344 Palache and Warren — Kr'6hn~kite, N atrochalcite. 



Cleavage. — Cleavage is perfect and easy parallel to J(010) 

 and good but not so easily produced* parallel to c(011). No 

 trace of a prism cleavage as recorded by Darapsky could be 

 detected. Hardness is a little less than 3, just scratched by 

 the finger nail. Specific gravity is 2*061 (Warren), determined 

 in absolute alcohol and calculated for water at 4°C. 



Optical Characters, determined hy H. E. Merwin. — The 

 principal indices of refraction, determined by means of the 

 refractometer, are : a = 1*5437, /3 = 1*5775, 7 = 1*6013 for 

 sodium light. 2V Da calculated from the refractive indices is 

 78° 36' ; from observation of the acute optic angle in oil 78°42 / . 



The plane of the optic axes is in the plane of symmetry, 

 with the acute bisectrix for yellow (ether-axis a) inclined 48° 45' 

 to .the crystallographic axis c in the obtuse angle /5. The dis- 

 persion, as determined by the colored hyperbolas of interfer- 

 ence figures, is inclined. The acute bisectrix for blue is 

 nearer c than the bisectrix for red. The optic axes also are 

 slightly dispersed, more for blue than for red, as indicated by 

 broader color fringes on the hyperbola emerging nearly per- 

 pendicular to c. 



Chemical Composition, with analysis hy C. H. Warren. — 

 Analysis of the very pure material available confirms the com- 

 position of the mineral as given by earlier writers. 



CuS0 4 .Na 2 S0 4 + 2H 2 0. 



Most of the water is given off below 150°. Small additional 

 amounts continue to come off up to 350°, when dehydration is 

 complete. The residue may be brought to complete fusion 

 without further decomposition, yielding a bright green enamel. 



Per cent. 



Mol. 



Eatio. 



Theory. 



CuO 23-25 



•292 



•98 



23-49 



Na 2 18*89 



•304 



1-02 



18-39 



S0 3 47*60 



•595 



2-00 



47*44 



H 2 U 10-72 



•595 



2 00 



10-68 



Atacamite trace 









100-46 100-00 



Paragenesis. — Krohnkite is the most abundant sulphate 

 in these specimens and the first to be formed. Atacamite 

 alone of the few associated minerals may be older, thin crusts 

 of it sometimes lying between the krohnkite and the vein 

 matrix. Crystals of krohnkite also show occasional inclusions 

 of copiapite, brochantite and atacamite ; none of the other 

 minerals mentioned above as occurring in the collection is 

 found with krohnkite. 



