THE MINERALS OF IDAHO 221 



CUSTER COUNTY 



Calcite of the argentine variety is included in a number of speci- 

 mens of mordenite and associated minerals from south of Challis, 

 which were collected for the National Museum by Dr. Charles L. 

 Kirtley, and which have previously been described in detail. 30 The 

 largest specimen of this argentine has plates 16 cm. in diameter and, 

 while containing quartz and analcite, shows no mordenite. A portion 

 of this specimen with analcite crystals resting upon the argentine is 

 shown in Plate 12, right. Another large specimen, which is shown in 

 Plate 12, lower, is evidently a remnant from partial solution and con- 

 sists mainly of argentine, including analcite, quartz, and mordenite. 

 A third specimen, in which the platy character is not so evident, 

 consists of a rounded cast of the central portion of a large geode 

 and is coated on the outside with fibrous mordenite. When clean 

 rhombic cleavages from the interior of this mass are dissolved in acid, 

 long silky fibers of mordenite remain behind. The broadly platy 

 specimens are snow-white in color. The characteristic pearly luster 

 on the basal pinacoid, usually shown by argentine, is lacking, doubt- 

 less because the plates have a thin coating of reticulated quartz. 

 A specimen in the National Museum from the Republic mining dis- 

 trict, Washington, is identical with the Challis argentine in appear- 

 ance and like it has the calcite plates coated with thin reticulated 

 layers of quartz. The Washington argentine is associated with 

 laumontite. 



The large argentine specimens are clearly later in age than both the 

 associated mordenite and the spherulitic quartz, but the argentine 

 is older than the reticulated quartz and analcite which rest upon it. 

 In the specimen of mordenite shown in Plate 10, lower, however, the 

 calcite plates are embedded in the mordenite and thin sections show 

 that the argentine plates are earlier than both the mordenite and the 

 crust of small heulandite crystals which preceded the mordenite. 

 Elsewhere in the mordenite specimens there are flat gashes now lined 

 with analcite, which are apparently casts left by the removal by solu- 

 tion of early calcite plates. There are clearly two generations of the 

 argentine calcite in these specimens from this locality. The tabular 

 habit of the mineral may be due to the fact that the solutions from 

 which it crystallized were highly saturated with silica and the calcite 

 plates and their surfacing films of quartz are probably nearly con- 

 temporaneous, the silica presumably having deposited in colloidal 

 form. 



In the Yankee Fork district calcite occurs in the gangue of silver 

 \eins in volcanic Tertiary rocks. The calcite forms lamellar crystals, 



3» Clarence S. Ross and Earl V. Shannon. Mordenite and associated minerals from near Challis, Custer 

 County, Idaho. Proc. U. S. Nat. Mus., vol. 64, No. 19, 1924. 



