EPS 101 Spring 2003– Adrian Brearley - Lecture 6

Minerals - Building blocks of rocks III

Keywords and concepts

Rock-forming minerals

Silicates – composed of oxygen (O) and silicon (Si) in combination with other cations, e.g. olivine (Mg,Fe)2SiO4.

Carbonates – minerals made up of the carbonate ion (CO3)2- in combination with other cations (mainly Ca and Mg) e.g. calcite CaCO3.

Oxides – compounds consisting of oxygen and metallic cations e.g. magnetite Fe3O4.

Sulfides – compounds of the sulfide anion (S2-) and metallic cations e.g. pyrite FeS2.

Sulfates – compounds of the sulfate anion (SO4)2- and metallic cations e.g. anhydrite (CaSO4).

Silicates

Silica tetrahedron – basic building block of silicates – Si atom surrounded by 4 O atoms arranged in a tetrahedral configuration. Complex ion – (SiO4)4-. Bonding of tetrahedra can occur in different ways – i.e. to cations such as Na+, K+, Ca2+, Mg2+, Fe2+ or tetrahedra can bond to one another (see below).

Isolated tetrahedra – (SiO4)4- tetrahedra are isolated from one another and are bonded to cations – example, olivine

Ring linkages – tetrahedra are linked in a closed ring pattern e.g. metamorphic mineral cordierite. Two oxygens of each tetrahedra are linked to adjacent tetrahedra.

Single chain linkages – tetrahedra arranged in a chain rather than a closed loop. Two oxygens of each tetrahedra are linked to adjacent tetrahedra. e.g. minerals of the pyroxene group such as enstatite (MgSiO3)

Double chain linkages – two single chains linked to each other by shared oxygens, e.g. minerals of the amphibole group, such as hornblende, a complex silicate containing Ca, Na, Mg, Fe and Al.

Sheet linkages – each tetrahedron shares three of its oxygens with adjacent tetrahedra and the structure extends infinitely in two dimensions to form a sheet.

Physical Properties of Minerals.

Hardness - measure of how easily a mineral can be scratched – relative scale of hardness called Mohs scale of hardness.

Cleavage – tendency of mineral to break along planar surfaces. Depends on bond strength along particular planes in the crystal.

Luster – way that mineral reflects light – controlled by the type of atoms and how they are bonded.

Color – color of mineral is produced by transmitted or reflected light. Determined by the type if ions present.

Specific Gravity and Density – density = mass per unit volume (g/cm3). Specific gravity = weight of mineral in air divided by equal volume of pure water at 4° C.

Crystal habit – shape in which minerals grow. Usually common geometric shape – blades, plates, needles, etc.