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BULLETIN 131, UNITED STATES NATIONAL MUSEUM 



It gave qualitative reactions for copper and arsenic and was found by 

 A. Rodolfo Martinez to have optical properties in agreement with 

 those given in Larsen's tables for trichalcite as shown by the following 

 comparison : 



Comparison of optical properties of trichalcite 



Pine Creek, Idaho (Martinez). 



Color pale bluish-green. 



Nonpleochroic. 



Biaxial. 



Sign negative (— ). 



2V large. 



1=1. 



Birefringence medium low. 

 X normal to plates. 



Turginsk, Urals (Larsen). 



Color pale bluish-green. 



Nonpleochroic. 



Biaxial. 



Sign negative ( — ). 



2V large. 



a=h 67 ±0.01. 



0=1. 686 ±0. 003. 



7 = 1. 698 ±0. 003. 



Birefringence 0.028. 



X normal to plates. 



Upon examination of the specimen under a binocular microscope 

 it was found that the mineral was in thin tabular crystals of hexagonal 



aspect and although these 

 were very minute it was 

 found possible to measure 

 two of them on the 2- 

 circle goniometer. The 

 basal pinacoid gave good 

 signals but the very nar- 

 row prismatic planes were 

 more or less curved and 

 irregular, yielding only 

 approximate measure- 

 ments accurate, perhaps, 

 to 1°. These indicated 

 60° angles for the pris- 

 matic zone, the mineral thus simulating hexagonal crystallographic 

 symmetry. Examination of the measured crystals in polarized 

 light shows them to be made up of biaxial orthorhombic sectors as 

 shown in Figure 138. The sectors have interlocking boundaries and 

 each sector has well-defined cleavage parallel to its edge. The end 

 sectors give parallel extinction and in convergent light give a per- 

 fectly centered biaxial interference figure. The lateral sectors have 

 a confused structure as though made up of overlapping plates. It is 

 evident that the several apparently prismatic planes of the pseudo- 

 hexagonal tablet are pinacoids of several orthorhombic units which 

 go to make up the twinned group. It is probable that trichalcite 

 is orthorhombic, with a prism angle near 60°, and that it has a 

 tendency to form pseudohexagonal twins like aragonite, cerusite, 

 chalcocite, etc. Figure 139 is drawn to show in orthographic and 



Figs. 138-139.— Trichalcite. Liberal King Claim, Pine 

 Creek, Shoshone County 



