native elements and sulfides mineralogy november 9, 2012

Native Elements and Sulfides

MineralogyNovember 9, 2012

Native elements

Native Metals

3 groups:

Gold Group

Platinum Group

Iron Group

Native Semimetals

Native Nonmetals

Native Metals

Gold group

weak metallic bonds

all belong to same group in the periodic table (Ib, the transition metals.

isostructural

FCC lattice: 12-fold coordination

bonds intermediate between covalent and metallic; stronger and more directional bonds, leading to lower symmetry

Au and Ag: same ionic radius (1.44 A): complete SS

Cu (1.28 A)---limited SS with Au and Ag

Occurrence

Platinum Group

harder, higher melting points

Pt and Ir: CCP

Iron Group

isometric, includes pure Fe (very rare—why?)

Ni-Fe metals: kamacite and taenite

Fe: 1.23 A, Ni: 1.24 A

kamacite: BCC 5.5 wt% Ni; taenite: FCC 27-65 wt% Ni

Fe-Ni phase diagram

Native Semimetals

arsenic, antimony, bismuth

structures can’t be represented as simple packing of spheres—why

bond types intermediate between

Native Nonmetals

sulfur, diamond, graphite

Sulfur: orthorhombic, stable at atm p below 95.5 deg C; above that, monoclinic, melts above 119 C. 128 atoms in unit cell. Rings of 8 atoms form molecules. Rings bonded with…

Diamond: covalent; insulator; not close packed; sheets of C parallel to {111}; synthetic diamonds: who and when; lonsdalite; kimberlites

Graphite: conductor; 6-membered rings, each C with 3 nearest neighbors; 3 of 4 valence electrons in each C are locked in tight covalent bonds; the fourth wanders; organic mtl in mm rx

diamond

graphite

Sulfides and Sulfosalts

Sulfides: most ore minerals, generally opaque, many have IV or VI coord; all types of bonds

Some important ones:

Sphalerite (low T) and wurtzite (high T) : both: Zn in IV coord, in sphalerite, Zn are in a FCC lattice; in wurtzite, HCP

Chalcopyrite: structure can be derived from the sphalerite structure: sub Cu and Fe for Zn

Pyrite: cubic; derivative of NaCl structure: replace Cl with 2S

Occurrence

sphalerite

wurtzite

chalcopyrite

Pyrite