THE RKI.ATIOX OF MINERALS TO Till. I.I.EMI.NTS 235 



lu>ite. it decomposes, forming muscovite. The formula of mu- 

 covite is that of ;m ort hosiliciite, mid when written in the form of a 

 substitution product in the m>rm:il ;iluniinium orthosilicate is as 

 follows: 



Muscovite Andalusite 



/H /Al = 



Si0 4 < H / Si0 4 / Al = O 



X K \A1 = 



Al -<: - SiO 4 =Al Al ~SiO 4 = Al 



SiO 4 =Al 



In the formation of muscovite from andalusite, hydration attacks 

 the Al = O group, forming gibbsite, A1(OH) 3 , which at the same 

 time may be carried off in solution. The necessary potassium for 

 substitution in the formula of andalusite to form muscovite is fur- 

 nished by the potassium carbonate dissolved in the ground water. 



2 (Al s (SiO 4 ) 3 (AlO) 3 ) + H H 2 O + K 2 CO 3 



= 2 (Al 3 (SiO 4 ) 3 H 2 K + 6 A1(OH) 3 + C0 2 . 



If the formula as written for muscovite is true, it would follow 

 that the formula as written for andalusite would be most probable 

 and it would be an orthosilicate. 



In the case of pyroxene and amphibole, from their analysis alone 

 both could be expressed by the formula, R"SiO 3 , as metasilicates, 

 in which R" represents practically the same group of isomorphous 

 elements. Their crystallographical constants and physical prop- 

 erties are, however, quite different, indicating entirely different 

 molecular structures. Pyroxene is more dense than amphibole, 

 which taken with the evidence of uralitization, a process in which 

 amphibole is formed from pyroxene, with but little chemical change, 

 would indicate that the molecule of pyroxene is more compact than 

 that of amphibole and uralitization is the breaking down of a com- 

 plex but compact molecule into less dense and simple molecules. 

 Pyroxene is therefore usually considered as a metasilicate, while 

 amphibole may be written as a mixed silicate composed of the 

 orthosilicic acid and the trisilicic acid, preserving the ratio of silicon 

 to oxygen as found in analyses. 



Pyroxene 



n(R"Si0 3 ) 



Mg 



Amphibole 



= tremolite. 



Ca 



