MINERALS. 359 



SECTION 4.— GENERAL STATEMENTS. 



PHYSICAL-CHEMICAL FACTORS ON WHICH NATURE OF ALTERATIONS 



DEPENDS. 



As inferences from the foregoing treatment it may be said that the more 

 important physical-chemical factors on which the alteration of an individual 

 mineral depends are (1) the chemical composition of the mineral, (2) the 

 chemical composition of the adjacent minerals, (3) the chemical composi- 

 tion of the circulating solutions, (4) the specific gravity, (5) the symmetry, 

 (6) the heat effect of the reaction, and (7) the pressure and volume. 



CHEMICAL COMPOSITION. 



Certain chemical compounds are stable under a great variety of 

 conditions; others are stable only under certain definite conditions; and thus 

 the chemical composition influences the stability of minerals. As an illus 

 tration of minerals which have stability under widely varying conditions 

 may be mentioned quartz, which forms alike from a magma and from water 

 solutions, and also at the surface and at great depth. Nephelite and soda- 

 lite are examples of minerals which can exist only under a comparatively 

 narrow rang*e of conditions. 



CHEMICAL COMPOSITION OF ADJACENT MINERALS. 



It has been seen that mineral particles may react upon one another, 

 either through the medium of contained solutions or by direct rearrange- 

 ment under the influence of pressure. Therefore, it is clear that the nature 

 of a mineral which is mainly secondary to another mineral is influenced 

 by the chemical compositions of adjacent minerals. 



CHEMICAL COMPOSITION OF CIECULATING SOLUTIONS. 



It has already been shown that the secondary minerals are dependent 

 not only upon the adjacent minerals, but upon the material carried by the 

 underground solutions. The amount of such material is dependent upon 

 the vigor of the circulation. As explained on pages 507-518, 655-656, 

 764-766, the material added or abstracted may be great in the zone of 

 katamorphism, but is usually rather limited in amount in the zone of 

 anamorphism. 



