THE MINERALS OF IDAHO 353 



soda and potash are in excess, both molecularly and by weight, of 

 the lime. 



A form of heulandite of somewhat different properties occurs 

 rarely in the amygdaloid rock as small solid pebble-like bodies coated 

 on the outside with deep green chlorite and, where fractured, show- 

 ing coarse granular structure, well-defined cleavage, pearly luster, 

 and a faintly pink color. A similar material occurs as drusy crusts 

 of brilliant crystals of faintly pink color lining small flat open cavities 

 in the rock adjacent to the large masses of mordenite. These 

 crystals, when examined optically, are seen, like the preceding, to 

 be made up of sectors not identical in extinction angle, the angle 

 between adjacent sectors varying 10° or more. Moreover they are 

 strongly zoned, the outer zone having the following optical properties: 

 Biaxial, positive ( + ), 2V = 68°, a = 1.494, 0=1.496, 7 = 1-501, 

 7 — a = 0.006, while the properties of the cores are: Biaxial positive 

 ( + ), 2V = 90°, a = 1.483, 0=1.487, 7 = 1.490, 7~a = 0.007. Crystal- 

 lographically these are, like the others, in fair agreement with the 

 heulandite angles, the forms identified being 6(010), w(110), c(001), 

 e(101), s(T01), ic(011), and w(032). The last is a rare form previ- 

 ously noted only on heulandite from Scotland. In habit these pale 

 crystals are much stouter than the others, as shown in Figures 111 and 

 112, and they are grown together in parallel groups over considerable 

 areas, the plane of contact being 6(010). 



STILBITE (443) 



Hydrous sodium-calcium aluminium sil- Orthorhombic. 



icate, (Na 2) Ca)O.Al 2 3 .6Si02.6H 2 0. 



Stilbite has been noted in specimens from Adams, Blaine, Custer, 

 and Kootenai Counties. In two of the localities the mineral has the 

 habit of epidesmine, but the essential identity of epidesmine and 

 stilbite has been suggested by the writer in a previous paper. 72 

 Dana lists stilbite as monoclinic because of the internal twinned 

 structure which many of the crystals show in polarized light, but 

 Goldschmidt regards the mineral as orthorhombic and, since the 

 crystals always show orthorhombic symmetry, the crystallization is 

 considered orthorhombic below. 



ADAMS COUNTY 



Stilbite occurs with heulandite and chabazite on a specimen of 

 massive garnet from the Blue Jacket claim in the Seven Devils 

 district, The stilbite forms rectangular prisms and slightly di- 

 vergent groups of prisms up to 2 mm. long between the chabazite 

 crystals resting on the brown garnet. The crystals are bounded by 



n Earl V. Shannon. Mineralogy and Petrography of Intrusive Triassic Diabase at Goose Creek, Lou- 

 doun County, Va. Proc. U. S. Nat. Museum vol. 66, art. 2, pp. I-SG, pl<. 1-9, 1<J24. 



54347— 26t 24 



