308 



BULLETIN 131, UNITED STATES NATIONAL MUSEUM 



BINGHAM COUNTY 



Forsterite occurs as a constituent of nepheline basalt in Bingham 

 County, 24 miles east of Blackfoot, which has been described in 

 detail. 30 The forsterite is in clear grains of the usual form up to 2 

 mm. in diameter. The optical properties are biaxial negative, 

 2V (computed from the refractive indices) 87°; refractive indices 

 a = 1.641, 0=1.661, 7 = 1.680. In convergent light the bars of the 

 interference figure are nearly straight and the dispersion of the 

 optic axes is barely perceptible with r>v. In addition to embay- 

 ments filled with the groundmass the forsterite 

 carries a few inclusions of iron ore and chains of 

 gas or liquid inclusions. The mineral shows no 

 signs of alteration except for a narrow reddish 

 border which probably represents incipient alteration 

 to iddingsite. A Rosiwal determination on two thin 

 sections showed 24.9 per cent by volume or 26.4 

 per cent by weight of olivine. The nepheline basalt 

 carrying the forsterite is a rare rock. The rock 

 forms a little knoll on a ridge that descends north- 

 east along the north side of Wood Creek in the SW. 

 }4 of the SE. % sec. 18, T. 3 S., R. 38 E. Boise meri- 

 dian. 



LEMHI COUNTY 



Forsterite occurs as a contact-metamorphic min- 

 eral in two places in the Spring Mountain district 

 in Lemhi County. In Dry Gulch on the Colorado 

 group of claims where a quartz diorite dike invades 

 the limestone the contact is cut by a tunnel which 

 shows the metamorphic effect of the diorite intrusion. 

 The metamorphically developed minerals include cal- 

 cite, coarse flecks of muscovite and some biotite, 

 all readily recognized megascopically. In addition 

 pepperlike specks are scattered through much of 

 he. Bruce es- the rock which, when examined microscopically, 



tateLemhi 



county prove to be forsterite, the iron-free olivine, and a 



little garnet. Ore minerals do not occur associated 



with these contact minerals, but a vein a short distance up the hill 



contains argentiferous galena, manganese oxide, pyrite, and chal- 



copyrite in a siliceous gangue. 31 



The third locality for forsterite, and, to judge from the single 

 specimen available, the most important mineralogically, is the 

 Bruce Estate, which, according to Umpleby, 32 is a large low-grade 



30 George R. Mansfield and E. S. Larsen, jr. Journ. Wash. Acad. Sci., vol. 5, pp. 463-468, 1915. 

 si J. B. Umpleby. U. S. Geol. Survey, Bull. 528, p. 88, 1913. 

 « J. B. Umpleby. U. S. Geol. Survey, Bull. 528, p. 89, 1913. 



Fig. 77.— Forster- 



