MINERAL FAMILIES

Aim: Describe the Properties of the Two Major Groups of Minerals: Silicates and Non-Silicates Classified into 7 Groups Based upon their chemical composition

I Native Elements: consist of a single element Examples: Silver Ag, Copper Cu, Graphite C

II Sulfides: - - compounds of metal and sulfur - Most are heavy with a metallic luster - examples: Galena, Pyrite, sphalerite

III Oxides: - compounds of metals and oxygen Contain an oxygen and an element other than Silicon - examples: Hematite, corundum

IV Halides - salts made of a combination of metals and the nonmetallic elements known as halogens. - examples: Sodium Chloride

V Carbonates : - compounds of metals with carbon and oxygen. - carbonates dissolve easily in acid. examples: calcite (calcium carbonate)

VI Sulfates: compounds of metals with sulfur and oxygen hydrous sulfates are soluble in water and contain water molecules. - anhydrous sulfates are less soluble and do not contain water molecules.

VII Silicates: - compounds of various elements with silicon and oxygen. - examples: Quartz, clay, talc, mica - the most abundant group - make up 90% of Earth’s Crust - elements that make up are Si and O - these elements form a unit called a tetrahedron All silicates have the same fundamental building block which is Silicon and Oxygen tetrahedron (this is not a compound, it is an ion) In nature, the tetrahedra form neutral chemical minerals producing chemically stable structures. Individual tetrahedra are linked by pos. ions. When oxygen atoms are shared between silica atoms in adjacent tetrahedra a variety of structures are formed.

Tetrahedron molecular formula SiO4 structural formula

The Silicates Single chain – Si surrounded by 4 O atoms – 2 are shared => the linkage across shared O atoms joins the tetrahedra into a chain. There can be single chains, double chains, 3-D frameworks - => changing the ratio of Si to O makes all the difference. Isolated tetrahedra 1:4 Single chain 1:3 2 shared = 1, 2 unshared = 2 => 3(O) : 1 (Si) 3-D => 4 shared = 2:1 O:Si More O shared => less Si present => minerals have a high or low Si content based on this ratio. Each mineral group has a particular silicate structure which detemines the cleavage it exhibits. Si-O bonds are strong=> silcate minerals cleave between Si-O structures not across them so Mica – sheet structure cleaves easily in one direction, quartz has equally strong bonds in all directions so it fractures.

As the mineral forms, more silica tetrahedra are layered on As the mineral forms, more silica tetrahedra are layered on. The weaker the bonds between the layers of tetrahedra, the more likely a mineral is to cleave along those planes. Write this at the bottom Mica – perfect cleavage – smooth Olivine – poor/no cleavage - rough

The kind of ion that bonds with the tetrahedron will determine the kind of rock forming silicate that is produced – the arrangement of these atoms determines the crystal shape that results

Because most of the common minerals are silicates, and because silicates are formed by combinations of the silica tetrahedral with other elements, the silica tetrahedral is the fundamental unit of the earth’s crust.