Systematic Description of Minerals

Oxides, Hydroxides, Carbonates, Phosphates

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=Tix- +4 cation Pyrolusite X=Mnmost 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=Cror orthorhombic Chrysoberyl X=Be, Y=Al

Ulvospinel X=Ti+4, Y=Fe+2

Hematite, ferric Irons Fe+3

in octahedral coordination(C.N. = 6) with Oxygen ions O-2

Hematite GroupX2O3

For example this one

Rutile, Titanium ions Ti+4

in octahedral coordination(C.N. = 6) with Oxygen ions O-2

Rutile Group

TiO2

Fe-Ti oxides

FeFe+2+2FeFe+3+3

Anatase

Spinel Group (XY2O4)

e.g Magnetite Fe+2Fe+32O4

Gem-quality Gem-quality SpinelsSpinels

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

Rutile (TiORutile (TiO22))

Rutile needles in Rutile needles in QuartzQuartz

Pyrolusite dendritesPyrolusite dendriteson surfaceon surface

Pyrolusite (MnOPyrolusite (MnO22))

Mass of botryoidal Mass of botryoidal PyrolusitePyrolusite

Corundum (AlCorundum (Al22OO33))

Hardness=9Hardness=9

Red = RubyRed = RubyBlue = SapphireBlue = Sapphire

Chrysoberyl (BeAlChrysoberyl (BeAl22OO44))

Cyclic twinningCyclic twinningin Chrysoberylin Chrysoberyl

Hydroxides

(OH)- main anionic group forming octahedrally coordinated sheets with weak bonds between.

Two structural types: Brucite-type Brucite-type Mg(OH)2 trioctahedral sheets (all trioctahedral sheets (all octahedral cation sites are octahedral cation sites are filled with Mgfilled with Mg++++))

Gibbsite-type Gibbsite-type Al(OH)3 dioctahedral sheets (only two dioctahedral sheets (only two of three octahedral sites are of three octahedral sites are filled with Alfilled with Al+3+3) ) WHY?WHY?

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)

Al+3 radius 0.61 A OH- radius 1.37 A ratio 0.445 octahedral (CN = 6)

Think of the missing OH- below as neutralizing the Al+3 below

Common Types of HydroxidesBrucite Brucite Mg(OH)Mg(OH)22 Gibbsite Gibbsite Al(OH)Al(OH)33

Manganite Manganite MnO(OH)MnO(OH) DiasporeDiaspore AlO(OH)AlO(OH)

GoethiteGoethite FeO(OH)FeO(OH) Bauxite Bauxite Al-hydroxide* Al-hydroxide*

*mixture of diaspore, gibbsite, and boehmite (*mixture of diaspore, gibbsite, and boehmite (AlO(OH))AlO(OH))

Pronunciations:Gear-tightGo-eth-thiteGurrr-tite

Seem to vary with region

CarbonatesCarbonates

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

Carbonates

AragoniteAragonite(High-P) Orthorhombic(High-P) Orthorhombic

CalciteCalcite(Low-P - Hexagonal)(Low-P - Hexagonal)

Most are Hexagonal Most are Hexagonal

Calcite

Calcite Structure

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 - CuAzurite - Cu33(CO(CO33))22(OH)(OH)2 2 (Blue)(Blue)

Malachite – CuMalachite – Cu22COCO33(OH)(OH)22 (Green) (Green)

Rhodochrosite – MnCORhodochrosite – MnCO33

Borates

Kernite – Na2B4O6(OH)2·3H2O

Borax - Na2B4O5(OH)4·8H2O

Ulexite – NaCaB5O6(OH)6·5H2O

H – 2-2.5; SG – 1.7H – 2-2.5; SG – 1.7

H – 3; SG – 1.95H – 3; SG – 1.95

H – 1-2.5; SG – 1.96H – 1-2.5; SG – 1.96

““Television Rock”Television Rock”

Tungstates & Molybdates

Wolframite – (Fe,Mn)WOWolframite – (Fe,Mn)WO44

Scheelite – CaWOScheelite – CaWO44

Wulfenite – PbMoOWulfenite – PbMoO44

SG: 7-7.5SG: 7-7.5

SG: ~6SG: ~6

SG: 6.8SG: 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: CrocoitePbCrO4Monoclinic 2/m.Commonly in prismatic crystals, vertically striated=102o33’Cleavage {110} perfectH 2.5 – 3G 5.9-6.1Luster AdamantineColor bright red to orange- redStreak orange-yellow