676 CX OSS, ID DINGS, PIRSSON, WASHINGION 



The ratio 



(Mg,Fe)0 _ D 



CaO d ± CaO 



must be established as well as that of 



MgO = 

 FeO 



since the latter is highly characteristic. 



The adjustment of Si0 3 follows the ratio 



Si0 o 



CaO H ± CaO, 



= D 



and the requirements of the other molecules. 



For the calculation of ferromagnesian mica. — The process is 

 essentially the same for various kinds, the values of the ratios 

 varying for biotite, lepidomelane, and phlogopite. 



In most cases Al 3 O s is in excess of K 2 0, and in transferring 

 K 3 from feldspathic molecules the accompanying equal A1 2 3 

 is not sufficient to satisfy the ratio 



A1 2 Q 3 = D 



K 2 - * 



Extra A1 2 3 may be derived from corundum when estimated 

 present, and this will be the case in numerous rocks in which 

 muscovite and biotite have developed. 



If there is no extra Al 2 O s of this kind, A1 2 3 must be 

 transferred from anorthite until the ratio in question is satisfied. 

 The liberated CaO must be transferred to diopside molecules. 



Fe 2 3 is to be transferred from Fe 3 3 (hematite) or FeO. 

 Fe 2 3 (magnetite), and the liberated FeO is to be added to 

 (Mg.Fe)O, to satisfy the ratio 



Fe 2°3 = r> 

 K 2 



(MgFe)O must be transferred from hypersthene or olivine 

 to satisfy the ratio 



(Mg,Fe)0 



K 2 



= D, 



