474 Mr, L. Fletcher's Crystallographic Notes. 



These give illustrations of the results accomplished; but of 

 course many other experiments have been made. I have not 

 yet been able to decide whether the definition of the concave 

 grating fully comes up to that of a flat grating ; but it evi- 

 dently does so very nearly. 



Baltimore, May 2o, 1882. 



LXIT, Crystallographic Notes. By L. Fletcher, M.A., of 

 the Mineral Department, British Museum*. 



[Plate X.] 

 IX. Skutterudite. 



THE first mention of this mineral was made in 1827 by 

 Breithauptf, to whom it had been submitted by his 

 brother-in-law on returning from a voyage in Norway. 

 Though none of the specimens presented crystal-faces, Breit- 

 haupt found that there were distinct cleavages parallel to the 

 sides of a cube, for which reason he assigned to the species 

 the name of Tesseral-Kies. Cleavages, more or less interrupted 

 and indistinct, were found to exist parallel to planes trunca- 

 ting the edges and quoins of this cube, indicating that the 

 crystallisation was that characteristic of the cubic system. 

 Breithaupt went a step further, and, from traces of separation 

 which manifested themselves in the direction of the octahe- 

 dral planes, hazarded the conjecture, since (curious to say) 

 verified, that the crystals would prove to present the octahe- 

 dron as the predominant form. The specimens were of a 

 bright metallic lustre and of a tin-white colour. The specific 

 gravities of five different fragments were determined to be 

 6-659, 6-681, 6-718, 6'748, 6-848, and thus had a consider- 

 able range. As, however, after breaking up the fragments, 

 it was seen that the first three included particles of quartz and 

 actinolite, Breithaupt regarded the true specific gravity as 

 lying between 6*748 and 6*848. The hardness was greater 

 than that of cobaltite or Cobaltkies; and as blowpipe examina- 

 tion had indicated that cobalt and arsenic were the chief con- 

 stituents, Breithaupt suggested the secondary title of Hartko- 

 baltkies. The associated minerals were cobaltine, copper 

 pyrites, glassy actinolite, serpentine, quartz, and sometimes 

 also cobalt-bloom. 



* Communicated by the Author, having been read before the Crystallo- 

 logical Society, May 80, 1881. 



t "Ueber erne neue Ivies-Species you Skutterud," Pogfir. Ann. vol. ix 

 p. 115(1827). Bo 



