The Rock Forming Minerals

It is convenient to distinguish between minerals which are essential constituents of the rocks in which they occur, their presence being applied by the rock name, and others which are accessory. Secondary minerals are those which result from the decomposition of earlier minerals, often promoted by the action of water in some form, with the addition or subtraction of other material, and with the formation of by-products.Atomic Structure The arrangement and spacing of the atoms of which a crystal is proposed control its regular form and properties.The unit of silicate structure is SiO4-tetrahedron, and is built into different structure as follows;1. Separate SiO4 groups2. Single Chain Structure Si2O63. Double Chain Structure Si4O114. Sheet Structure Si4o105. Three-dimensional Framework SiO2

Identification of Minerals in the Hand Specimen Many minerals can be identified in a rock specimen with some degree of certainly, but it is not always possible to do this, for two reasons: (i)the mineral may be too small to be identified easily, (ii) only one or two characteristics features may be seen , and these together may be insufficient to determine the mineral. It should be possible to identify the common rock-forming minerals in the hand forming specimen with pocket lens where one dimension of the mineral grain is not less than about 1 mm. With practice much smaller grains can be determined. The most useful characteristics for this purpose are:1. General shape of grains, depending on the crystallization of the mineral; the faces of well-formed crystals can often be observed, but where grains have been modified (e.g. by rounding) other properties must be used.2. Colour and transparency.3. Presence of absence of cleavage.4. Presence or absence of twinning, and type of twinning.5. Hardness.

THE SILICATE MINERALS

Olivine Common olivine has the composition (MgFe)2Si4, in which Fe replaces part of Mg.Crystals: Orthorhomboric; pale olive-green or yellow; vitreous lustre; conchoidal fracture.Properties in thin section: Porphyritic crystals commonly show 6- or 8-sided sections, the outline generally somewhat rounded. Cleavage rarely seen; irregular cracks common.Colour: None when fresh. Alteration