228 GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA. 



are often intercrystallized, each mineral being perfectly fresh. That, however, 

 they depend upon slightly different conditions for their formation is indicated by 

 the fact that some profoundly altered specimens show the feldspar almost entirely 

 altered to adularia without muscovite, while others show complete alteration to 

 quartz and sericite without adularia. Adularia requires more silica than musco- 

 vite, but its formation in preference to the latter does not necessarily depend on 

 this fact, for when muscovite is formed in these rocks an amount of free quartz 

 is separated out equivalent to the quantity which would have gone into the adu- 

 laria, as is shown by the analyses of rocks 5, 6, and 7, of which 5 and 7 are altered 

 chiefly to quartz and sericite, and 6 chiefly to adularia. This difference is not 

 shown in any way by the bulk analysis of the rocks, the relation of the elements 

 harmonizing in the two cases. 



FORMATION AND OCCURRENCE OF ADULARIA. 



CONDITIONS REQUIRED FOB THE FORMATION OK ADl'LARIA. 



Adularia is a variety of orthoclase, which is a silicate of alum'num and potas- 

 sium. It is distinguished from ordinary orthoclase chemical^ by being nearly 

 pure," while ordinary orthoclase contains a variable and often large amount of 

 soda. Crystallographically adularia has usually an entirely different habit from 

 ordinary orthoclase, and this crystallographic difference is apparently controlled 

 by the difference in chemical composition. While ordinary orthoclase is one of 

 the commonest primary minerals in igneous rocks, especially in the more siliceous 

 varieties, the writer is not aware of adularia occurring in this way. On the other 

 hand it is known as a secondary mineral in metamorphosed rocks and in veins. 

 Still, experimental investigations do not seem to show any essential difference in 

 the conditions of formation. 



Orthoclase, muscovite, and quartz are all minerals which have not yet been 

 artitically reproduced by the cooling of dry melts, in spite of many careful attempts.* 

 All these may, however, be formed in the presence of such agents as water, chlorides, 

 fluorides, boron compounds, tungstic acid, etc., without which they apparently can 

 not crystallize. These agents, so potent in the formation of minerals, but entering 

 into their composition slightly or not at all, are called "mineralizers" (agents 

 mineralisateurs). 



Friedel and Sarasin heated a mixture of potassium carbonate, alumina, silica, 

 and water in a platinum-lined iron tube to about 500 C., for fourteen to thirty -eight 

 hours, and obtained tiny quartz crystals and rhomboidal tablets of feldspar. Similar 

 more abundant feldspar crystals were obtained by heating aluminum chloride, 



" It usually contains, however, a little soda, lime, etc. 



* Vogt, J. H. L., Mincralbildung in Silikatschmelzlosungen, p. 6. 



