SULFIDES
Galena | PbS | Isometric |
Sphalerite | ZnS | Isometric |
Pyrrhotite | Fe(1-x)S | Monoclinic/
Hexagonal |
Chalcopyrite | CuFeS2 | Tetragonal |
Molybdenite | MoS2 | Hexagonal |
Pyrite | FeS2 | Isometric |
Covellite | CuS | Hexagonal |
Bornite | Cu5FeS4 | Tetragonal/
Isometric |
Marcasite | FeS2 | Orthorhombic |
Sphalerite - ZnS
isostructural with diamond, face-centered cubic lattice with 1/2 of carbon represented by Zn and the other half by S; radius ratio for Zn:S (0.32) predicts tetrahedral coordination of Zn within 4 S.
Sphalerite structure - analogous to the diamond structure, in which 1/2 of the C are replaced by Zn and the other half by S - mixture of ionic and covalent bonding results in ionic complex with very different properties than diamond
Sphalerite-Chalcopyrite Structure - formed by regular substitution of Fe and Cu for Zn, leading to a doubling of the unit cell. Chalcopyrite structure is twice the sphalerite structure, with 1/2 of the Zn replaced by Fe and the other half by Cu.
Pyrrhotite (Fe1-xS) has vacancy substitutions to compensate for the fact that some Fe is +2 and some is +3; in 1:1 ratio, can only be +2. Electron interactions associated with the vacancy cause the magnetism. - many hexagonal and monoclinic polymorphs of pyrrhotite exist.
Pyrite-Marcasite - NaCl type structure, cubic, S-pairs for Cl ions in Halite-type structure, octahedral coor. - Marcasite, orthorhombic polymorph, octahedral coordination, single sulfur in coord. - arsenopyrite is derivative of marcasite structure with substitution of arsenic
Pyrite is isostructural with NaCl, with Na represented by Fe, and Cl by diatomic sulfur; Marcasite also has diatomic sulfur, but it is arranged around Fe in a body centered 'cubic' lattice (orthorhombic).
OXIDES
Hematite | Fe2O3 | Hexagonal |
Corundum | Al2O3 | Hexagonal |
Ilmenite | FeTiO3 | Hexagonal |
Periclase | MgO | Isometric |
Pyrolusite | MnO2 | Tetragonal |
Rutile | TiO2 | Tetragonal |
Spinel | MgAl2O4 | Monoclinic |
Magnetite | Fe3O4 | Isometric |
Chromite | FeCr2O4 | Isometric |
Simple oxides have a formula either of XO2, XO, or X2O3; multiple oxides have the formulas XY2O4 with two different metal sites (one octahedral and one tetrahedral). Oxides include minerals of great economic importance, as well as some diagnostic of geological environments The bond type in metals is strongly ionic, meaning that is fairly accurate to consider the radius ratio constraints and packing models for viewing structures and coordination.
Hematite Group (X2O3):
Hematite Fe2O3 Hexagonal
Corundum Al2O3 Hexagonal
Ilmenite FeTiO3 Hexagonal
HCP of oxygens with all cations in octahedral coordination, and only 2/3
of the octahedral sites filled with trivalent cations. Each oxygen is shared
between four octahedra, a result of the local charge balance. Octahedra
within layers share edges, and faces are shared across layers.
Simple Oxides (XO):
Periclase MgO Isometric
Simple cubic packing, identical to halite, in which each layer of octahedra is filled. The structure is analogous to the hematite group structures, yet with no vacancies as each oxygen is shared among six octahedra (cations are 2+ rather than 3+).
Rutile Group (XO2):
Pyrolusite MnO2 Tetragonal
Rutile TiO2 Tetragonal
Based on hexagonal closest packing of oxygen, with Ti filling half of the octahedral interstitial positions, ABABABAB stacking of octahedra in 'chains' is evident. The +4 cation are arranged in chains of octahedra, parallel to the c-axis; rutile has a prismatic habit due to the chains parallel to c.
Spinel Group (XY2O3):
Spinel MgAl2O3 Monoclinic
Chromite FeCr2O4 Isometric
Magnetite Fe3O4 (Fe+2Fe+32O4)
Isometric
Oxygen is in CCP along (111) planes in the spinels. The differing cations are either in octahedral or tetrahedral coordiantion, depending on radius ratio considerations. In 'normal spinels', the X cations occupy the tetrahedral sites (8 per unit cell) and the Y cations occupy the octahedral sites (16 per unit cell). In the 'inverse spinels' the X cations are in octahedral coordination, and 8 of the 16 Y cations are in tetrahedral coordination.
Solid solution among the oxides is variable. Among the spinels it is extensive.
HYDROXIDES
Goethite | aFeO(OH) | Orthorhombic |
Gibbsite | Al(OH)3 | Monoclinic |
Brucite | Mg(OH)2 | Hexagonal |
All characterized by a hydroxyl (OH)- group or H2O molecules. The presence of the hydroxyl group causes bond strengths to be weaker than for oxides.