THE MINERALS OF IDAHO 361 



cite. The analcite occurs as transparent colorless crystals up to 2 

 mm. in diameter coating a fracture in a fine granitic rock. 



CUSTER COUNTY 



Analcite is common in association with the mordenite, described 

 in detail above, from the locality south of Challis, as noted in a 

 preceding paper. The analcite occurs as single crystals and groups 

 of aggregated crystals embedded near the centers of the larger 

 masses of cottony mordenite and as crusts and druses of crystals 

 lining cracks in the adjacent rock or filling gashes in the mordenite 

 which probably have resulted from solution of calcite, as shown in 

 the upper right picture of Plate 10. Individual crystals of the anal- 

 cite reach an extreme diameter of 14 mm. Where free from inclu- 

 sions the analcite is clear and colorless, but the crystals commonly 

 inclose fibers of mordenite and grains of quartz giving them a milky 



120 121 



Figs. 120-121.— Analcite, Challis, Custer County. 120, Simple trapezohedron. 121, Trape- 



zohedron modified by cube and trisoctahedron 



appearance. They are for the most part simple trapezohedrons of 

 form q (112), which is a common form for analcite as shown in Figure 

 120. Occasionally, however, one shows faces of the cube a(100), and 

 of the trigonal trisoctahedron w(233), which though previously 

 known are rare forms on analcite. A modified crystal is illustrated 

 in Figure 121. * 



Certain very interesting crystals occur between the plates of the 

 associated argentine calcite. These are attached to the calcite 

 plates by the equatorial plane and, where space was available, have 

 developed completely in their top half. Where the space between 

 the calcite plates was thin the analcites are very mucli flattened and 

 are bounded only on their edges by narrow crystal planes. Both 

 undeformed and greatly flattened crystals are shown attached to 

 calcite in the photograph at the right of Plate 12. These disklike 

 plates may reach 1 cm. in diameter with a thickness of only 0.2 mm. 

 although most of them are thicker than this, with a smaller diameter. 

 Selected ones of these are ideal thin sections, and show admirably 

 the birefringence and division into sectors. Such a crystal photo- 



