Volcanic environments, clay minerals

Clays and clay minerals occur under a fairly limited range of geologic conditions. The environments of formation include soil horizons, continental and marine sediments, geothermal fields, volcanic deposits and weathering rock formations. Most clay minerals form where rocks are in contact with water, air or steam. For example, weathering boulders on a hillside, sediments on sea or lake bottoms, deeply buried sediments containing pore water, and rocks in contact with water heated by magma. Extensive alteration of rocks to clay minerals can produce relatively pure clay deposits that are of economic interest. 

Most of the solid components of the Earth s crust, i.e. rocks, sediments, clays etc. are largely made up of various mineral species. Amineral species is deiined as A naturally occurring, inorganic, homogeneous solid, having a definite (but not necessarily fixed) chemical composition and a fixed ordered internal structure, i.e. crystalline. This ordered internal structure is reflected in the external morphology when the mineral has a well crystalline form. When the crystalline form is not perceptible externally called cryptocrystalline, its crystalline nature can be detected by scientific analytical techniques like X-Ray Difiraction. Amorphous natural solids like coal, volcanic glasses etc. do not qualify as minerals. The abimdance of any mineral in the Earth is decided by the availability of the constituent elements of the mineral in the earth s crust and also the stability of that mineral in the surface or near-surface environment. 

---