Systematic Description of Minerals Oxides, Hydroxides, Carbonates, Phosphates Systematic Description of Minerals

Oxides Three main groups based on metal cation/oxygen ratios (impurities not shown) Hematite Group (X2O3) Corundum X=Al+3 most phases hexagonal Hematite X=Fe+3 Ilmenite X= (Fe+2,Ti+4) solid solution) Rutile Group (XO2) Rutile X=Ti x- +4 cation Pyrolusite X=Mn most phases tetragonal Cassiterite X=Sn Uraninite X=U Spinel Group (XY2O4) Spinel X=Mg, Y=Al X- +2 ; Y- +3 cation Magnetite X=Fe+2, Y=Fe+3 most phases isometric Chromite X=Fe+2, Y=Cr or orthorhombic Chrysoberyl X=Be, Y=Al Ulvospinel X=Ti+4, Y=Fe+2

Hematite Group X2O3 Hematite, ferric Irons Fe+3 in octahedral coordination (C.N. = 6) with Oxygen ions O-2 For example this one

Rutile Group TiO2 Rutile, Titanium ions Ti+4 in octahedral coordination (C.N. = 6) with Oxygen ions O-2 TiO2

Fe-Ti oxides Anatase Fe+2 Fe+3

Spinel Group (XY2O4) e.g Magnetite Fe+2Fe+32O4 Gem-quality Spinels

Magnetite Fe3O4 Magnetite cubic unit cell = Fe24O32. Oxide ions CCP = FCC: 32 octahedral and 64 tetrahedral holes. Three types Iron ion present: Fe3+ in tetrahedral holes (1/8 of the tetrahedral holes filled); Fe3+ in octahedral holes (1/4 of the octahedral holes filled); and Fe2+ in octahedral holes (1/4 of the octahedral holes filled).

Other Common Oxides in color Corundum (Al2O3) Rutile (TiO2) Hardness=9 Rutile needles in Quartz Pyrolusite (MnO2) Blue = Sapphire Red = Ruby Chrysoberyl (BeAl2O4) Mass of botryoidal Pyrolusite Pyrolusite dendrites on surface Cyclic twinning in Chrysoberyl

Hydroxides (OH)- main anionic group forming octahedrally coordinated sheets with weak bonds between. Two structural types: Gibbsite-type Al(OH)3 dioctahedral sheets (only two of three octahedral sites are filled with Al+3) WHY? Brucite-type Mg(OH)2 trioctahedral sheets (all octahedral cation sites are filled with Mg++)

Gibbsite Al(OH)3 Basic building block shown is Al(OH)3, shown as Al2(OH)6, electrical neutrality is satisfied, so every third OH- octahedron must be empty (no Al+3) Think of the missing OH- below as neutralizing the Al+3 below Al+3 radius 0.61 A OH- radius 1.37 A ratio 0.445 octahedral (CN = 6)

Common Types of Hydroxides Brucite Mg(OH)2 Gibbsite Al(OH)3 Manganite MnO(OH) Diaspore AlO(OH) Goethite FeO(OH) Bauxite Al-hydroxide* *mixture of diaspore, gibbsite, and boehmite (AlO(OH)) Pronunciations: Gear-tight Go-eth-thite Gurrr-tite Seem to vary with region

Carbonates Reason for electrostatic valency calculations: Amount of residual charge indicates relative strength of bonds with cations, which are reflected in the hardness of the mineral

Carbonates Most are Hexagonal Aragonite (High-P) Orthorhombic Calcite (Low-P - Hexagonal) Most are Hexagonal

Calcite Structure Calcite

The structure of calcite is described as a "modified NaCl" structure, but calcite is not cubic. The carbonate groups stretch the atomic planes and distort the cube into a rhombohedron.

Aragonite Group Carbonate minerals with a single divalent cation of radius > 1.00 Å. With increasing radius the species are Aragonite (CaCO3), Strontianite (SrCO3), Cerussite (PbCO3), and Witherite (BaCO3). Aragonite is denser than calcite, and is the high P polymorph. It crystallizes at ambient conditions and persists metastably for millions of years. The orthorhombic structure is nearly hexagonal with c as the unique axis.

Dolomite CaMg(CO3)2 Forms during seasonal high tides that flood limestone (calcite) islands with seawater. Mg++ in the seawater replaces some of the Calcium ions. Has very distinctive crystals

Other Carbonates Azurite - Cu3(CO3)2(OH)2 (Blue) Malachite – Cu2CO3(OH)2 (Green) Rhodochrosite – MnCO3

Borates Kernite – Na2B4O6(OH)2·3H2O Borax - Na2B4O5(OH)4·8H2O Ulexite – NaCaB5O6(OH)6·5H2O H – 3; SG – 1.95 H – 2-2.5; SG – 1.7 H – 1-2.5; SG – 1.96 “Television Rock”

Tungstates & Molybdates Wolframite – (Fe,Mn)WO4 SG: 7-7.5 Scheelite – CaWO4 SG: ~6 Wulfenite – PbMoO4 SG: 6.8

Phosphates Apatite – Ca5(PO4)3(F,Cl,OH) prismatic hexagonal crystals common in igneous rocks and hydrothermal deposits - variable colors “the deceiver”

Other Common Phosphates Monazite – (Ce,La,Y,Th)PO4 Ore mineral for Rare Earth Elements Useful mineral in U-Pb and Th age dating Wavellite – Al3(PO4)2(OH)3·5H2O Radiating globular aggregates Turquoise – CuAl6(PO4)4(OH)8·5H2O

A rare Chromate: Crocoite PbCrO4 Monoclinic 2/m. Commonly in prismatic crystals, vertically striated b=102o33’ Cleavage {110} perfect H 2.5 – 3 G 5.9-6.1 Luster Adamantine Color bright red to orange- red Streak orange-yellow