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

2. 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

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

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

5. Fe-Ti oxides
Anatase
Fe+2
Fe+3

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

7. 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).

8. 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

9. 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++)

10. 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 octahedral (CN = 6)

11. 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

12. 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

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

14. Calcite Structure
Calcite

15. 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.

16. 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.

17. 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

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

19. 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”

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

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

22. 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

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