ALMANDITE AND ITS SIGNIFICANCE 657 



COMPOSITION CHANGES DURING METAMORPHISM 



For the purpose of studying the metamorphism that has taken 

 place, it is advantageous to recalculate all the ultimate analyses 

 to molecular proportions. Molecular proportions are calculated 

 by dividing the percentage weight by the molecular weight and 

 multiplying by i ,000 to dispense with decimal points. The advan- 

 tage of molecular proportions over percentage weights is that in 

 chemical reactions, to form definite mineral compounds, a definite 

 number of molecules of one oxide combine with a definite number of 

 molecules of another. If the relative number of molecules of each 

 oxide present is known, it is an easy matter to see the changes that 

 have taken place. Although the results could not be recalculated 

 with accuracy to mineral percentages on account of the mineral com- 

 plexity, the method was deemed advisable in order to adhere more 

 closely to the chemical principles in distinguishing recrystallization 

 from replacement. Where simple minerals such as biotite and 

 garnet are discussed, this method can be applied with obvious 

 advantage. 



Since the changes in the rock are indicative of the conditioning 

 agencies, it is desirable to discuss them at some length before 

 proceeding to discuss the changes in the garnet. The analyses 

 of all the rocks recalculated to molecular proportions is given 

 below: 



Limestone Garnet Rock Biotite Gneiss Granite Gneiss 



SiOz 266 880 929 993 



AI2O3 I 239 i-8o 173 



FeA \ 18 15 12 



FeO J ^ 156 103 38 



CaO 832 21 62 45 



MgO 137 60 90 39 



NaaO o 47 34 62 



K2O o 16 12 92 



From the analyses it will be seen that the change is in the 

 direction of equilibrium with the granite. For the purpose of 

 calculating the additions and subtractions, some constituent of the 



