Dec, 1908. Mineralogical Notes 155 



been hitherto described. The largest of the crystals measure 20 

 millimeters in length by 17 millimeters in width, and from this 

 they diminish to about one-half this size. They are arranged 

 upon the matrix in a nearly parallel position though not exactly 

 so. The mode of attachment may be in general stated to be that 

 of the lower end of the vertical axis, though this attachment varies 

 somewhat. The crystal planes do not present brilliant surfaces, 

 but though dull are not rounded. They do not afford sharp signals 

 with the reflecting goniometer, but give tokens sufficiently well de- 

 fined so that very close estimates of the angular values can be ob- 

 tained. The cleavage parallel to the clinopinacoid is, as usual, very 

 strongly marked. This cleavage affords reflections which are sharp 

 but vicinal. The color of the crystals is a dark orange-red, on 

 cleavage surfaces bright golden-yellow. The crystals are opaque. 

 In development the crystals exhibit monoclinic symmetry throughout 

 and leave little doubt that orpiment should be considered as crystal- 

 lizing in this system. They are all of the same habit and one which 

 seems to be new for this mineral. It is especially characterized by the 

 large development of the positive pyramid v (343). This occurs in 

 broad planes, sometimes 1.5 cm. in length by 1 cm. in width. 

 Grouped with this pyramid occurs in greater or less development 

 the pyramid w(l2i). Accompanying this occur several prisms and 

 in less prominent development several other pyramids. The 

 habit generally exhibited is illustrated in Fig. 1, PI. LI II, the crystal 

 being drawn in the normal position. As this position is not, however, 

 favorable to exhibiting the positive pyramid, Fig. 2, PI. LIII, shows 

 the crystal drawn in reverse position. Three forms new to orpiment 

 were detected upon the crystals. These were the 3/2 clinodome, 023, 

 designated as /, the 1/3 negative pyramid I33 designated as n, and 

 the 1/3 positive orthodome I03 designated as d. In addition two 

 forms were noted which had been observed by Stevanovic,* but to 

 which he had assigned no letters, apparently because he did not 

 regard his results as conclusive. These forms were the 1/3 negative 

 orthodome 103 to which the letter e has been assigned, and the posi- 

 tive pyramid T23 to which the letter k has been assigned. The 

 habit of the Mercur crystals, it may be noted, somewhat resembles 

 that of one figured by Stevanovic f from Allchar except that in the 

 crystal figured by him, the prisms are the prominent forms instead 

 of the pyramids. A basal projection showing the usual development 

 of the different forms found upon the Mercur specimen is given 



*Zs. Kr.. IQ04. 39, P- 14. tLoc. cit. Fig. 3. 



