%66 Dr Turner on Lithion-Mica. 



gives rise to the same phenomena as the Zinnwald-mica, but 

 the redness is less distinct. 



The analysis itself is so precisely similar to the preceding, 

 that it would be mere repetition to describe it. The result is 

 given below. 



Analysis of a Greyish White Mica from Cornwall. — The 

 laminae of this mica are small and brittle, and it is hence 

 easily pulverized. Its density is 2.814 when first put into 

 water, and 2.897 after being boiled. 



Its. loss from ignition did not exceed a quarter per cent. It 

 melts readily before the blow- pipe, and bears a close analogy, 

 with respect to the phenomena it then exhibits, with the Zinn- 

 wald mica, as well in its fusibility and power of reddening the 

 flame, as in forming a black scoria indicative of iron. 



The following Table contains the result of the analysis of 

 the three preceding micas. The result of a new analysis of 

 the brown Cornwall-mica is likewise added. 



Zinnwald. Grey Cornw. Altenb. Brown Cornw. 



Silica, 44.28 50.82 40.19 40.06 



Alumina, 24.53 21.33 22.72 22.90 



Protoxide of iron, 1 L.33 9.08 black oxide. 19.78 peroxide. 27.06 



Protox. of manganese, 1.66 a trace. 2.02 1.79 



Fluoric acid, 5.14 4.81 3.99 2.71 



Potash, 9.47 9.86 7.49 4.30 



Lithia, 4.09 4.05 3.06 2.00 



100.50 99.95 99.25 100.82 



The state of oxidation of the iron in these micas is estimated 

 by their colour. 



I have been unable, in any of these varieties, to detect the 

 presence of titanium ; nor do lime and magnesia enter into 

 their composition. The essential ingredients, if we may judge 

 from the constancy with which they occur, are silica, alumina, 

 oxide of ii'on, fluoric acid, and the two alkalies. It is exceed- 

 ingly curious, as was remarked on a former occasion, that all 

 these micas are found in tin districts; and, if justified by fu- 

 ture observation, the occurrence of lithion-mica may even, 

 perhaps, assist the judgment of the practical miner in his 

 search after veins of tin. 



