LARSEN AND WHERRY! LEVERRIERITE FROM COLORADO 209 



leverrierite, and partly of cherty character in coatings and reni- 

 form masses attached to the manganese oxides. 



Physical properties. The mineral has many of the properties 

 of ordinary " clay-gouge," but it differs in possessing a prominent 

 micaceous cleavage, some of the cleavage pieces being several 

 inches across and commonly bent. When dry it is rather brittle 

 and has a hardness of about 1^. If large fragments are immersed 

 in water they gradually break up, chiefly along the cleavage, 

 into smaller pieces. After soaking for some time the mineral 

 becomes smooth and plastic, and rather sticky. Under the 

 microscope the fragments can be seen, however, to have retained 

 their optical properties. On kneading the plastic mass the 

 fragments are broken up, but still retain their properties. The 

 plasticity can not be accounted for on the basis of the presence 

 of any colloidal material, but seems rather to be due to the super- 

 position of innumerable minute, soft, flexible plates, separated 

 by films of water. The Colorado leverrierite has a vitreous 

 luster and waxy appearance, and varies in color from nearly 

 colorless to dark olive-buff (Ridgway 21'") or russet-vinaceous 

 (Ridgway 9'"). 



Optica properties. A microscopic examination of a number of 

 specimens showed them all to be essentially identical. The 

 pale russet-vinaceous specimen selected for analysis is in bent 

 plates up to an inch across, and appears to be homogeneous. 

 The acute bisectrix is normal to the plates, the axial angle is 

 sensibly 0, and the mineral is optically negative. The indices of 

 refraction were measured by the immersion method. Fragments 

 immersed in mixtures of clove oil and petroleum oil and quickly 

 compared with the oil gave apparent values of a = 1.470 ± 0.005, 

 /3 and y = 1.515 ± 0.005, but on standing in the oils the indices 

 of refraction slowly increased. After standing for two days in 

 mixtures of clove oil and cinnamon oil they reached a maximum 

 constant value and matched the liquids in which they were im- 

 bedded with values of <x = 1.558 ± 0.003, and 7 = 1-602 ± 

 0.003. On removing the fragments from the liquid that had 

 stood and placing them in a fresh liquid of the same index they 

 still matched. 



