400 BULLETIN 131, UNITED STATES NATIONAL MUSEUM 



tered to iddingsite, but some of the smaller ones show outer borders 

 of iddingsite around cores of olivine. The iddingsite has very per- 

 fect cleavage which, however, is masked by the extreme brittleness 

 of the mineral. Cleavages parallel to the three pinacoids are well 

 developed and there is probably a fourth cleavage parallel to a dome 

 face. The optical orientation is X = a, Y=6, Z = c. The indices of 

 refraction are: a=1.710±0.005, j8= 1.722±0.005, 7= 1-754 ±0.005. 

 The optic axial angle varies from 20° to 65°, with an average of about 

 50°. The optical character is negative, dispersion strong; color red- 

 brown; pleochroism slight. 



As has been noted from other localities, there occurs with this 

 iddingsite an amorphous or cryptocrystalline substance derived from 

 the olivine in the same manner as the deeper-colored crystalline 

 material. In some specimens this forms at the core and in others 

 occurs as a border around the crystalline material. A sample of this 

 material, separated from the iddingsite, was analyzed with the fol- 

 lowing results: 



Analysis of cryptocrystalline material associated with iddingsite 



(E. V. Shannon, analyst) 



Per cent 



Silica (Si0 2 ) 44. 40 



Titanium dioxide (Ti0 2 ) . 16 



Alumina (A1 2 3 ) 2. 28 



Ferric iron (Fe 2 3 ) 29. 00 



Lime (CaO) 2. 20 



Magnesia (MgO) 7. 12 



Water (H 2 0) above 110° C 6. 96 



Water (H 2 0) below 110° C 8. 40 



Total 100. 52 



This is apparently an amorphous equivalent of the crystalline 



iddingsite. 



TITANITE (510) 



Calcium titano-silicate, CaO.Ti0 2 .Si0 2 . Monoclinic. 



Titanite has been noted at several places in Idaho as crystals in 

 placer concentrates and as small crystals in rocks, but no locality 

 of consequence as a source for mineralogical specimens of the mineral 

 is yet known. The following occurrences have been noted: 



ADAMS COUNTY 



Titanite is listed by Livingston and Laney 11 as a mineral of the 

 contact deposits of the Seven Devils district. 



BOISE COUNTY 



Titanite is common as an accessory mineral in rock from the 

 Silver Wreath mine in the Willow Creek district. 12 



11 D. C. Livingston and F. B. Laney. Idaho Bur. Oeol. and Mines., Bull. 1, p. 62. 

 i 2 Waldemar Lindgren. U. S. Geol. Survey, 18th. Ann. Rept, pt. 3, p. 709. 1898. 



