I9I4] 



HYDROUS CALCIUM VANADATES. 



37 



elongation. The refractive indices a and /? were determined with 

 difficulty, a was obtained from groups of crystals seen edgewise; 

 fi is so high that it could barely be matched without heating by im- 

 mersion in a mixture of tin iodide, methylene iodide and the com- 

 pound of arsenic sulphide and methylene iodide, y could not be 

 obtained except after expelling water from the mineral by heat. 



Fig. I. Optical orientation of metahewettite. 



For Li-light a^i.70, ^ = 2.10 and 2V (calculated) ==52°. a is the 

 acute bisectrix. No measurable diiiferences between the optical prop- 

 erties of specimens from different localities were found. Fig. i 

 shows the optical and crystallographic relations. The axial ratio 

 a: b = .54: i. 



The density of the mineral containing 9 molecules of water — 

 determined in xylol — is 2.51 1; after the loss of 6 molecules of 

 water it is 2.942. The loss of this water is not accompanied by any 

 perceptible breaking down of crystal structure. The same fact was 

 observed with hewettite. Like hewettite, metahewettite melts 

 readily and is slightly soluble in water. The solubility, slight as it 

 is, affords a means, by the use of much boihng water, of separating 

 from metahewettite the associated minerals mentioned on pages 34 

 and 35. 



Differing strikingly in external aspect from the ores described 



