MINERALOGICAL AND CHEMICAL CHARACTERS, 25 



the smooth even sheets appear to be slightly softer than those which 

 have been damaged by earth pressure, and a high degree of hardness 

 is generally looked upon with disfavour in the trade ; it should be just 

 possible to scratch the mica with the finger nail. The loss of hardness 

 following hydrous decomposition is accompanied by reduction in 

 strength, elasticity and consequent commercial value. 



Although their remarkable physical characters mark the micas as 

 a distinct and natural family amongst minerals, they vary so greatly in 

 chemical composition that considerable difficulty has been experienced 

 in discriminating the essential features in chemical constitution which 

 are peculiar to the family and common to all its members. The subject 

 has been elaborately studied by Tschermak (1878), Rammelsberg 

 (1878-1889) and F.W. Clarke (1886-1889), who have expressed different 

 views of the chemical constitution of the family. 



Tschermak x regarded the micas as isomorphous mixtures of the 

 following fundamental molecules— 



tf=Si 6 A1 6 K 2 H 4 24 

 J/=Si 6 Mg 12 24 

 5'=Si 10 H 8 24 



Rammelsberg 2 considered them to be mixtures in different 

 molecular ratios of the three silicates R 2 Si0 3 , R t Si0 4 and R & Si0 6 , 

 and divided them into an alkaline group and a ferromagnesian group. 



Clarke 3 regards the micas, and in fact all such aluminous orthosili- 

 cates, as substitution products of the compound Al 4 (Si0 4 ) 3 in which 

 the-Al-atom is successively replaced by an univalent (A 1 ) atom and 

 Mg or other divalent metal. If we take magnesium as a generic 

 representative of the bivalent metals, and give univalent elements or 



1 Tschermak," Die Glimmergruppe : II Theil, Chemiscre Zusammenset- 

 zung", Sitzber. Ahad. Wien, 1878, LXXVIII, 5-60. 



* Rammelsberg, " Ueber die chemische Natur der Glimmer," Sitzber. ATcad. 

 Berlin, 14th February 1899, 99—109. 



3 F. \V. Clarke, " A theory of the mica group". Amer. Joum. Sci., (3) 

 XXXVIII (1889), 38+— 393. See also same Joum., XL (1890), 410 and XLI 

 (1891), 242. 



( '5 ) 



