Olivine Group
Olivine is the only mineral of this group of sufficient importance to require mention; it is a silicate of magnesia and iron 2 (MgO, FeO)Si02, though the percentage of iron varies greatly.
Sp. gr. = 3.2-3.5; H = 6.5-7. Olivine crystallizes in the orthorhombic system, and occurs in prisms, flat tables, or irregular grains. Hydrochloric acid decomposes the mineral, with separation of gelatinous silica. The colour varies from olive green to yellow, or it may be colorless, and usually the irregular grains look like fragments of bottle glass.
Other Silicates, Chiefly Decomposition Products
Many of the complex silicates, when long exposed to the action of the weather and of percolating waters, become more or less profoundly changed. One of the commonest of these changes is hydration, or the taking up of water into chemical union, and this may be accompanied by the loss of soluble ingredients, or the replacement of some constituents by others.
Zeolites
In this group are included a large number of minerals, which are hydrated silicates of alumina, potash, soda, lime, etc. They all contain large quantities of water and hence boil and effervesce when heated before the blowpipe. All these minerals are products of decomposition and d) not occur as original constituents of rocks.