346 Palache and Warren — Krohnkite, Natrochalcite. 



The table contains the forms found, the angles calculated from 

 the elements and the observed angles with their range of vari- 

 ation. It will be seen that the calculated and observed angles 

 show a very satisfactory agreement. 



Natrochalcite. — Table of Angles. 

 Calculated. Measured. Limits. * 



<t> P <f> P <f> P of 



c 001 90°00' 28°42' 90°00' 28°4l' 28°39' 23°42' 3 



b 010 00 00 90 00 00 00 90 00 4 



m 110 38 41 90 00 38 41 90 00 38°40'-38°43' 12 



p 111 51 23 62 47 51 23 62 47 51 15-51 31 62°45 -62 50 11 



v 112 59 35 50 10 59 35 50 09 59 28-59 44 50 07-50 12 3 



u 221 45 45 73 58 45 55 73 39 45 41 -46 01 73 14-74 11 5 



w 331 43 34 78 45 43 35 78 55 _. 1 



q 111 19 16 52 08 19 16 52 08 19 12-19 20 52 06-52 10 7 



x 221 29 54 70 21 29 56 70 17 29 50-30 04 70 13-70 21 5 



The habit of the crystals is always dominated by the forms 

 chosen as prism and pyramid, m and p ; all other forms are 

 developed with but small faces and often without the full 

 number of their faces. 



Cleavage. — Cleavage is perfect and easy parallel to the base, 

 (001). Hardness 4*5, scratching fluorite easily and not scratched 

 by it. Specific gravity 2*33, determined by Warren. 



Optical Characters determined by H. E. Merwin. — The 

 principal indices of refraction, determined by means of the 

 reilectometer, and the optic angle for sodium light, are as 

 follows : a = 1-6491, /3 =1-6555, 7 = 1-7143. 2V na , calculated 

 from the refractive indices, 36° 52'; by observation of obtuse 

 optic angle in oil, 36° 48'. 



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

 acute bisectrix for yellow being inclined to the crystallographic 

 axis c 12° in the acute angle /3. The acute bisectrix is the 

 axis c ; the mineral is therefore Optically positive. Dispersion 

 of the optic axes is strong, the acute optic angle for the 

 strongest blue rays transmitted by cobalt glass being 3° greater 

 than the corresponding angle for yellow. There is also a slight 

 inclined dispersion of the acute bisectrix, that for blue lying 

 nearer c than that for red. 



Chemical Composition. — The composition of the mineral 

 may be expressed by the formula 



Na 2 S0 4 .Cu 4 (OH) 2 (S0 4 ) 2 + 2H 2 0. 



