20 /S. L, Penfield — Pearceite^ a Sulpharsenite of 



from whicli the axial ratio was calculated : 



«:^>:c = l-YSOO : 1 : 1-6199 

 /?= 001/^100 = 89° 51'. 



The crystals are quite highly modified, and it seems best 

 before giving a list of the forms to explain the different kinds 

 which were observed and to state something concerning their 

 occurrence. The basal pinacoid c, 001 is prominent, is hexa- 

 gonal or triangular in shape, and is characterized by triangular 

 markings and vicinal planes, fig. 1, so that it was often impos- 

 sible to obtain accurate measurements from it. The prism 

 m, 110 and the pinacoid a^ 100 are nearly at right angles to c 

 and 60° from one another, so that the combination approaches 

 very closely to an hexagonal prism, and it is sometimes impos- 

 sible to distinguish a from m, or without accurate measure- 

 ments to decide whether the forms between c and a or c and m 

 modify the acute or obtuse angles. It is very probable that a 

 twinning is present, similar to that of the micas and chlorites, 

 where the twinning plane is at right angles to c in the zone 

 TTi/^c and where the parts are superimposed upon one another 

 with G as the composition face, but no absolute proof of this 

 was obtained. The crystals are opaque, so that optical tests 

 could not be applied as was done by Miers,* who has described 

 this kind of twinning on polybasite. If the twinning occurs 

 on pearceite, as it probably does, it must cause uncertainty as 

 to the identification of some of the forms in the zones between 

 c and a and g and m, and it may also account in part for the 

 decidedly rhombohedral aspect of many of the crystals. As 

 far as could be observed, similar faces are often developed 

 about equally above and below m and a in the zones between 

 the basal planes, but to what extent this is di;e to twinning it 

 is impossible to state. The faces in these zones are moreover 

 commonly striated parallel to their mutual intersection, and 

 while T and p, r° and j9°, n and t and n° and t° (compare fig. 1 

 and the list of forms beyond), are the most prominent, other 

 faces, especially e and e°,fsindf°, s and s° and u and n°, are 

 very often present. When q was observed it was always a 

 prominent, dull face, not sharing in the horizontal striations of 

 the other faces of the zone. It was only occasionally that 

 forms were observed between 6', 001 and I, 310, and they were 

 always small, while the corresponding forms were not observed 

 between 001 and 310. The pinacoid b, 010 was identified, not 

 only by the symmetrical arrangement of the forms with refer- 

 ence to it, but also by the similarity of the angles measured 



*Min. Mag., viii, p. 204, 1889. 



