THE MINERALS OF IDAHO 387 



obvious that this mineral is of very common occurrence and Dr. E. S. 

 Larsen has found, by optical examination, that the clays which have 

 been called bentonite, ardmorite, etc., and which are commonly con- 

 sidered to be altered volcanic dust, are related in their properties 

 but differ somewhat chemically, agreeing with montmorillonite. 



The leverrierite and montmorillonite are completely crystalline, 

 under the microscope, and are biaxial and negative. They have the 

 micaceous platy structure also possessed by kaolinite and sericite. 

 Their optical properties are sufficiently distinctive to permit them to 

 be determined microscopically. Chemically these minerals are char- 

 acterized by a large water content, a very considerable proportion 

 of which is released below 110° C, as contrasted with sericite and 

 kaolinite, the water of which is not given off much below a red heat. 

 When the mineral powders are dehydrated at 110° C. and allowed 

 to stand in damp air they regain the water which has been lost. 



When moist the leverrieritic clays are very plastic. Many of them, 

 especially the varieties of montmorillonite called bentonite, absorb 

 great quantities of water and expand remarkably into a jellylike 

 mass. 



One occurrence of leverrierite and one of montmorillonite have been 

 noted in Idaho, the latter a bentonite and the other a gouge clay. 



MONTMORILLONITE (bentonite) 



BOISE COUNTY 



A lot of clay having the appearance and general properties of ben- 

 tonite was sent to the National Museum for identification by Edward 

 Schwerd, of Boise. A letter directed to Mr. Schwerd asking for 

 details as to the source and occurrence of this material failed to elicit 

 a reply, but it is presumed that the locality is somewhere in the 

 vicinity of Boise. 



In the hand specimen this material, dried in the air, appears as a 

 fine-grained compact and homogenous clay of a yellowish to grayish- 

 white color. It is traversed by numerous contraction cracks and has 

 a tendency to conchoidal fracture. It is lusterless until rubbed with 

 a hard object, when it assumes a soap-like polish. The hardness is 

 less than that of gypsum. Fracture surfaces bear small manganese 

 oxide dendrites. When placed in water the material swells very 

 strikingly to a sticky glutinous mass. When stirred with water it 

 settles with moderate rapidity, leaving the water clear. It is almost 

 free from grit. When examined optically the leverrierite of this 

 sample is found to be very fine-grained and micaceous, with moder- 

 ately high birefringence. It is found to contain a considerable amount 

 of very fine-grained calcite as impurity. The apparent index of re- 

 fraction is about 1.580, and the material is brown and filled with air 

 inclusions. A sample treated with hydrochloric acid to remove cal- 



