408 MINERALOGY 



Adularia is a transparent orthoclase ; the most noted locality is 

 at St. Gothard in Switzerland. The name is derived from Mount 

 Adula of that region. It occurs in crystals of an apparent rhom- 

 bohedral habit, being a combination of the prism m and the base, 



FIG. 458. Adularia from St. Gothard, Switzerland. 



nearly equally developed. The base may be distinguished by the 

 horizontal striations. 



Beautiful specimens coated with chlorite and associated with 

 epidote occur in the Sultzbachthal, Tyrol. Adularia when cut 

 and polished en cabochon is the moonstone of the jewelers. 



Feldsite is a compact flint or jasper-like rock, showing no cleav- 

 age, of which the varieties rhyolite, trachyte, and phonolite have 

 a chemical composition of an alkalic feldspar. Perthite denotes a 

 structure made up of interlaminated potassium and sodium feld- 

 spars ; when the laminae are exceedingly thin it forms microper- 

 thite. 



Potassium and sodium feldspars intercrystallize, forming alkalic 

 feldspars, while sodium and calcium feldspars intermix, forming 

 plagioclases, or calcic feldspars ; although the potassium and cal- 

 cium spars are isomorphous, in nature they are seldom found in- 

 tercrystallized. 



Sanidine is a clear, glassy, granular orthoclase, usually associated 

 with lavas and the later eruptive rocks. It is found in the lavas 

 of Monte Somma, Vesuvius, in clear, colorless grains. 



