Descriptions of Rock Forming Minerals: All the following minerals are made of silica tetrahedrons, either alone or combined with other elements
Quartz • Quartz (SiO2)is silicon dioxide. It has a glassy or greasy luster. • 7 on Mohs hardness scale • Colour varies froms colourless or white, and pink rose quartz, purple amethyst, and brown or gray smoky quartz. • Quartz is the second most abundant mineral in Earth’s crust. • Quartz makes up much of sand and granites.
Feldspar • The most abundant family of minerals in Earth’s crust. (over 60%) • 3 major properties: • Two directions of cleavage • 6 on Mohs hardness scale • Pearly luster • Al ions have replaced some of the Si ions in the basic silica tetrahedron • The also have K, Na, and Ca ions. • Two major groups of feldspars: potassium feldspars and sodium calcite feldspars. • The most common potassium feldspar is orthoclase • The sodium-calcite feldspars are called plagioclase
Mica • Soft silicates found in many rocks • Flat shiny flakes are easily picked out of rocks such as granite and gneiss • Muscovite mica is silvery white • Biotite mica is dark brown or black • 2.5 on Mohs hardness scale
Talc • The softest mineral • 1 on Moh’s hardness scale • White, gray, or greenish in colour • One good cleavage and a soapy feel
Amphiboles • Complex silicate minerals that tend to form long, needlelike crystals • Hornblende is the most common, it is shiny dark green, brown, or black. It has two good cleavages at oblique angles, and a hardness of 5 to 6. • Hornblende is an example of a ferromagnesian silicate which all contain atoms of iron and magnesium.
Pyroxenes • Right angle cleavage surfaces • Augite is the most common member of the pyroxene family. It is a ferromagnesian silicate. Dark green, brown, or black. Two good cleavages, hardness 5 to 6. • Poorer luster than hornblende, short stout crystals
Olivine • Olive-green ferromagnesian silicate • Glassy shell-like in fracture • Very hard (6.5-7.5) • Found in some meteorites
Garnets • Dark red, brown, yellow, green, or black • Very hard (6.5-7.5) • Used as abrasives and gems
Kalinite or kaolin • Aluminum silicate • Formed from weathering of feldspar or other silicates • Principle mineral in clay and shale • Pure kaolin is white, but impurities make it yellow • Earthy (crumbly) fracture • Hardness between 1 and 2.5 • Feels greasy and has earthy odour
Carbonate Minerals:Calcite and Dolomite • These minerals are not silica tetrahedrons • The carbonate group is CO32- • A carbonate mineral is made of carbonate groups joined with various metal ions • Limestone and marble are carbonate minerals
The most common carbonate mineral is calcite, calcium carbonate (CaCO3 ) • Pure calcite is colourless or white, impurities make it almost any colour • Hardness of 3 • Three perfect oblique cleavages • Dolomite is calcium magnesium carbonate
Iron Oxides and Sulfides • Some minerals have large amounts of iron • Iron tends to be combined with either oxygen or sulfur to form an oxide or a sulfide • Hematite is the most common iron oxide mineral, it has a hardness of 5-6, is red with an earthy luster and crumbly fracture.
Magnetite is a black magnetic iron oxide • Hardness 5.5-6.5 • Lodestone is a highly magnetic variety • Pyrite is iron sulfide, it is the most common sulfide mineral • It has a golden colour and a metallic luster, and is commonly known as fool’s gold