282 BULLETIN 131, UNITED STATES NATIONAL MUSEUM 



fractive index, /3 of 1.737. Sections of maximum extinction measure 

 Zac = 48°. These properties indicate an iron pyroxene near the 

 hedenbergite end of the series. 



OWYHEE COUNTY 



Hedenbergite occurs in contact-metamorphic deposits in limestone' 

 in the South Mountain district. It is the most abundant lime- 

 silicate mineral in the Golconda mine where it is associated with 

 ilvaite, calcite, sphalerite, chalcopyrite, etc. It ranges from a fine- 

 grained massive rock to coarse masses of fibrous blades as much as 

 3 cm. long by 1 cm. thick. Some specimens from the Golconda 

 dump are composed of finely fibrous silky-lustered rock of pale-green 

 to brown color containing coarse calcite patches in which are em- 

 bedded large crystals of ilvaite. The fine fibers tend to turn brown 

 and oxidize rapidly with the formation of limonite, upon weathering. 

 Examined under the microscope, this fine material is found to consist 

 wholly of pyroxene dusted and stained with limonite. When the 

 powdered mineral is treated with warm 1 : 1 hydrochloric acid the 

 limonite is dissolved leaving the pyroxene better suited for optical 

 examination. It is then pale-green, faintly colored, and nonpleo- 

 chroic. Optically it is biaxial positive with 2V medium large, r>v 

 perceptible. The maximum extinction is approximately Z Ac = 42°. 

 The refractive indices are, roughly, a = 1.715, /3 = 1.722, 7 = 1.738. 

 The coarser varieties are optically identical. A medium coarse 

 specimen of olive-green hedenbergite contains abundant chalcopyrite 

 and one of the coarsest seen includes some coarse granular dark- 

 colored sphalerite. The brownish olive-green color, fibrous ap- 

 pearance on cleavage surfaces, and especially the tendency to quickly- 

 weather brown upon exposure, readily identify this pyroxene. 



AUGITE 



ALUMINOUS PYROXENE 



Variable .silicate of lime, magnesia, fer- Monoclinic. 



rous and ferric iron and alumina. 

 CaO.Mg0.2Si0 2 with some CaO.FeO. 

 2Si0 2 and (Mg.Fe)0.(Al,Fe) 2 3 .Si0 2 . 



Although augite is widespread in occurrence in Idaho as a pyroxe- 

 nic constituent of igneous rocks, especially basalts, diabases, and 

 andesites, no locality furnishing specimens of interest to the collector 

 or mineralogist has yet been described in the State. In an area 

 containing such quantities of eruptive rocks some localities will, in 

 all probability, be discovered in which augite occurs in a basic igneous 

 rock in phenocrysts of such size and quality as to make interesting 

 mineralogical specimens. 



