ABSTRACT
Features are generalized, and exceptions do occur; crystal faces are displayed on museum specimens of most minerals, but are rarely seen in normal rocks. H = hardness, on a scale of 1-10, from talc the softest mineral of hardness 1, to diamond the hardest of hardness 10. Steel and glass have hardnesses between 6 and 7. D = density, measured in grams/cm 3 or tonnes/m3-
CLASSIFICATION OF IGNEOUS ROCKS This simple classification covers the great majority of Chemical composition is determined by what rocks had igneous rocks. It is based on two parameters which are melted to form the original magma; silica-rich magmas both significant and recognizable. The main types of are referred to as acidic (unrelated to pH) and are igneous rocks can therefore be identified by just colour generally low in iron, so have few black iron minerals, and grain size. and are therefore lighter in colour than basic rocks. The form of occurrence determines the structure of the Porphyritic rocks have scattered larger, older crystals rock in the ground; also, lavas may cool in hours or (called phenocrysts) in a finer groundmass. days while a batholith may take a million years to In fine grained rocks, grains cannot be seen with the crystallize, and the cooling rate determines the grain naked eye; the limit of 0-1 mm is effectively the same as size of the rock. the limit of 0-06 mm used in soils and sediments.
