LARSEN AND WHERRY: LEVERRIERITE FROM COLORADO 215 



within moderate limits. This similarity, together with the con- 

 tinuous variation in chemical composition, shows with reasonable 

 certainty that all the minerals are members of a continuous 



TABLE 4 

 Properties of Leverrierite and Related Minerals 



Batchelorite 

 Kryptotile" 

 Leverrierite 6 

 France 



Rectorite c , 

 Arkansas 



Material from 

 Colorado 



Delanouite d 



RATIO 



AI2O3: 



SiOu 



1: 1.86 

 1: 1.94 

 1:2.11 



1:2.34 



1 : 2.76 



1:3.95 



OPTICAL 

 ORIENTATION 



X J. cleav. 

 X j_ cleav. 



X X cleav. 



X j_ cleav. 



X _|_ plates 



0-45° 

 0-50° 



38° = 



Small 



1.558 

 1.554 



1.550 



1.558 



1.54 







1.593 



1.582 



1.588 



1.602 



1.570 



1.594 

 1.582 



1.590 



1.602 



1.570 



REMARKS 



Foliated 



Fibrous, platy 



Vermicular, mica- 

 ceous 

 'White, dull, 



tough. Resem- 

 bles mountain 

 leather. Inter- 

 woven fibers 

 and plates. 

 Rather plastic 

 when wet 



\ Luster vitreous 

 to waxy, rather 



\ brittle. Platy. 

 Very plastic 

 when wet 



Not entirely homo- 

 geneous. Feath- 

 ery plates 



a New data on specimen from Waldheim, Saxony, kindly furnished by Colonel 

 Roebling. The material closely resembles sericite. 



6 Published data of Wallerant. Other data gives the birefringence as low as 

 0.008. 



c New data from U. S. Nat. Mus. specimen No. 80607. The indices vary. 

 When first measured a = 1.500, = 1.541, y = 1.543. The indices appear to 

 increase on standing in oils. (Cf. the Colorado mineral.) 



d New data on "montmorillonite, Millac." Specimen kindly furnished by 

 Prof. A. Lacroix. A red, clay-like mineral. Under the microscope it is seen to 

 be made up of rather coarse, feathery plates or fibers. The best interference 

 figures show that the mineral is optically negative and has a small axial angle. 

 Some fibers are optically positive and have a large axial angle. 



series with varying proportions of Si0 2 and A1 2 3 . This would 

 not require any greater variation in the chemical composition 

 than that due to the change in the water content, which, as was 



