ALMANDITE AND ITS SIGNIFICANCE 



66s 



The change in composition shows quite clearly the effects of 

 magnesia bearing alkaline solutions. Apparently these found 

 easy channels along the fracture planes of the garnet where altera- 

 tion is most intense. 



If the garnet is corrected for the amount of quartz, which 

 appears to be high in the molecular proportions but is really very 

 small, the molecular composition would be : 



Garnet 



SiOa 603 



AI2O3 201 . 5 



Fe^Oj 



FeO 419 



CaO 42 



MgO 142 



Na^O 



K.O 



H2OI 

 F 



Biotite 

 613 



2I9-S 

 14.8 

 19.6 



48.7 

 228.5 



50-3 



39-3 



292.0 



Difference 



ro* 

 18* 



38s 



5-7* 

 "' 5 

 3 

 3 



50 



39 



292 



* These differences are so small that they might readQy arise through errors in sampling and in analysis 

 as well as other variations which cannot be guarded against. For these reasons they cannot be regarded as 

 a part of the changes during the reaction. 



The F2O3 and FeO of the biotite are grouped together, the only 

 significance of the ferric iron being that it indicates a very slight 

 oxidation which, although showing in the mineral, does not occur 

 so prominently in the rock. The outstanding differences then are 

 the replacement of the ferrous iron by MgO, Na20, K,0, H2O, and F. 

 Although replacement has increased the number of molecules by 

 almost exactly one-third, which is a remarkable feature, it does not 

 seem that the reaction could be due to simple reorganization. All 

 of the variables are'divalent and with the exception of hydroxyl 

 and fluorine are chemically positive elements. The definite 2-3 

 ratio between the positive elements, alkalies and magnesia, and the 

 negative elements, hydroxyl and fluorine suggests that a rather 

 definite hydrated chemical compound had replaced the FeO during 

 this reaction. If such is the case, the compound was one of the 

 solutes in the highly heated emanations from the granite magma. 

 Whether the fluorine and water existed separate at the existing 

 temperature and pressure is unknown, but the oxidation of the 



