Van Horn and Cook — New Occurrence of Pearceite. 521 



triangular figures. According to Hintze* the sides of these, 

 figures are parallel to the faces (111), (111), and (101). 



The new twinning plane, which is practically present on all of 

 the specimens, is likewise parallel to one of the sides on the tri- 

 angle. It must, therefore, lie in either the unit prism-basal pina- 

 coid zone or the ortho-basal pinacoid zone and the twinning 

 plane must be either a pyramid or an orthodome. 



The examination of the twinning planes of a large number 

 of monoclinic minerals shows that twinning in the ortho-basal 

 pinacoid zone is very common, whereas the pyramids function 

 but rarely as twinning planes. The uew twinning plane on 

 pearceite has, therefore, been tentatively assumed to be par- 

 allel to the orthodome (702) on the basis of the twinning angle, 

 which is 34° 42'. The average of the measurements, made on 

 a number of different crystals, gives for the angle between the 

 basal pinacoid (100) and the orthodome (702) a value which 

 agrees quite closely with the calculated value, as may be seen 

 from the following : 



Observed Calculated 



(100) : (702) 72° 39' 72° 53' 



Fig. 1 shows the general character of the crystal aggregates 

 as well as the presence of reentrant angles. The triangular 

 striations on the basal pinacoid are seen on numbers 1, 2, and 

 3. Parts of the best crystal measured are shown under 8, 9, 

 and 10, but unfortunately the original was broken while deter- 

 mining the specific gravity. Nevertheless the reentrant angles 

 are still visible on all three fragments. Number 7 is an aggre- 



fate of small thin plates and closely resembles some specular 

 ematites in micaceous appearance. 



Chemical Composition. 



The empirical formula which was proposed in 1896 by Pen- 

 field,f and was accepted, is (Ag,Cu),(As, Sb) S„ or 9(Ag,Cu) 2 S. 

 (As, Sb),,S s . This is analogous to the formula for polyba- 

 site, (Ag,Cu),(Sb, As) S 6 , which was proposed by Heinrich Rose 

 in 18294 A portion of one of the Sierra Mojada crystals which 

 seemed free from all impurities was analyzed by Dr. JS\ A. 

 Dubois of the Chemical Department of Case School of Applied 

 Science with the following results : 



* C. Hintze, Handbuch der Mineralogie, 1904, p. 1167. 



f Op. cit., page 18. 



X H. Rose, Pogg. Ann., xv, 575, 1829. 



