672 CROSS, IDDINGS, P IRS SON, WASHINGTON 



it is convenient to compare other components with K 2 0. The 

 micas included in Table XIV are of three kinds — biotite proper, 

 lepidomelane, and phlogopite. 



a) For biotite : 



A1 2 3 ranges from 1.3 to 1.9, and the ratio of Fe 2 3 to K 2 

 is 0.28 to 0.33, that is, nearly constant. 



The ratio of MgO + FeO to K 2 is 4.5 to 5.9 and magnesia 

 is in excess of ferrous iron. The ratio of MgO to FeO is 1.1 to 

 to 1.6. Average, 1.5. 



The Si0 2 nearly conforms to the theoretical molecule 

 (K, H) 2 0. (Al, Fe) 2 O s 2Si0 2 +n 2(Mg, Fe)O.Si0 2 . 



d) For lepidomelane : 



The ratio of A1 2 3 to K 2 is nearly the same as in biotite, 

 but that of Fe 2 3 to K 2 is higher in two cases. 



(Mg, Fe)0 is lower, and ferrous iron dominates over magnesia. 



c ) For phlogopite : 



The ratio of A1 2 3 to K 2 is nearly the same as for biotite 

 and lepidomelane, but that of Fe 2 3 to K 2 is lower. 



The ratio of MgO + FeO to K 2 is nearly the same as in 

 biotite, but magnesia greatly predominates over ferrous iron. 



The ratio of SiO g to K 2 0, although nearly the same as in 

 biotite, does not conform to the formula given for biotite. 



From these relations we may deduce the following method of 

 transferring aluminous molecules from salic and non-aluminous 

 molecules from femic to form molecules of augite, amphibole, 

 and mica. 



For the calculation of aluminous pyroxene : The kind of pyrox- 

 ene to be calculated should depend upon the kind of rock in 

 which it occurs. 



a) If the femic minerals already calculated for the rock 

 include acmite molecules, sufficient Na 2 and equal Fe 2 3 are 

 to be combined with diopside molecules to satisfy the ratio 



Na 2 Q 

 CaO 

 (D standing for datum, the value given in the pyroxene or 

 other mineral to be calculated); CaO in the ratio being equal to 



