THE MINERALS OF IDAHO 



309 



contact-metamorphic copper deposit extending south from Dry 

 Gulch in the Spring Mountain district. Ludwigite from this property 

 is also described elsewhere in this volume. 



The specimen in question is a banded granular mass made up of 

 about equal parts of granular magnetite and pale brown forsterite. 

 There are small cavities in the forsterite lined with minute tabular 

 forsterite crystals and later filled with calcite. These minute tabular 

 crystals, when lying on the 010 face, give an optical interference 

 figure indicating the obtuse bisectrix perpendicular to this plane 



Figs. 7S-79.— 78, Olivine. 



Vertically elongated crystal from black sand from Minidoka. 

 Olivine. Crystal elongated on the o axis 



with the axial plane across the vertical elongation. The mineral is 

 therefore optically biaxial and positive with X = 6, Y = c, Z=a; 2\ 

 very large. The refractive indices are: a = 1.640, /3= 1.653, 7 = 1.675. 

 Several trials were made with various dilute acids with a view to 

 dissolving out the calcite filling of the cavities in order to secure crys- 

 tals for measurement. Practically all acids which dissolve the cal- 

 cite attack the forsterite sufficiently to destroy the luster, but with 

 acetic acid some measurable crystals were obtained, although they 

 were so dulled as to give very poor measurements. The habit of 

 these is shown in Figure 77 and they give the following forms and 

 angles : 



