Billebrand and Merwin— Calciovolborthite (t ) 443 



The extremely strong inclined dispersion is noteworthy 

 .Between the optic axes for Li-light and Tl-light there are 

 angles of 28° and 7°. The axis emerging in the field of the 

 microscope is dispersed 7°. The bisectrices are dispersed 18°. 

 I he plane of the optic axes bisects the angle of the triangular 

 plates. This relation and inclined dispersion are strong evi- 

 dence of monoclinic symmetry. The absorption of light by 

 the crystals was studied in a general way by observing them 

 in light of different wave lengths. JSTo sharp absorption bands 

 were disclosed and there is little absorption in the bright part 

 of the spectrum, but beyond 530/u/* and 630/*/* absorption in 

 all three of the principal directions increases steadily and 

 sensibly equally, except that beyond 470 w differences are 

 noticeable. 



2. Greenish Yellow Variety.— The arsenical variety of the 

 mineral has the same reticulate appearance as that just de- 

 scribed. The color is dull green (G-, shades 1 and 2), and is in 

 some cases only a thin film covering crystals of the yellow- 

 green mineral. This fact was not known when the analyses 

 were made, hence it is possible that the composition found rep- 

 resents a mixture. Under the microscope the green crusts are 

 seen to be very minute, doubly refracting crystals of refractive 

 index about 1-92. They absorb at the ends of the spectrum, 

 noticeably beyond 500/^ and 660/*/*. This mineral is pseudo- 

 morpnic after the yellow-green variety. 



Yellow- Green 

 Percentages Mol. Eatio 



V a 5 30-6* ) 



££::::;:;:;::; *\\ ^ 



CuO 48-4*^ 



CaO. 3.9* I 



BaO 2-7 



Mgo .... .3 y 713 



§ft::::::::. : :::} - j 



H s O,105°- 1-8 100 



H s O, 105° + 6-4 355 



CO, 2-4 ) 



SiO, . 6 f 60 



Fe.0, - 8 



Mn O 

 Co,0 4 J. f 

 Al,O s 



Green-Yellow 

 Percentages Mol. Eatio 



16-0'" 



17-2* 



•8 



37-1* 



15*3 



2-3 



•5 



168 



y 769 



"2 



1-0 



43 

 •9 



3-2 



J 



55 

 239 



32 



100-0 



100-0 



*Mean of two closely agreeing results, 

 f As weighed after precipitation together. 



Original condition unknown. 



