ORTHOCLASE AND MICROCLINE. 253 



M0N0CL1NIC OR TSEUDOMONOCLIMC. 



ORTHOCLASE, MICROCLINE, AND AXOKTHOOLASE. 



Orthoclase: 



KAlSi 3 8 . 



Monoclinic. 



Sp. gr. 2.57. 

 Microcline: 



KAlSi 3 O s . 



Triclinic. 



Sp. gr. 2.54-2.57. 

 Anorlhoclase : 



mNaALSijOs-nKAUSiaOa. (Na-silicate:K-silicate:: 2:1 or 3:1, usually.) 



Pseudomonoclinic or triclinic. 



Sp. gr. 2.57-2.60. 



ORTHOCLASE AND MICROCLINE. 



occurrence. — Orthoclase and microcline have a very widespread occur- 

 rence as chief pyrogenic constituents. The minerals also are allogenic 

 constituents of the clastic rocks. They further have a very widespread 

 occurrence in the metamorphic rocks, being chief constituents both as 

 allogenic and as autogenic constituents of the schists and gneisses of both 

 aqueous and igneous origin. In the development of the feldspars as 

 autogenic constituents it is usually necessary that two or more minerals 

 unite, except in the case of the derivation of the acid feldspars from the 

 more basic ones or from leucite. As a metamorphic mineral orthoclase is 

 derived from analcite, heulandite, leucite, laumontite, and stilbite. Micro- 

 cline is recorded as derived from spodumene. 



Alterations. — -Que of the most important alterations of orthoclase and 

 microcline is to kaolinite (monoclinic; sp. gr. 2.60-2.63). The most prob- 

 able reaction, for reasons given below, is believed to be: 



(1) 2KAlSi 3 8 +2H 2 0+CO,=H 4 Al,SiA+4Si0 2 +K 2 C0 3 +k. 



The decrease in volume, supposing the freed silica to separate as quartz, 

 and K 2 C0 3 dissolved, is 12.57 per cent. If all of the freed silica be dis- 

 solved, the decrease in volume would be 54 44 per cent. In calculating 

 these volume changes and those which follow, the specific gravity of 

 orthoclase is used. 



While the ordinary alteration of the potash feldspars to the kaolin 

 group is to kaolinite as indicated, the alteration may be to other minerals 

 of this group; for instance, to pyrophyllite (monoclinic (?); sp. gr. 2.8-2.9), 

 halloysite (massive; sp. gr. 2.1), newtonite (rhombohedral ; sp. gr. 2.37), 



