Many of the complex silicates, when long exposed to the action of the weather and of percolating waters, become more or less c profoundly changed chemically, a change which is known as alteration and forms an early stage of decay. 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.

I. Zeolites

In this group are included a large number of minerals, which are hydrated silicates of alumina, potash, soda, lime, etc. They all contain water and hence boil and effervesce when heated before the blowpipe. All these minerals are products of decomposition and do not occur as original constituents of rocks.