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 Bhmite - AlO(OH) Al is a residual phase and bauxite occurs where weathering is extreme and thick layers of aluminum oxyhydroxide are left over Aluminum concentration controlled by pH-dependent mineral solubility 2
0 Gibbsite -4 Al +++ - -6 Al(OH)4
-8 ++ 2 3 4 5 6
+ ++ + AlOH Al(OH)2 -10 Diagram Al , T = 25 C, P = 1.013 bars, a [H2 O] = 1 log a Al +++ -2
25oC 7 pH 8 9 10 11 12
Greg Mon Nov 01 2004 6 Muscovite Quartz 7 Amorphous silica Activity diagram showing the stability relationships among some
minerals in the system K2O-Al2O3-SiO2-H2O at 25C. The dashed lines represent saturation with respect to quartz and amorphous silica. log (aK+/aH+) 5 K-feldspar 4 3 Gibbsite Kaolinite
2 1 Pyrophyllite 0 -6 -5 -4 -3
log aH SiO 0 -2 -1 Mn 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) grayblack botroyoidal, massive Mn oxides XRD analyses do not easily distinguish different minerals, must combine with TEM, SEM, IR
spectroscopy, and microprobe work Mn Oxide minerals (not all) Romanechite
Mn2+Mn23+O4 Mn2O3 Mn(OH)2 MnO Wad Iron 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 ferrihydrite
Fe2+ O2 Ferrihydrite Ferrihydrite (Fe5O7OH*H2O; Fe10O15*9H2O some argument about exact formula) a mixed valence iron oxide with OH and water Goethite Ferrihydrite recrystallizes into Goethite (FeOOH) There are other polymorphs of iron oxyhydroxides: Lepidocrocite -FeOOH Akaganeite -FeOOH
Iron 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 tetrahedral Banded Iron Formations (BIFs) HUGE PreCambrian
formations composed of hematite-jasper-chalcedony bands Account for ~90% of the worlds 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 minerals Sulfide Minerals Minerals with S- or S2- (monosulfides) or S22- (disulfides) as anionic group Transition metals bonded with sulfide anion groups Sulfides Part 1 Substitution into sulfides is very common As and Se substitute for S very easily Au can substitute in cation sites
(auriferrous minerals) Different metals swap in and out pretty easily Cu and Fe for instance have a wide range of solid solution materials Complexes Minerals Metals in solution are coordinated with ligands (Such as H2O, Cl-, etc.) Formation of a sulfide mineral requires direct bonding between metals and sulfide requires displacement of these ligands and deprotonation of the sulfide Molecular Clusters are the intermediates between aqueous species and nanocrystals
Sphalerite Wurtzite Structures Zn3S3(H2O)6 aggregation with or without excess bisulfide affects the structure of the resulting nanoscale cluster 5 Zn3S3 (H2O)6 + 3 HS- 3 Zn4S6 (H2O)4 4- + 3 sphalerite structure Zn(H2O)62+ + 3 H+ 2 Zn3S3 (H2O)6 Zn6S6 (H2O)9 + 3 H2O wurtzite structure Pyrite  face Iron Sulfides
Pyrite FeS2 (cubic) Marcasite FeS2 (orthorhombic) Troilite FeS end member Pyrrhotite Fe1-xS (slightly deficient in iron) how is charge balance maintained?? Greigite, Mackinawite FexSy Arsenopyrite FeAsS Chalcopyrite CuFeS2 Other important sulfides
Galena PbS Sphalerite/wurtzite ZnS Cinnabar HgS Molybdenite MoS Covellite CuS Chalcocite Cu2S Acanthite or Argenite AgS Stibnite Sb2S3
Orpiment As2S3 ; Realgar AsS Sulfides are reduced minerals what happens when they contact O2? This is the basis for supergene enrichment and acidic mine drainage Actively 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+ + 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 100s 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!
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