THE MINEEALS OF IDAHO 313 



It is uniaxial, negative, and the refractive indices are approximately 

 €= 1.545, o)= 1.562, birefringence 0.017. These identify the material 

 as probably wernerite. 



The scapolites often show more or less imperfect crystal form, 

 especially where developed along joints and seams, but the character- 

 istic forms are the rounded shot-like masses (pi. 7, upper). One 

 rock from between Moline and Eagle Creeks contains a large pro- 

 portion of white fine granular patches associated with scattered 

 spheroids of scapolite 2 mm. in average diameter. The scapolite 

 of this specimen, likewise extremely poikilitic, has e = 1.550, co = 1.570, 

 birefringence 0.020 and is likewise wernerite. 



Typical crystals of the mineral, developed along a joint, are 

 shown in the specimen illustrated in Plate 7, lower. These are not 

 terminated, although some of them show prismatic faces. These 

 have the indices e = 1.562, co= 1.547, birefringence 0.015. A sim- 

 ilar specimen labeled "Lower middle Wallace," from the ridge 

 between the forks of Gold Creek consists of a banded fine grained 

 rock which, in certain bands, contains very minute spherical masses 

 of scapolite. One face of the specimen is coated with dull white 

 scapolite crystals up to 1 mm. by 3 mm. in size, probably originally 

 developed in a seam of calcite later removed. Under the micro- 

 scope these are clear and transparent as contrasted with the poikilitic 

 character of the spherical masses, and show good prismatic cleavage. 

 The refractive indices are e= 1.540, co = 1.562, birefringence 0.022. 



The poikilitic character of most of the scapolite is probably due 

 to its mode of formation, but may in part be due to alteration. A 

 specimen labeled " Wallace hornfels with large scapolites," from the 

 basin north of Trimmed Tree Hill, consists of rude lusterless white 

 square prisms up to 8 by 30 mm. in size in a schistose micaceous 

 matrix. When crushed and examined under the microscope these 

 are found to contain comparatively very little scapolite, although 

 they probably are the remains of large scapolite crystals. They 

 now consist in the main of what appear to be feldspar, quartz, 

 kaolinite, biotite, and a little diopside. 



The optical measurements indicate that the scapolite of the 

 metamorphosed rocks of this area is, in the main, near wernerite in 

 composition. 



r VALLEY COUNTY 



A scapolite near wernerite (Ma 30 Me 70 ) occurs as a constituent 

 of a lime-silicate hornfels formed by the metamorphism of a band 

 of marble in the Yellow Pine mercury mining district. Associated 

 with this in making up the hornfels are quartz, orthoclase, diopside, 

 tremolite, carbonates, and more or less phlogopite, epidote, titanite, 

 magnetite, and apatite. 38 



« E. S. Larsen and D. C. Livingston. U. S. Qeol. Surv., Bull. 715, p. 77, 1920. 

 54347— 26t 21 



