Slide 1Slide 2Slide 3Slide 4Slide 5Slide 6Slide 7Slide 8Slide 9Voltammetry at 7,500 ft depthSlide 11Slide 12Slide 13Sulfides are reduced minerals what happens when they contact O2?Actively Oxidizing PyriteAMD neutralizationSlide 17Slide 18Oxides - OxyhydroxidesAl oxidesMn oxides - oxyhydroxidesMn Oxide minerals (not all…)Iron OxidesFerrihydriteGoethiteSlide 26Slide 27Banded Iron Formations (BIFs)BIFs and bacteriaOther important oxidesThe sphere is 6 ft in diameter, the pilot and 2 observers sit in that space1.5-2 hours going down, 1.5-2 hours coming up bottom time usually limited by battery consumption…Wand, nicknamed ‘Fat Albert’ contains 2 electrodes and a thermistorVoltammetry at 7,500 ft depth•Black Smoker chemistry – Scans up to 220º C lots of Fe2+, HS-, FeS(aq), tens to hundreds of millimolar Fe and S•Riftia field chemistry – Scans up to 20º C little/no Fe2+, FeS(aq), few mM HS-,Sulfides are reduced minerals what happens when they contact O2?•This is the basis for supergene enrichment and acidic mine drainageActively Oxidizing Pyrite•FeS2 + 3.5 O2 + H2O Fe2+ + 2 SO42- + 2 H+•FeS2 + 14 Fe3+ + 8 H2O 15 Fe2+ + 2 SO42- + 16 H+•14Fe2+ + 3.5 O2 + 14H+ 14 Fe3+ + 7 H2O•Sulfur species and H+ generation:–FeS2 + 2 Fe3+ 3 Fe2+ + ¼ S8 + 0 H+–FeS2 + 7 Fe3+ + 3 H2O 8 Fe2+ + 0.5 S4O62- + 6 H+AMD neutralization•Metals are soluble in low pH solutions – can get 100’s of grams of metal into a liter of very acidic solution•HOWEVER – eventually that solution will get neutralized (reaction with other rocks, CO2 in the atmosphere, etc.) and the metals are not so soluble but oxidized S (sulfate, SO42-) is very soluble•A different kind of mineral is formed!Oxides - Oxyhydroxides•FeOOH minerals Goethite or Limonite (FeOOH) important alteration products of weathering Fe-bearing minerals•Hematite (Fe2O3) primary iron oxide in Banded Iron Formations•Boehmite (AlOOH) primary mineral in bauxite ores (principle Al ore) which forms in tropical soils•Gibbsite (Al(OH)3) – common Al oxide forming in aqueous sysems•Mn oxides form Mn nodules in the oceans (estimated they cover 10-30% of the deep Pacific floor)•Many other oxides important in metamorphic rocks…Al oxides•Aluminum occurs in economic deposits principally as bauxite•Bauxite is a mixture of Al oxides and oxyhydroxides:–Diaspore - AlO(OH)–Gibbsite - Al(OH)3–Böhmite - AlO(OH)•Al is a residual phase and bauxite occurs where weathering is extreme and thick layers of aluminum oxyhydroxide are left overMn oxides - oxyhydroxides•Mn exists as 2+, 3+, and 4+; oxide minerals are varied, complex, and hard to ID–‘Wad’ soft (i.e. blackens your fingers), brown-black fine-grained Mn oxides–‘Psilomelane’ hard (does not blacked fingers) gray-black botroyoidal, massive Mn oxides•XRD analyses do not easily distinguish different minerals, must combine with TEM, SEM, IR spectroscopy, and microprobe work•Romanechite Ba.66(Mn4+,Mn3+)5O10*1.34H2O Psilomelane •Pyrolusite MnO2•Ramsdellite MnO2•Nsutite Mn(O,OH)2 •Hollandite Bax(Mn4+,Mn3+)8O16 •Cryptomelane Kx(Mn4+,Mn3+)8O16 •Manjiroite Nax(Mn4+,Mn3+)8O16 •Coronadite Pbx(Mn4+,Mn3+)8O16 •Todorokite (Ca,Na,K)X(Mn4+,Mn3+)6O12*3.5H2O •Lithiophorite LiAl2(Mn2+Mn3+)O6(OH)6 •Chalcophanite ZnMn3O7*3H2O •Birnessite (Na,Ca)Mn7O14*2.8H2O •Vernadite MnO2*nH2O •Manganite MnOOH •Groutite MnOOH •Feitknechtite MnOOH •Hausmannite Mn2+Mn23+O4 •Bixbyite Mn2O3 •Pyrochroite Mn(OH)2 •Manganosite MnOMn Oxide minerals (not all…)WadIron Oxides•Interaction of dissolved iron with oxygen yields iron oxide and iron oxyhyroxide minerals•1st thing precipitated amorphous or extremely fine grained (nanocrystaliine) iron oxides called ferrihydriteFe2+O2Ferrihydrite•Ferrihydrite (Fe5O7OH*H2O; Fe10O15*9H2O some argument about exact formula) – a mixed valence iron oxide with OH and waterGoethite•Ferrihydrite recrystallizes into Goethite (-FeOOH)•There are other polymorphs of iron oxyhydroxides:–Lepidocrocite -FeOOH–Akaganeite -FeOOHIron Oxides•Hematite (Fe2O3) – can form directly or via ferrihydrite goethite hematite•Red-brown mineral is very common in soils and weathering iron-bearing rocks•Magnetite (Fe3O4) – Magnetic mineral of mixed valence must contain both Fe2+ and Fe3+ how many of each??•‘Spinel’ structure – 2/3 of the cation sites are octahedral, 1/3 are tetrahedralBanded Iron Formations (BIFs)•HUGE PreCambrian formations composed of hematite-jasper-chalcedony bands•Account for ~90% of the world’s iron supply•Occur only between 1.9 and 3.8 Ga many sites around the world Hammersley in Australia, Ishpeming in Michigan, Isua in Greenland, Carajas in Brazil, many other sites around the world…BIFs and bacteria•Early earth did not have free O2, as microbial activity became widespread and photosynthetic organisms started generating O2, the reduced species previously stable (without the O2) oxidized – for Fe this results in formation of iron oxide mineralsOther important oxides•Periclase, MgO – Met. mineral from dolomite•Brucite, Mg(OH)2 – weathering product•Rutile, Anatase, brooksite- TiO2 polymorphs•Corundum, Al2O3 – sapphire and ruby•Ilmenite, FeTiO3 – common in igneous/met. rx•Cuprite, Malachite, Azurite – copper oxides •Uraninite, UO2 – important U ore•Spinel, MgAl2O4 – High-P met.
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