130 Penfield and Forbes — Chrysolite-Fay alite Group, etc. 



pegmatite. It occurred at one side of the pegmatite vein in 

 the massive hornblende-biotite granite, as a lenticular shell of 

 varying thickness, about 12 to 16 inches in diameter, filled on 

 the inside with loose earthy material and enveloped by a layer 

 of magnetite about one inch thick. 



The material showed do crystal faces, but two distinct cleav- 

 ages at right angles to each other. The color of the mineral 

 on a fresh fracture is a dark resinous green, but thin edges 

 transmit a yellowish light. On examining fragments with a 

 microscope it was found that they were fresh and transparent, 

 but permeated with grains of magnetite. The material for 

 analysis was therefore prepared by pulverizing and sifting to 

 a uniform grain and freeing from magnetite with a magnet. 

 The material thus prepared was of exceptional purity. The 

 specific gravity taken with the pycnometer and the results of 

 the analysis are as follows : 



Specific gravity 4*323, 4-316, 4-317. Average 4*318 



I. II. Average. Ratio. 



Si0 2 _.. 30-11 30-05 30-08 -501 TOO 



FeO.__ 68-04 68-19 68'12 '946 



MnO.._ -77 -65 -72 -010 \- 



H„0 ... -88 -87 -80 -048 



Total... 99-80 99'76 99-80 



The ratio indicates almost pure ferrous ortho-silicate Fe 2 Si0 4 

 with only a trace of manganese and no magnesia, although a 

 careful test was made for the latter. The small amount of 

 water is considered as basic. A high temperature is needed to 

 expel it, and limonite, the most likely mineral to be formed by 

 decomposition, was wanting. If the water is disregarded, the 

 analysis shows a slight excess of silica. 



The cleavages served for the orientation of the optical proper- 

 ties and plates cut parallel to these, as described by Penfield and 

 Pratt,* proved to be parallel to the pinacoids 001 and 010. 

 The first of these gave no interference figure in polarized light, 

 while the second snowed the emergence of the acute bisectrix. 

 The divergence of the optical axes was measured on a large 

 Fness axial angle apparatus, and although it could be 

 measured in air, it was found best to measure it in a-mono- 

 bromnaphthalene. The plates also served for determining the 

 pleochroism. 



The indices of refraction were obtained from prisms carefully 

 oriented by the cleavages. The results are as follows : for yel- 



*This Journal, vol. 1, p. 387, 1895. 



