83 



Si0 2 .. 

 A1 2 3 . 

 Fe203 

 FeO.. 

 CaO. 

 MgO 

 K 2 0.. 

 Na 2 

 CO2.. 

 H 2 0.. 



99-66 



II. 



58-32 

 23-80 

 1 -09 

 1-67 

 6-67 

 I.- 14 

 1-44 

 5-83 



III. 



Molecular Ratios 



•972 



•233 

 •007 

 •023 

 •119 

 •029 

 •015 

 • 094 



•972 

 •240 



■280 



Deducting the excess of alumina present as corundum, 

 which was determined by trial, neglecting the loss on 

 ignition (H 2 0) and deducting the amount of lime (CaO) 

 necessary to form calcite with the CO2 in the rock, the 

 results given under II are obtained. This is the compo- 

 sition of the residual or alumina-saturated magma. 



Morozewicz [9, p. I and 105] has shown by direct 

 experiment that in super-saturated alumino-silicate mag- 

 mas, whose general composition is RO, n AI2O3, Si0 2 (where 

 R = K 2 , Na 2 or Ca; and n = 2), the whole of the excess 

 of alumina separates out (1) as corundum if no consider- 

 able amount of MgO or FeO is present and if n is less 

 than 6; (2) as sillimanite and corundum if n is greater 

 than 6; (3) when the magma is rich in magnesia, as spinel 

 or spinel and corundum, if n is less than 6 ; (4) as cordierite 

 or cordierite with one or more of the other minerals if n 

 is greater than 6. The absence of corundum in the neph- 

 eline syenites of India is explained by Holland as due to 

 the fact that this rock, as shown by analysis, contains too 

 much MgO and FeO, and he refers to the abundance of 

 iron-magnesia minerals in the nepheline syenite, and the 

 scarcity of such minerals in the corundum syenite, as amply 

 accounting for the abundance of free alumina in the latter 

 and its absence in the former. A similar low content of 

 iron and magnesia is noticeable in the Canadian corundum 



35064— 6| 



