SOMERS: MICROSCOPIC EXAMINATION OF CLAYS II9 



linite, but the presence of mica is less often commented on, and 

 yet, judging from the nature of the clays described in this report, 

 it seems to be quite abundant. 



The same might possibly be inferred from the chemical com- 

 position of many kaolins which show a small percentage of pot- 

 ash; for since feldspar seems to be very scarce, the former may 

 be regarded as belonging to mica. 



Vogt, for example, in 1906, concluded that china clay consisted 

 of kaolinite, muscovite, and quartz, although he based his con- 

 clusions on the chemical composition of the material." 



Later, Hickling,-^ after studying the china clay of Cornwall, 

 states that in the finest washed clays, kaolinite, mica, quartz, 

 and tourmaline are present, but that the first two make up 90 

 per cent of the mass. He adds, however, that the relative 

 amounts of kaolinite and muscovite are difficult to estimate. 



He differentiates the mica into two classes, viz, primary mica 

 and secondary mica derived from feldspar. 



He refers to the kaolinite as occurring as irregularly hexagonal 

 prisms, with rough faces, which show strong transverse stria- 

 tions corresponding to the basal cleavage. "These prisms are 

 usually curved, sometimes quite vermiculiform. The shorter 

 prisms commonly present a fanlike arrangement and exactly 

 resemble the similar forms of mica, from which they can be dis- 

 tinguished only by their lower interference tint." 



Isolated plates or very short prisms may occur, and then "even 

 with convergent polarized light, it is not easy to judge the amount 

 of birefringence under such circumstances, and, consequently, 

 to decide to which mineral a given fragment belongs; hence the 

 difficulty of estimating their relative proportions. Both kinds 

 show the same irregular form (due probably to development 

 within decaying feldspars) and the same evidence of corrosion 

 on the edges. The low interference-tint and low index of re- 

 fraction definitely distinguish these crystals from mica. 



- Vogt, G. De la composition des argiles. Memoires publics par le Societe 

 d'Encouragement pour I'lndustrie Nationale, Paris, 1906, pp. 193-218. 



^ HiCKLiNG, G. China clay. Its nature and origin. Trans. Inst. Min. Eng. 

 36. 1908-09. 



