FORMATION OF MUSCOVITE. 231 



rarely absent in waters) breaks up the potash silicate in the adularia and removes the 

 new-formed soda silicate with the separated alumina silicate, while the sodium 

 silicate contained in the adularia, with the combined alumina silicate, remains as 

 albite. 



" On the other hand, potassium carbonate decomposes sodium silicate. It is 

 therefore possible that water containing potassium carbonate may either transform 

 soda feldspar into potash feldspar or that the alteration products of the former 

 may be taken up and removed. Such water brings about the opposite of that in 

 the first case.'" a 



This explanation corresponds with the conclusion as to the excess of potash 

 in the mineralizing waters, derived from a comparison of the rock analyses. 



FORMATION AND OCCURRENCE OF MUSCOVITE. 



CONDITIONS REQUIRED FOR THE FORMATION OF MUSCOVITE. 



Muscovite, as previously noted, has never been formed artificially by cooling 

 from dry fusion. Concerning its formation, as well as that of other micas, Doelter 

 observes:* 



"Mica results from heating aluminum silicate with potassium fluoride or mag- 

 nesium fluoride; the fluorides seem to assist on the one hand because the fluoric 

 vapors which form bring about the crystallization, and so play the same part as in 

 the transformation of amorphous alumina into corundum; on the other hand the 

 influence is also chemical, since small quantities of fluorine enter into the composition 

 of the mica." 



Brauns remarks : c 



"Any mica can be easily formed if one melts any mineral containing its 

 elements with any fluoride at a temperature below 800 C. ; for in higher temper- 

 atures the micas are not stable." 



Of the micas, biotite or magnesia mica is found in many volcanic rocks, such 

 as rhyolites, dacites, and andesites, while muscovite is not; neither does muscovite 

 occur in the deeper seated igneous rocks save in granites, where it is common, rf 

 and generally occurs together with quartz and potash feldspar/ Evidently, then, 

 muscovite demands for its formation special conditions not present in lavas or 

 in ordinary rock magmas and different from those necessary for biotite. 



MUSCOVITE AS AN ALTERATION PRODUCT. 



Muscovite is common as a secondary mineral the alteration product of many 

 other minerals, such as feldspar, nepheline, leucite, etc. and in these cases is 

 evidently the result of the action of waters, probably heated. It is very abundant 



"The italics are the writer's (J. E. S.). 



& Doelter, C., Allgemeine Chemische Mineralogie, p. 161. 



o Brauns, R., Chemische Mineralogie, p. 247. 



dRosenbusch-Iddings, Microscopical Physiography of the Rock-making Minerals, p. 2W. 



r Brauns, op. cit., p. 301. 



