TALC AND CHLORITE GROUPS 21 



Sp. gr. = 3.2-3.5 ; H = 6.5-7. Olivine crystallizes in the ortho- 

 rhombic system, and occurs in prisms, flat tables, or irregular 

 grains. Hydrochloric acid decomposes the mineral, with sepa- 

 ration of gelatinous silica. The colour varies from olive green to 

 yellow, or it may be colourless, and usually the irregular grains 

 look like fragments of bottle glass. 



C. 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. 



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 large quantities of water and hence boil and effervesce 

 when heated before the blowpipe. All these minerals are prod- 

 ucts of decomposition and do not occur as original constituents 

 of rocks. 



II. Talc and Chlorite Groups 



1. Chlorite. — Under this name are grouped a number of closely 

 allied minerals, which are hydrated silicates of alumina, magnesia, 

 and iron. They are soft minerals, with a hardness of 1-1.5 and a 

 specific gravity of 2.6-2.96, and are of a green colour. The crystal- 

 line form is somewhat uncertain, but is now generally regarded as 

 monoclinic, with a pseudo-hexagonal symmetry. These minerals 

 are laminated and split readily into thin leaves, as do the micas, 

 from which they may be distinguished by the fact that the leaves 

 are not elastic. 



The chlorites result from the decomposition of hornblende, 

 augite, or the magnesian micas. 



