v /.. Penfield — Spangolite^ <t new <■<>/</»/• mineral. 



low and very delicate rhombohedral depressions with curved 

 contours. In part b we have again Bcalenohedral, surrounded 

 bv shallow and more delicate rhombohedral, depressions. The 

 obtuse angle of the scalenohedral sections measured 129°, from 

 which it> relation on the Lateral axes a : can be cal- 



culated, which requires an angle of L29 26'. In part 

 we have again represented simple scalenohedron depressions 

 which are about •n-_ ) .v in " in diameter and were produced by a 

 very dilute acid. I concentrated EC! diluted with L60 00 of 

 water. The obtuse angle of the scalenohedron measured about 

 142°, Indicating a relation on the lateral axes y>{ a:$a:3a, 

 which requires an angle of 141° 48'. With nitric acid the fig- 

 ures are very similar to those produced by hydrochloric; in 

 all il was observed that with very dilute acids there 



was a tendency to form scalenohedral and with stronger rhom- 

 bohedral depressions. When we compare the position of these 

 rhombohedral and scalenohedral etchings to the outer hexagon, 

 which in figs. 3, 4 and 5 indicates the outline of the crystal sec- 

 tion. we Bee at once that the pyramids on the mineral must be 

 of the second order. It should be stated here that the etchings 

 were of very great beauty and perfection, the outline of the 

 scalenohedral cross sections being in almost all cases very dis- 

 tinct and free from distortions of any kind, so that the angles 

 could he measured with comparative accuracy. 



Optical properties. — The color of the mineral by reflected 

 light is dark green, cleavage plates by transmitted light are light 

 en. Prof. H. L. Wells examined a basal section of the 

 mineral 4 mm thick with the spectroscope. When the slit was 

 very narrow the light transmitted by the mineral gave a nar- 

 row spectrum with a maximum of light in the green at about 

 A 526. There was a total absorption of the red and yellow, run- 

 ning well into the yellowish-green. At the other end of the spec- 

 trum there was a decided absorption of the blue and a total 

 absorption of the violet. Pleochroism is not very marked. 

 The ordinary ray is green while the extraordinary i> a de- 

 cided bluish green. Cleavage plates show perfectly normal 

 optical properties In convergent polarized light they yield a 

 black cross surrounded by rings which are bordered by green 

 and blue. The double refraction is quite Btrong and negative. 

 asiderable difficulty was obtained in making a prism from a 

 small crystal of the mineral with its edge at right angles to the 

 perfect basal cleavage, but a small one was obtained, with an 

 angle of 37° 48', from which the indices of refraction were de- 

 mined. The prism was opaque t<> the red and yellow lights 

 of lithia and soda flames, even to the yellowish-green light of a 

 thallium flame. With an ordinary kerosene flame the prism 

 yielded two narrow green spectra with a minimum deviation 



Am. Jouh. Sci.— Third - Vol. XXXIX. No. 2H4.— May. 1S90. 



25 



