350 



BULLETIN 131, UNITED STATES NATIONAL MUSEUM 



ADAMS COUNTY 



Associated with chabazite and stilbite on a specimen of massive 

 brown garnet rock from the Blue Jacket claim in the Seven Devils 

 district are a few minute crystals of a zeolite which is probably 

 heulandite. This mineral under the microscope shows an apparently 

 six sided form and is strongly zoned in layers parallel to its outline 

 and also shows some division into irregular sectors. The appearance 

 of one of the crystals between crossed nicols is indicated in Figure 113. 

 All sections lie on a perfect cleavage which is perpendicular to the 

 acute bisectrix and such sections show a good interference figure which 

 is biaxial positive with 2V medium. The /3 index of refraction aver- 



109 110 



Figs. 109-112.— Heulandite. 



Ill 112 



From Challis, Custer County 



ges about 1.50, although different zones vary considerably. The 

 angles between the edges marked 1, 2, and 3, in Figure 113 were 

 measured under the microscope and gave: 1A 2 = 65° which corre- 

 sponds to c(001) A 2(101) =63° 40' and 2 A 3 = 49° which cor- 

 responds to lS0°-t A s(129° 40') =50° 20', so that 1 is probably 

 c(001), 2 is 2(101), and 3 is s(101). This makes the trace of the 

 axial plane parallel to c(001) as contrasted with the heulandite from 

 Custer County described below in which the optic plane is approxi- 

 mately parallel to one of the domes, either s or t. 



CUSTER COUNTY 



The occurrence and properties of heulandite associated with 

 mordenite and other minerals near Challis have been fully described 



