216 LARSEN AND WHERRY: LEVERRIERITE FROM COLORADO 



pointed out above, takes place without a break in the crystal 

 structure, as shown by the optical properties. Muscovite and 

 other micas (to which leverrierite is probably related), show 

 almost as wide a range in the ratio between the Si0 2 and A1 2 3 , 

 as leverrierite ; they also show like the latter but small variations 

 in their optical properties. 



Leverrierite and some of the minerals here united with it have 

 usually been included in text books on mineralogy as varieties 

 of kaolinite. However, Termier 5 has shown that it is undoubtedly 

 a distinct mineral. He concluded that it is a muscovite in which 

 the K 2 is replaced by H 2 0, but since this conclusion is based 

 on an analysis of the mineral dried at 110° it seems hardly justi- 

 fied; yet the cleavage and optical properties are certainly near 

 those of the mica group, to which the leverrierite group is be- 

 lieved to be closely related. 



Chemically it differs from kaolinite, chiefly in the fact that 

 it retains only 7 per cent of its H 2 at 110° and very little at 350°, 

 while kaolinite retains nearly all of its 14 per cent of H 2 up to 

 400 degrees. Optically leverrierite has higher indices of re- 

 fraction, much stronger birefringence, and much smaller axial 

 angle than kaolinite, and it is commonly found in larger plates. 



The leverrierite group includes then, the micaceous hydrous 

 silicates of aluminium with small amounts of Fe 2 3 , RO, and 

 R 2 0, in which the ratio A1 2 3 : Si0 2 varies at least from 1.85 to 

 3.95; the H 2 content under normal conditions is from 15 to 24 

 per cent, of which all but about 7 per cent is given off below 110°. 

 In physical and optical properties leverrierite resembles musco- 

 vite, but its cleavage is less prominent, it is rather brittle when 

 dry and very plastic when wet, and its axial angle is commonly 

 very small. 



Summary. Leverrierite occurs in the veins of quartz and 

 manganese oxide at Beidell, Saguache County, Colorado, in 

 cleavage plates up to several inches across. It has a very per- 

 fect basal cleavage. It becomes plastic when wet. It is opti- 

 cally negative, practically uniaxial, and the optic axis emerges 

 sensibly normal to the cleavage. The indices of refraction as 



5 Termier, P. Bull. Soc. Min., 22: 27. 1S99. 



