420 W. Lindgren — Orthoclase as a gangue mineral. 



simple crystals. Crusts with projecting crystals of clear quartz 

 and whitish feldspars with broken or curved faces often occur. 

 The second mode of occurrence is as small, more or less 

 perfect, almost transparent crystals l-3 min in diameter, coating 

 cracks and crevices in the altered granite adjoining the 

 vein. 



The third occurrence is as abundant small and perfect crys- 

 tals coating a lamellar quartz, itself a pseudomorph after some 

 other mineral, probably calcite. 



The vein is apparently richest in orthoclase where traversing 

 the granite. Much of it was also found where it cuts the 

 rhyolite, but none was observed in the basalt. The vein where 

 entering the basalt contracts and becomes very narrow. 



All these occurrences demonstrate beyond doubt the aqueous 

 origin of the mineral. The crystals have the habit of adularia, 

 though the only strongly developed faces are 110 and 101 (m 

 and x, Dana), producing a distinctly wedge-shaped appearance. 

 One of the larger crystals from the first described occurrence 

 on the main vein was analyzed by Dr. W. F. Hillebrand with 

 the following result : 



Si0 2 66-28 



A1 2 3 17-93 



K 2 15-12 



Na 2 -25 



Undetermined _. -42 



100-00 

 Specific gravity, 2-54. 



The analysis indicates typical adularia. No orthoclase has 

 thus far been found in the other veins near Silver City; many 

 of them, though, possess interesting structural and mineralogicai 

 characteristics which will be described in the full report of 

 the district. 



Orthoclase has been artificially produced by the wet way by 

 Friedel and Sarasin in 1881. In 1890, Ch. and Gr. Friedel 

 obtained orthoclase in small crystals by heating pulverized 

 muscovite with a solution of potassic silicate. It is easily seen 

 that the interesting experiment has a direct bearing on this 

 occurrence. 



The reason why orthoclase is not more frequently found on 

 mineral veins may possibly be found in the abundant and 

 usual presence of carbon dioxide in thermal waters. Under 

 such conditions the more stable compound — muscovite or seri- 

 cite — would be formed, orthoclase being rapidly attacked by 

 waters under pressure containing C0 2 . Orthoclase could then 

 only result if C0 2 were absent or present in only small quan- 

 tities. 



Stanford University, California, March, 1898. 



