Vol. 5] 



Louderback. — Benitoite. 



345 



about fifteen crystals in a lot of about 500 examined. It is always 

 small and may occur without other planes of the second order, 

 as in plate 38, figure 4, but it is commonly associated with d 

 (2241) as in figure 3 of this plate. This form, a, may be dull, 

 and if bright is not so lustrous as either m or fi. The second 

 order pyramid, d (2241) was found on eleven crystals in the 

 same lot. It is always small and with luster inferior to that 

 of a. It generally accompanies a, but on two crystals showing 

 an oscillatory growth between c and p, it occurs without a. 



The basal plane is generally a very brilliant face, as also 

 the pyramid p. The negative pyramid is frequently dull, and 

 even when at its best is never as smooth and brilliant as the 

 positive. Its surface is almost always uneven and often irregu- 

 larly curved. This results sometimes in the production of a 

 wedge-like form for the prism face, the edges n A /x and w A fx 

 converging towards the right or left. The prisms are generally 

 quite bright, and if any difference appears, m shows a better 

 surface than p.. Vertical striations of oscillatory growth are 

 common on /x. 



Symmetry . — The habit of benitoite is very characteristic of 

 its trigonal symmetry, and of the presence of a plane of sym- 

 metry parallel to the base. The planes at each end of the 

 vertical axis are always similarly developed, and the identity 

 of the planes in the upper half with those vertically below are 

 clearly shown, both by geometrical development and physical 

 character. There may still be a doubt, however, as to whether the 

 symmetry is that of the trigonal-bipyramidal or the ditrigonal- 

 bipyramidal group. The simple forms that characterize the 

 benitoite crystals are common to both groups. The planes of 

 the second order occurring in like development at the end of 

 each lateral axis, and vicinal form x, which is occasionally found, 

 would indicate the group of highest symmetry. 



Etch Figures. — Etching was also resorted to in the investi- 

 gation of the symmetry. Good figures may be obtained on the 

 base and the positive unit forms by treatment with hydrofluoric 

 acid, and also with fused caustic potash, the results being sim- 

 ilar in both cases. The figures with the acid are somewhat 

 sharper and easier to handle and will be especially described. 



