Mineral Groups

Major Silicates More than 90% of the minerals in Earth’s crust are members of the silicate family. A silicate is a compound of silicon, oxygen, and usually one or more metallic elements, such as aluminum or iron. In all silicates, the basic building block is the silica tetrahedron, consisting of four oxygen atoms bonded to central silicon atoms.

Figure 1. Three ways of drawing the silica tetrahedron: a) At left, a ball & stick model, showing the silicon cation in orange surrounded by 4 oxygen anions in blue; b) At center, a space filling model; c) At right, a geometric shorthand.

Quartz Quartz is made entirely of tightly bound silica tetrahedral. It has the chemical formula SiO2.

Some Physical Properties of Quartz:  glassy or greasy lustre  colourless or white (pure quartz)  can be rose coloured, amethyst, and smoky  exhibit either conchoidal or irregular fracture  hardness of 7 on Mohs scale (hardest of the common minerals)

Some uses of quartz include being used in watch movements, prisms, heat lamps, glass, and paints. The crystal quartz is a semiprecious gemstone often worn as jewellery. Quartz is found in granite and is the second most abundant family of minerals. Feldspars The feldspar family of minerals makes up 60% of Earth’s crust.

Some Physical Properties of Feldspars:  two directs of cleavage  hardness of 6  pearly lustre

Feldspar differs from quartz because aluminum atoms replace some of the silicon atoms in the silica tetrahedra. The replacement creates a net electrical charge in these tetrahedral, which is balanced by the addition of other metals, such as potassium, sodium, or calcium. Because of this feldspars are classified into two major groups – potassium feldspars and sodium-calcium feldspars.

The most common potassium feldspar is orthoclase (light- coloured – pink or salmon, cleavage surfaces meet at right angles, commonly found in granite) and the most common sodium-calcium feldspar is plagioclase (white to gray, cleavage surfaces meet at slightly less than a right angle, one cleavage surface is often marked by fine parallel lines called striations).

Some uses of feldspar include being used in glass and ceramics.

Other Silicates Pyroxene Family  have cleavage surfaces that meet nearly at right angles  Augite (ferromagnesian silicate) is the most common member of this family  Augite is dark in colour  Augite has two good cleavages and a hardness between 5 and 6

Mica Family  hardness of 2.5 and perfect cleavage  can easily be picked out of rocks such as granite and gneiss  form flat crystals  cleave in only one direction

Amphibole  complex silicates that form long, needlelike crystals  most common amphibole is hornblende  hornblende can be shiny dark green, brown, or black  hardness from 5 to 6  has two good cleavages that meet at oblique angles  ferromagnesian silicate  found in igneous and metamorphic rocks

Olivine  olive-green colour  hardness of 6.5 and glassy  shell-like fracture  some meteorites contain olivine  ferromagnesian silicate

Kaolinite  aluminum silicate  formed by the weathering of feldspars and other silicates  white  hardness of about 2  perfect cleavage in one direction Carbonates Summarize the information on Carbonates in the chart below (page 110). Minerals Description General Information

Calcite

Dolomite Oxides and Sulfides Summarize the information on Oxides and Sulfides in the chart below (pages 110 -111). Minerals Description General Information

Hematite

Magnetite

Pyrite