202 A TREATISE ON METAMORPHISM. 



SECTION 2.-GEKEEAL NATURE OF ALTERATIONS. 



Minerals may be altered (1) without chemical change and (2) with 

 change of chemical composition. 



ALTERATION WITHOUT CHANGE IN CHEMICAL COMPOSITION. 



The alterations which occur without changes in chemical composition 

 are (a) molecular rearrangement and (b) simple recrystallization. 



MOLECULAR REARRANGEMENT. 



Molecular rearrangement alone means passage from one crystalline 

 form to another crystalline form. Such change of form may result from 

 changed physical conditions, as, for instance, change in temperature or 

 pressure or movement. As an example of molecular rearrangement due 

 to change of temperature may be mentioned leucite, which crystallizes 

 from a hot magma in the regular system, but which changes upon cooling 

 to ordinary temperatures to a complex twinned anisometric form. An 

 example of a change due to pressure is furnished by orthoclase, which is 

 said for this reason to alter to microcline. a Molecular readjustments such 

 as above are simply changes of form, and are thereforo called paramorphism. 



SIMPLE RECRYSTALLIZATION. 



Simple recrystallization usually but probably not always occurs through 

 the medium of a certain amount of water, which is able to take material into 

 solution and deposit it from solution. Changing pressure and comparatively 

 high temperatures are favorable conditions for such recrystallization. Per- 

 haps the most common example of recrystallization without chemical 

 change is that of the transformation of amorphous or finely crystalline 

 calcium carbonate to crystalline or more coarsely crystalline calcium 

 carbonate, such as occurs in limestones and marble. This process has 

 been called marmorosis. Another instance of recrystallization without 

 change in chemical composition which takes place, on an extensive scale, is 

 alteration of flinty or finely crystalline quartz to coarsely crystalline 

 quartz. 



ALTERATION WITH CHANGE IN CHEMICAL COMPOSITION. 



Alterations with chemical change may take place (1) without the 

 addition or subtraction of material or (2) with the addition or subtraction 



" Dana, J. D., A system of mineralogy, Descriptive mineralogy by E. S. Dana, Wiley & Sons, New 

 Yors, 6th ed., 1892, p. 318. 



