Geology 111G/
Lecture 4. Minerals
Definition
Properties
Rock-forming minerals
Rocks
I. Mineral: Naturally occurring, solid crystalline substance, generally inorganic,with a specific chemical composition.
A. Crystal structure is characteristic of a specific mineral.
B. Chemical composition may vary within limits.
C. Minerals form by crystallization, which is crystalline assembly of atoms in a gas orliquid. This may take place duringcooling of a hot melt or by precipitation from a concentrated solution.
II. Mineralproperties. Physical properties of minerals determined by internalarrangement of atoms, chemical formula and bond types.
A. Crystal habit. Shape of a crystal is determined
byatomic structure. The
atomicstructure repeats itself throughout the mineral; this is the crystallattice.
1. Cubic habit of halite (NaCl) is determined by the equallyspaced disposition of sodium and chlorine ions in the crystal lattice.
B. Polymorphs: Different crystal structures with thesame chemical composition. Theclassic example is carbon.
1. Graphite: Sheets of carbon with covalent bonds,the sheets connected by weak bonds. In sheets, carbon is arranged in hexagons.
2. Diamond: Three-dimensional network of carbonatoms with covalent bonds. Innetworks, carbon is arranged in tetrahedra.
C. Cation substitution: Same crystal structure, differentchemical composition.
D. Cleavage: Tendency of a mineral to break incertain preferred directions along smooth planar surfaces. Number of cleavage planes and theirorientations are determined by crystal structure and bond types. Quite different from crystal habit; forexample, quartz forms six-sided crystals, but breaks like glass.
E. Hardness: relativeease with which mineral can be scratched. Mohs hardness scale:
1. Talc
2. Gypsum (fingernail=2.5)
3. Calcite (penny=3)
4. Fluorite
5. Apatite (glass, steel=5.5)
6. Orthoclase (K-feldspar)
7. Quartz
8. Topaz
9. Corundum
10. Diamond
III. Rock-forming Minerals
A. Minerals are classified according to their chemicalcomposition.
1. Native elements: minerals that occur in nature as singleelements
a. Copper, carbon, gold, sulfur.
2. Halides: minerals composed of cations combinedwith various of the halogen elements, such as chlorine, iodine, bromine,fluorine.
a. Halite, NaCl.
b. Commonly formed by precipitation from brines.
3. Oxides: cations combined with oxygen.
a. Hematite, Fe2O3
b. Magnetite, Fe3O4
4. Sulfides: cations incombination with sulfur.
a. Pyrite, FeS2
5. Carbonates and sulfates: anions are complex.
a. Carbonates: cationplus CO3--; exaample, Calcite, CaCO3
b. Sulfates: cation plus SO4--; example, Gypsum,CaSO4x2H2O
6. Silicates: Class ofmost common minerals, composing >90% of the rock-forming minerals.
a. Fundamental constitutent is the silica tetrahedron (SiO4-4),which is a combination of Si and O with covalent bonds. Behavior of silicate minerals is determinedby the arrangement of these tetrahedra in the crystal lattice.
b. Ferromagnesian silicates. SiO4tetrahedra in combination with iron and/or magnesium atoms. Tetrahedra bonded covalently, iron andmagnesium bonded ionically. Theseare classified by their crystal structure.
1. Olivine: individualtetrahedra linked by Fe+2 andMg+2; (Mg, Fe)2SiO4.
2. Pyroxene: singlechains of tetrahedra. In addition to Fe and Mg, Ca, Na and Al substitute intothe structure.
3. Amphibole: doublechains of tetrahedra. Differ frompyroxene in that they contain a hydroxyl group.
4. Biotite: sheets of tetrahedra.
c. Non-ferromagnesian silicates: Tetrahedra linkedprimarily by cations other than Fe and Mg. Generally lighter in color and of lower density than ferromagnesiansilicates.
1. Muscovite: potassiumrich sheet silicates.
2. Feldspars: 3-Dnetwork of tetrahedra in which 1/4 to1/2 of silica ions are replaced by Al (KALSi3O8). Calcium, sodium and potassium are the primary cationsoutside the tetrahedra. Mostabundant minerals in crust, about 54% by weight.
3. Quartz: 3-D network of tetrahedra: SiO2.
IV. Rocks: Rocksare combinations of minerals. Theyare classified and named by the minerals present. Different rock types form in different geologic environmentsand thus are key to understanding geologic processes.