442 Hillebrand and Merwin — Calciovolborthite ( f ) 



besides ferric and other oxides, foreign to the vanadates, but 

 complicating the analysis and interfering with deductions. 



Both minerals occur, sometimes on the same hand specimen, 

 in rosettes and patches of minute reticulated scales on surfaces 

 of sandstone, often intimately associated with thin coatings 

 and stains of other copper minerals — carbonates and possibly 

 chrysocolla — which made it impossible to secure material of 

 great purity for analysis. Mr. Boutwell mentions occasional 

 " carnotite and small yellow crystals, slightly darker than this 

 carnotite," as being "intimately associated with the green 

 crystals, 1 ' but we have observed neither of these on the speci- 

 mens now available. The rosette forms spoken of are often 

 distributed singly, but more often grouped and intermingled, 

 presenting then a very handsome appearance, especially in the 

 case of the yellow green mineral. 



1. Yellow-Green Variety. — Rosettes and patches of reticu- 

 lated scales. The aggregates have a dark yellow-green color 

 (YG), Shade 1, of the Bradley Color Scale.* In powder and 

 in scales under the microscope the color is brilliant green 

 yellow (G-Y). But few data as to crystalline form could be 

 obtained from the material from Paradox Valley, Colorado, 

 but that from Richardson, Utah, furnished a few triangular, 

 hexagonal and elongated scales, which, within the limits of 

 error (3°), possessed hexagonal symmetry. However, the min- 

 eral is not hexagonal, but probably monoclinic, as the optical 

 study showed. On account of the aggregated character of the 

 material nothing definite can be said concerning the hardness 

 and cleavage of the mineral, although the thinnest scales 

 appear to be cleavage fragments. Lack of material of suffic- 

 ient purity prevented the determination of the specific gravity. 



An optic axis and the bisectrix 7 emerge at angles of about 

 50° in air on opposite sides of the normal to the plane of the 

 scales. The optic axis is strongly dispersed, with the red 

 fringe of the hyperbola on the convex side. Measurements 

 of the optical axial angle, 2 V, gave the following average 

 results from values that varied 5° for different sections : in 

 Li-light 68°, Na-light 83°, Tl-light 89°. The mineral is, there- 

 fore, optically positive for red and negative for violet. The 

 measurements indicate that the bisectrices are dispersed. 

 Determination of the extinction position of plates for light of 

 different colors established the essential coincidence of the 

 optic planes for all colors, although most sections do not extin- 

 guish completely in white light. The dispersion is, then, 

 inclined, and the mineral is probably monoclinic. The indices 

 of refraction, a and /3, for Na-light, measured by immer- 

 sion in highly refracting liquids, are 2*01 and 2*05. 7 calcu- 

 lated == 2-10. 



*This scale can be found in Mulliken's Identification r of Pure Organic 

 Compounds ; Wiley & Sons, New York. 



