176 PERIDOTITE. 



Sectiox VII. — ChromHe and Picoilfe. — Their Relations. 



An interesting question is tlie relation of cliromite to picotite. Professor 

 H. Fischer described the former as opaque, j'et sometimes magnetic from the 

 contained magnetite ; * but in a later paper he stated that chromite, in the 

 finest dust and under a high power, was translucent and of a red-brown to 

 red color.f 



Dr. E. Dathe found that the chromite from Baltimore was, in thin splinters, 

 translucent and of a brown color much like brown obsidian glass, and that 

 this phenomenon could be observed with a low power. The same was also 

 found by him to be true of the chromite from Waldheim.t From this, Dathe 

 concluded that the ordinary microscopic distinction between picotite and 

 chromite, based on the translucence of the one and the opaqueness of the 

 other, was incorrect. M. J. Thoulet also found that the cliromite from Roraas 

 in Norway, and that from Negropont, were translucent, showing in trans- 

 mitted light a color mixed with yellow and red. The grains were traversed 

 by fissures which were impregnated with the surrounding rock material, — 

 serpentine in the first case, calcite in the second. In reQected light the 

 chromite shows a violet-rose color or a gray, and in the portions impregnated 

 by magnetite the characteristic metallic-blue reflection of the latter mineral 

 was observed. § 



Although the translucency of chromite is considered by lithologists to 

 have been first noticed by Fischer, it would appear to have been earlier 

 remarked by C. H. Pfaff, who stated in 1825, in describing a compact mass 

 of chromic iron from Massachusetts, that it was characterized by a thin violet 

 rim on the plane of fracture. || 



In order to ascertain, so far as possible, the microscopic relations of 

 picotite to chromite and the other iron ores, a number of sections have 

 been re-examined with special reference to these points; and the powder of 

 picotite and chromite from a number of different localities has also been 

 microscopically studied. 



* Kritisclie mikroskopiscli-mineralogische Studicn, 1869, pp. 5, 6, 20-22. 

 t Ibid. 1873, pp. 44, 77. 

 X Neues Jahr. Min., 1876, pp. 247-249. 

 § Bull. Soc. Mill. Trance, 1879, ii. 34-37. 



II " Charakteristiscli fiir dieses Chromeiseu ist eiiie diiiine violette Riude auf dcu AblosungsHiichea. Die 

 Masse war derb." Jour. Clicmie Pliysik, 1825, xlv. 101-103. 



