324 



Rankin and Merwin — 



water may be present in certain unaltered cordierites, 10 

 others may be nearly or quite anhydrous. 11 After the 

 expulsion of water the residues of all the cordierites 

 analyzed approach very closely the formula 2(MgO. 

 Fe()).2A!.,( )..,")Si(). A hydrous iron-hearing cordierite 

 alter heating 30 minutes at 1440° consisted of glass and 



Fig. 8. 



sillimanite. Dehydration for 5 minutes at 600° caused a 

 decrease of -010 in the average refractive index of the 

 same material. Probably the escape of water was not 

 accompanied by T corresponding shrinkage, and the liquid 

 used to immerse the grains for the refractive index 



10 See ' ' Untersuchungen an Cordierite. ' ' Leopold Oppenheimer, Vehr. 

 Heidelb. Nat. Mod. Ver., N. F. 13, 257-302, 1914. 



11 H. Bucking, Neues Jahrb. Min. Geol., 1901, I, 72 (ref.). 



