g., Cu, Zn, Sn)ferrous or ferro-alloy metals (e.g., Mn, Co, W, Ti)special metals (e.g., gallium, tantalum, zirconium)Use of iron accounts for 95 wt.% of all metals consumed. Large proportion of remaining 5 wt.%is used to alloy with iron to make steel.Look over hand-outs concerning the types of minerals from which we derive useful materials.Look briefly at the metals, because they will be the focus of the first “minerals” that we study. Major carriers of metals we use daily are native metals, sulfides, oxides; lesser amounts ofcarbonates and hydroxides (increasingly used); minor use of silicates as metal sources.2Geological groupings of the metals – in part, for mineralogic reasons. Group by crustalabundance:geochemically abundant $ 0.1 wt. % of crust: Si, Al, Fe, Mn, Mg, Tigeochemically scarce # 0.1 wt. % of crust: e.g., Cu, Zn, Pb, Au, Ag, Pt, Ga, LaAbundant metals tend to form large, numerous ore deposits; often form their own minerals,rather than piggy-backing via solid solution.*** ESSENTIAL: K Read indicated parts of textbook chapters 15, 16, and 17.K Structures of Non-Silicate MineralsUsually classify minerals based on dominant anion or anionic group (e.g., carbonate, sulfide,halide): similar chemical and physical properties, similar geologic occurrence. See Klein andDutrow, list on p. 332 and Table 15.1 on p. 333.**Themes to recall and keep in mind as you study the mineral groups:elements of structure, e.g., CCP, HCP, BCCkinds of sites, e.g., tetrahedral and octahedralelectrovalency considerationsstructural and compositional analogsP Native Elements: metals, semimetals, non-metals (Chapter 15) Au, Pt, Fe As, Bi, Sb S, CMetals: want to give up electrons. Simple structures with high (cubic) symmetry.Au and Pt group: FCC lattice (CCP of atoms). Closest packing layers along {111}.Fe group: BCC packing. I-lattice. Each Fe has 8 nearest neighbors.Semimetals: will either donate or accept electrons; semiconductor industry. As, Sb, Bi all isostructural. Structure type is Rhombohedral R3mNot closest-packing of spheres. Each atom closer to 3 of its neighbors than to others; formpyramidal groups with covalent bonding. Layered structure along {0001}.Non-Metals: S, CSulfur: 2 polymorphs, with transition temperature at 95.5 °C. 128 S atoms in unit cell. Puckered8-fold rings, held together by van der Waals forces.3Uses of sulfur: fungicides, insecticides, vulcanization of rubber, H2SO4Graphite: sheets. (See earlier notes on covalent and hybrid bonding.) 6-member rings. 3 ofcarbon’s valence electrons form strong, covalent sigma bonds in plane. Fourth electronwanders over sheet – forms pi bond side-to-side with adjacent carbons. Van der Waals forcesbetween sheets: cleavage. Requires 3 sheets to define unit cell.Diamond: each C is tetrahedrally coordinated to 4 other carbons. Very strong, directionalcovalent bonds, but all bonds equal in strength.Uses of graphitic carbon: lubricant; non-reactive compound; refractory (to about 3000 °C); goodelectrical conductor; pencil lead; strong carbon fibers, especially in composites. Activatedcarbon.Uses of diamond: grinding and polishing metals and other hard objects; synthetic diamond filmsto create robust surface on materials. P Sulfides Difficult to organize/classify(Chapter 15)Can exhibit metallic, ionic, and covalent bonding – even within same mineral!In many cases, get regular tetrahedral and octahedral coordination of metal atoms by sulfur, butdistorted polyhedra are also common.In simpler sulfides, can view both ways: metal atom coordinated by sulfurs AND sulfur atomcoordinated by metals.Sphalerite, ZnS. Has diamond structure in which C’s replaced by Zn’s and S’s in 1:1 ratio. Symmetry thus drops from 4/m 3 2/m to 4 3 m (both are cubic).Sphalerite illustrates derivative structures:The (more chemically complex) sphalerite structure is a derivative of the (less chemicallycomplex) diamond structure, by the above replacementChalcopyrite, CuFeS2, is derived from sphalerite structure by substituting 1:1 of Cu:Fe for Zn. Symmetry then drops to 42m (tetragonal) Pyrite, FeS2. Also cubic, but only 2/m 3 symmetry. S’s are covalently bonded dumbbells (goback to your lab #2). Also consider this structure as a derivative of NaCl structure, only withseparate Fe and S2 groups. Structurally different from marcasite. Arsenopyrite is a derivative of the marcasite structure.Galena, PbS. Exactly the same as the NaCl structure.4Some sulfides have very complex structures and stoichiometries; combinations of bondingtypes.Uses of sulfides: sources of many metals, e.g., Cu, Co, Pb, Zn.Problems of sulfides: smelting of them releases sulfurous gases to the atmosphere (highlyreactive, acidic). Oxidation of pyrite occurs readily and ubiquitously in the natural and disturbedenvironment. Grinding of sulfides makes them especially reactive:FeS2 + 2.5 H2O + 3.75 O2 º 2H2SO4 +


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