58 



SCHALLER : CRYSTALLIZED TURQUOISE 



wollastonite, are known to me. The eutectic composition is at 

 69 molecular per cent CaSi0 3 . At the CaSi0 3 end occur a num- 

 ber of wollastonites, all lying within the experimentally deter- 

 mined limits of the solid solution (9 molecular per cent MgSi0 3 ). 

 The diagram illustrates very well how it may become possible 

 to classify minerals and rocks on a physico-chemical basis, start- 

 ing with chemical complexity as the fundamental character. 

 Such a classification means more to the chemist than to the 

 mineralogist and petrologist, but it is to be hoped that it will 

 throw increased light on the relations of the silicates from both 

 points of view. 



MINERALOGY. — Crystallized turquoise from Virginia. Walde- 

 mar T. Schaller, Geological Survey. To appear in the 

 American Journal of Science. 



A sample of a well crystallized, bright blue mineral from Camp- 

 bell County, Virginia, sent in for identification by Mr. J. H. 

 Watkins, proved to be turquoise, a species hitherto known only in 

 a cryptocrystalline condition. The mineral forms a secondary 

 botryoidal crust on quartz. The minute crystals composing this 

 crust have a rhombic shape and, like the crystals of chalcosiderite, 

 with which turquoise is isomorphous, are triclinic. As the angles 

 of the two species are very close, the crystallographic elements of 

 chalcosiderite are taken for turquoise: a :b : c = 0.7910 : 1 : 0.6051, 

 a = 92°58', /3 = 93°30', 7 = 107°41'. Crystal forms of turquoise : 

 a {100}; M {110}; k {Oil}, large; m {110}, small; b {010}, 

 line face. The mean refractive index is about 1.63. The double 

 refraction is high (0.04) and the pleochroism strong, pale blue 

 to colorless. 



Analysis and ratios of turquoise 



