278 BULLETIN 131, UNITED STATES NATIONAL MUSEUM 



East Fork of Wood River. The diopside is coarse granular and 

 varies from white to blue-gray in color. 



Near the large granitic instrusion west of Hailey the limestones 

 contain abundant white pyroxene, probably malacolite, with fibrous 

 wollastonite. 89 



CUSTER COUNTY 



In the Alder Creek district pyroxene, next after garnet, is the most 

 abundant product of the metamorphism in the vicinity of the copper 

 deposits. Diopside is the predominant constituent of the pyroxene 

 rock formed from the limestone by metamorphism. During the 

 earliest stages of the endomorphic alteration of the granite-porphyry 

 the primary hornblende and biotite were altered to diopside. Finely 

 disseminated diopside is scattered through much of the garnet rock 

 and diopside. together with augite and hedenbergite, comprises 

 bunches of pyroxene rock free from garnet. Such rock is dense, 

 greenish, of aphanitic texture and has resinous to vitreous luster. 

 In some thin sections of the massive pyroxene there occur a little 

 plagioclase feldspar (Ab 65 An 35 ) and chance grains of titanite, apatite, 

 and fluorite. A specimen of typical diopside rock from near the 

 second lateral, No. 3 crosscut, Alberta tunnel, analyzed by Chase 

 Palmer, of the United States Geological Survey, gave the following 



results: 90 



Analysis of diopside rock from Mackay 



(Chase Palmer, analyst) 



Per cent 



Silica (Si0 2 ) 51. 55 



Alumina (A1 2 3 ) 4. 00 



Ferric iron (Fe 2 3 ) I. 02 



Ferrous iron (FeO) 6. 65 



Magnesia (MgO) '_ 11. 38 



Lime (CaO) 24. 23 



Soda (Na 2 0) . 38 



Potash (K 2 0) . 18 



Water (H 2 0) below 110° C . 14 



Water (H 2 0) above 110° C . 25 



Titanium oxide (Ti0 2 ) . 32 



Carbon dioxide (C0 2 ) . 00 



Phosphoric acid (P 2 5 ) . 24 



Manganous oxide (MnO) . 30 



Total 100. 64 



The specific gravity of the analyzed material was 3.304. Assuming 

 that the analysis represents essentially a single mineral, the first six 

 constituents may be given, as the essential oxides, together with 

 their ratios, as follows: 



» 8 Waldemar Lindgren. U. S. Geol. Survey, 20th Ann. Rept., pt. 3, p. 195, 1900. 

 " J. B. Umpleby. U. S. Geol. Survey, Prof Paper 97, pp. 52, 58, etc., 1917. 



