Vol. 5] 



Louderback. — 



Benitoite. 



347 



c axis; occasionally minute lozenge-shaped figures may be seen 

 in certain light, symmetrical with respect to the edges fi A m 

 and c A hi. 



The figures on the basal plane show conclusively the trigonal 

 character, and those on the prisms the presence of the horizontal 

 plane of symmetry. It is also evident that three planes of sym- 

 metry pass through the vertical axis and bisect the pyramidal 

 and prismatic faces, fixing the symmetry as that of the ditri- 

 gonal-bi pyramidal group, and making benitoite the type of the 

 highest grade of trigonal symmetry. 



Natural Etching. — A large number of the crystals of beni- 

 toite show natural etching. The negative pyramids commonly 

 show various degrees of dulling. This sometimes produces very 

 striking results, especially when the crystal shows a large de- 

 velopment of these planes with the other planes quite small, 

 when the bulk of the surface may be very dull and even cov- 

 ered by a film of decomposition, in the midst of which the 

 small faces may appear clear and brilliant. Triangular figures 

 on the basal plane occasionally occur, sometimes as depressions 

 when the triangle is in reversed position with respect to the 

 triangle of w A c, sometimes elevations when they are placed 

 in the same attitude as triangle of jtAc, Occasionally hex- 

 agonal figures occur outlined by grooves parallel to the basal 

 edges. 



Physical Properties. 



Hardness 6]-6^; distinctly above orthoclase and labradorite, 

 and below chrysolite and quartz. Density 3.64-3.67. The high- 

 est value obtained was on a flawless gem stone of moderately 

 deep color weighing 1.53243 grams, and giving a value of 3.667. 

 The clearest obtainable colorless fragment gave 3.65. A num- 

 ber of intermediate values were obtained on clear blue material 

 and as low as 3.64. Material with flaws, cloudy white speci- 

 mens, etc., in general give lower values. 



Fracture, conchoidal to sub-conchoidal ; cleavage very im- 

 perfect, pyramidal. 



Refractive index by prism method referred to 1) line: w = 

 1.757; e about 1.804. Double refraction strong, and positive. 



