36 W. T. Schaller — Crystallized Turquoise from Virginia. 



are triclinic. None of the sections showed a good interference 

 figure, though such as were seen indicated biaxiality. One 

 cleavage plate, possibly parallel to M (110), showed extinction 

 of 12° against_the vertical direction and 12° against the other 

 edge (110 ^ Oil ?). A different cleavage section, of a rhombic 

 shape, showed extinction values of 5° and 34° respectively, but 

 the orientation of this piece could not be determined. The 

 double refraction of the mineral is high, about 0-04. The 

 refractive indices are about 1*61 for a and 1*65 for 7. Lacroix* 

 gives the value 1*63 for the mean index. 



Crystallography. 



The crystals are very minute and so closely grown together 

 that it was almost impossible to obtain any suitable for measure- 

 ment. One complete crystal was found that gave fairly good 

 reflections and the measurements were verified by those 

 obtained on a second, less perfect, crystal. A third incom- 

 plete one also yielded a few measurements. The size of the 

 first two crystals measured is as follows : 



n , XT , j -27 mm high (c axis) 



Crvst. No. l.__ ■{ „ rtmm p, >7 • { 



j ( -32 mm wide (0 axis) 



( -32 mm high (c axis) 



Cryst. No. 2___ _ -] -40 m:n wide (b axis) 



( •12 mm thick (a axis). 



The crystals are triclinic and in angles very near to those of 

 chalcosiderite. In fact, the angular values of turquoise and 

 chalcosiderite are so close that the crystallographical elements 

 of chalcosiderite have been adopted for those of turquoise, as 

 the crystals of the latter mineral are but poorly adapted for 

 accurate measurements. Were it not for the knowledge of the 

 crystallography of chalcosiderite (isomorphous with turquoise, 

 see beyond under chemical composition) which we possess, 

 it is doubtful if the orientation of the turquoise crystals could 

 have been interpreted. 



The values for turquoise are then : 



a-.b'.c — O-^IO : 1 : 0-6051 ; a = 92°58', /3 = 93°30', y = 107°41'. 

 Forms : b {010}, a {100}, m {110}, M {110}, h {011}. 



The comparison of the measured angles with the calculated 

 onesf are shown below. 



*Lacroix, A., Mineralogie de la France, vol. iv, p. 529, 1910. 



f These calculated values are, with one exception, taken from the values 

 calculated for chalcosiderite by Maskelyne, Journ. Chem. Soc, vol. xxviii, 

 p. 586, 1875. 



