354 



BULLETIN 131, UNITED STATES NATIONAL MUSEUM 



the three pinacoids and the angles between these are 90° within the 

 limits of error of the measurements. These simple crystals are 

 typical " epidesmine," as illustrated in Figure 114. The broadest 

 face here taken as the pinacoid 6(010) exhibits a pearly luster and the 

 best cleavage is parallel to this plane. Crystals lying on this face 

 are homogeneous with parallel extinction. There is a less perfect 

 cleavage parallel to (100) but the frayed ends of the fragments, when 

 the crystals are powdered, shows that there is no basal cleavage. 



s\r 



115 



116 



Pigs. 114-116.— Stilbite. 114, "Epidesmine" habit from Seven Devils district, Adams County. 

 115, Ordinary habit drawn m orthorhombic position and lettered according to Gold 

 schmidt, Minnie Moore mine, Blaine County. 116, Minnie Moore mine, drawn in mono- 

 clinic position and lettered according to Dana 



Sections parallel to &(010) show the emergence of the optic normal 

 and all fragments lying on (010) and (100) show negative elongation, 

 so the optical orientation referred to the figure is X = c, Y = 5, Z = a. 

 Sections lying on (100) show the emergence of the obtuse bisectrix, 

 so the mineral is optically negative, 2V medium. The refractive 

 indices average a = 1.488, = 1.495, 7 = 1.497, y- a = 0.017, all 

 0.003 (variable). 



BLAINE COUNTY 



Stilbite occurs in the Minnie Moore mine near Bellevue in the Wood 

 River district, where it was identified by D. F. Hewett of the United 

 States Geological Survey. It was found at several places in the mine, 



