CLA SSIFICA TION OF IGNEO US RO CKS 6 7 I 



will be necessary to consider special kinds of amphibole in dif- 

 ferent cases. 



a). 1. For hornblende occurring in rocks with lime-soda- feld- 

 spar the ratio of Na 3 to CaO ranges from 0.08 too.21, while 

 for hornblende in one alkali feldspathic rock it is 0.28, and 

 for somewhat similar amphiboles, barkevikite and hastingsite, 

 it reaches 0.35 and 0.43. 



2. The ratio of Al 2 O s to CaO ranges from 0.32 to 0.67 in the 

 first mentioned cases, common hornblende, and from 0.44 to 0.85 

 in the second. 



3. The ratio of MgO +FeO to CaO ranges from 1.73 to 2.72, 

 being nearly 2.2 in most cases. It is not 3, as given in the text- 

 books generally. 



4. It is to be noted that in hornblende in the more calcic rocks 

 the ratio of MgO to FeO is 2 or more (from 2 to 4), while in 

 those in the perfelic, alkalic rocks, this ratio is less than 1 (from 

 0.1 to 0.6). In other words, magnesia dominates ferrous iron in 

 amphiboles of the first kind, while ferrous iron largely prepon- 

 derates in those of the second kind, 



5. The Si0 2 + Ti0 3 is equal to the MgO +FeO + CaO plus 

 an amount sometimes equal to 4Na g O, but not always. 



6. The ratio of Fe 3 3 to CaO is quite variable. 



b) For soda-amphiboles of various kinds, excepting that occur- 

 ring in comendite. 



1. The ratio of Na 2 to CaO ranges from 1.23 to 4.6. That 

 is, the lime is considerably less than the soda. 



2. The ratio of A1 2 3 to CaO ranges from 0.23 to 1.3. 



3. The ratio of MgO + FeO to CaO ranges from 4.6 to 21.6, 

 and the FeO is greatly in excess of MgO. 



4. The variation in the Si0 2 indicates that Na 2 is partly 

 present in the riebeckite molecule, and partly replaces CaO and 

 (Mg, Fe)0 in the RO . Si0 3 molecule. 



Micas [Table XIV). — For these, as already said, the chemical 

 component associated with A1 2 3 which may be transferred from 

 the feldspathic minerals is K 2 0, and for purposes of calculation 



