ALTERATIONS OF MINERALS. 203 



of material. For either of these changes the presence of water is required 

 in most instances, the alterations taking place through solution and redepo- 

 sition, although it is not impossible that solids may act upon one another 

 to an important extent without the help of water. 



ALTERATION WITHOUT ADDITION OR SUBTRACTION OF MATERIAL. 



In the changes which occur under this case the material moves only 

 short distances. Such changes may be (a) a crystallization of an amor- 

 phous substance or (b) interior alteration of mineral particles. 



An instance of the crystallization of an amorphous substance is 

 furnished by the devitrification of glass. In this alteration the uniform 

 homogeneous solid glass changes into a heterogeneous crystalline solid, the 

 different mineral particles of which have differing compositions. This 

 involves segregation of the different elements in various proportions into 

 the different minerals. It is therefore clear that the materials have moved 

 very short distances. 



Interior alteration of mineral particles is effected by the . change of 

 one mineral into two or more minerals. This is illustrated by the change 

 of pyrope into enstatite, spinel, and quartz; the change of pyrope into 

 hypersthene, spinel, and quartz; the change of spoduraene into eucryptite 

 and albite; the change of almandite into hypersthene, spinel, and quartz; 

 and the change of titanite into perovskite and quartz. 



ALTERATION WITH ADDITION OR SUBTRACTION OF MATERIAL. 



The changes which take place with the addition or subtraction of 

 material may vary from those which involve the slightest addition or 

 subtraction to complete substitution. The added material may come from 

 afar or from the adjacent mineral particles. The subtracted material may 

 enter into an adjacent mineral particle or may be transported great 

 distances before entering into a new mineral. Reactions between adjacent 

 minerals may produce new minerals. Two or more minerals may unite to 

 produce a single mineral. For example, olivine and quartz may pass into 

 anthophyllite ; nephelite and halite into sodalite; albite and halite into 

 marialite. Or two or more minerals may unite to produce two or more 

 new minerals. For example, rutile and magnetite may pass into ilmenite 

 and hematite; diopside and magnetite into tremolite and calcite; sahlite, 

 siderite, and magnesite into actinolite and calcite; augite, siderite, and 



