24: S. L. Penfield — Pearceite^ a Sulpharsenite of 



been prominently noted in descriptions of the mineral. The 

 earlier measurements of poljbasite crystals by Breithaupt* are 

 untrustworthy, probably owing to the difficulty of securing 

 suitable material, and we are indebted to Miersf for the first 

 careful series of measurements, which were made upon a suite 

 of crystals in the British Museum. He also evidently experi- 

 enced some difficulty in obtaining reliable measurements, for 

 the angles between similar faces show a considerable variation 

 amounting usually from a quarter to one-half of a degree. The 

 crystals are regarded by him as orthorhombic and the follow- 

 ing forms were observed : 



c, 001 w, 109 t, 201 p. 111 



m,\\0 n, 101 r, 112 s, 221 



Prominent angles are m : m, 110 /\ 110 = 60° 10^ ; c^^n, 001 

 A 101 = 42° 24' and c^p, 001 a 111 = 61° 14', the axial ratio 

 being a:h'.G = 1*7262 : 1 : 1-634. He also observed that the 

 crystals when examined in polarized light did not become dark 

 in any position and concluded, therefore, that they were 

 twinned, as already explained on page 20 for pearceite. He 

 points out the impossibility of distinguishing the bevelling 

 faces r and j9 in the zone between c and m from n and t in the 

 zone between c and <2, and calls attention to the fact that the 

 only form in these zones at right angles to the base is the 

 prism m and that the pinacoid a, 100 fails. When six faces 

 occur in the zone of the prisms the ones which occupy the 

 positions of the pinacoid a are the prismatic faces m in twin 

 position. 



Dana:j: accepts the measurements of Miers as fundamental 

 but shifts the position of the crystal so that the obtuse angle 

 of the prism, mAm, is in front. 



Among the specimens in the Brush collection at IS'ew Haven 

 one was found from the Himmelfahrt mine, Freiberg in Sax- 

 ony, showing crystals like fig. 3, which is apparently a combi- 

 nation of base and rhombohedron. On revolving the crystals 

 in polarized light they never became dark, showing thus that 



a twinning was present, while in converg^ent light a confused 

 interference figure was usually obtained, although in places the 



* Charakteristik des Mineral-Systems, 1832, p. 266. 



f Min. Ma^., viii, p. 204, 1889." 



X System of Mineralogy, 6th edition, p. 146. 



