Melanterite and Chalcanthite Groups. 229 



The probable error of the indices of refraction should 

 be less than ± 0.003 ; zinc-copper chalcanthite, iron-copper 

 chalcanthite, and chalcanthite are very similar optically, 

 but differ slightly in color ; the other members of the chal- 

 canthite group can be readily distinguished by their color 

 and optical properties. 



Boothite dehydrates to chalcanthite as a specimen so 

 labeled, kindly furnished the author by Colonel Roebling, 

 had the optical properties of chalcanthite. Pisanite 

 dehydrates readily as four specimens so labeled proved on 

 microscopic examination to have changed to iron-copper 

 chalcanthite; artificial pisanite alters rather rapidly to 

 pentahydrate. Melanterite, either natural or artificial, 

 is commonly coated with a white powder of siderotil and 

 the fine powder will dehydrate after standing for some 

 months to the pentahydrate ; siderotil is nearly as common 

 a mineral under surface conditions as melanterite and has 

 been found in a number of specimens sent in to the author 

 (E.S.L) from the field. Two museum specimens labelled 

 bieberite proved on optical study to have changed to 

 cobalt chalcanthite. A specimen labelled mallardite had 

 apparently altered to an uncertain lower hydrate. 



On standing for a few days at about 45° zinc-copper 

 chalcanthite loses water and alters to a white powder 

 made up of an aggregate of minute fibers with the follow- 

 ing indices of refraction : 



a = 1.60 ± 0.01, parallel to the fibers, y = 1.656 ± 0.01 



The hydrous Sulphates of C 'oo alt. 



Artificial ' ' cobalt sulphate ' ' from the stock bottle in the 

 chemical laboratory of the U. S. Geological Survey is 

 made up of blue, vitreous crystals that are uniaxal — , and 

 very faintly pleochroic in pale pink with <o = 1.495 ± 0.003, 

 e — 1.460 ± 0.003. This form also crystallized from a 

 solution of CoS0 4 at 45°. A water determination on 

 material from the stock bottle gave 40.7 per cent, and the 

 material is no doubt CoS0 4 .6H 2 0, which should have 41.1 

 per cent water and is said to crystallize from a water 

 solution at from 40 to 50° C. 



It dehydrates rather rapidly on exposure to air to a 

 finely crystalline, pale rose pink (Bidgways 71 f ) powder 

 which has the following optical properties and is the pen- 

 tahydrate: Opt. — , 2V medium, dispersion not strong, 

 faintly pleochroic. 



