Benitoite Description

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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 as in figure 3 of this plate. This form, a, may be dull, and if bright is not so lustrous as either m or μ. The second order pyramid, dwas 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 irregularly curved. This results sometimes in the production of a wedge-like form for the prism face, the edges converging towards the right or left. The prisms are generally quite bright, and if any difference appears, m shows a better surface than Vertical striations of oscillatory growth are common on Symmetry.—The habit of benitoite is very characteristic of its trigonal symmetry, and of the presence of a plane of symmetry 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 investigation 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.

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