272 



BULLETIN 131, UNITED STATES NATIONAL MUSEUM 



mine small transparent crystals are found coating a lamellar quartz 

 probably pseudomorphous after argentine clacite. All of these occur- 

 rences demonstrate the aqueous origin of the mineral. Prof. L. V. 

 Pirsson examined the crystals and found them to have the habit of 

 adularia with the forms (110), (001), and (T01) as in crystals from 

 St. Gothard in Switzerland. In the Idaho crystals, however, he 

 found the base (001) usually very small and often wanting, the crys- 

 tals then bounded only by (110) and (T01) and having a strikingly 

 rhombohedral aspect. This rhombohedral appearance is shown by 

 the photographs of Plate 6 and the unwary mineralogist often 

 pronounces the mineral calcite at a glance and specimens are found in 

 collections labeled quartz pseudomorphous after dolomite, etc. The 



72 73 



Figs. 72-73.-72, Orthoclase (adularia). Crystal showing T (110) and x (101) simulating a rhom 



BOHEDRON. SILVER ClTY DISTRICT, OWYHEE COUNTY. 73, SAME. PENETRATION TWIN ON (100) 



faces of the crystals examined by Pirsson were too uneven and striated 

 to afford good material for measurement and the results served only 

 to indentify the forms. The face (T01) was strongly striated by 

 oscillatory combination with the base. One of the larger crystals 

 from Lindgrens illustrated specimen (pi. 6, lower) was analyzed by 

 W. F. Hillebrand. The results of this analysis are below compared 

 with the theoretical composition of orthoclase. 



Analysis of valencianite, Silver City 

 (W. F. Hillebrand, analyst) 



