Louderback, G. D. "Benitoite, Its Paragenesis and Mode of Occurrence." University of California Publications.
Page 344
Habit The most common habits are distinctly trigonal and pyramidal. Although prism faces are almost universally present they are always comparatively small, and no approach to a prismatic habit has yet been observed. The largest crystal yet seen is about two and a half inches across, but is of unusual size. The majority are less than one inch across. The negative pyramid generally shows the largest faces, p commonly not reaching to c, and thus giving a triangular outline to the basal plane. The prisms appear as narrow bands. The basal plane may more rarely be absent, and p is then quite small. This gives the two habits shown in figures 1 and 2, plate 37. The positive pyramids reach the basal plane in about 20 per cent. of the crystals and then a hexagonal outline is produced, the edges generally being distinctly in two alternate groups (pl. 37, fig. 3). Only one crystal w as seen wherein was produced a pseudohexagonal symmetry. A peculiar habit that has been found in a few crystals is produced by a predominant basal plane, the crystal being very thin, and, if growing from one side, has the appearance of an orthorhombic table.

Only one crystal was found not showing prism faces. The positive prism m may be narrower than the negative prism as in plate 38, figure 1, and in about 3 per cent. of the crystals examined it was absent, as in plate 38, figure 4. On the other hand, it may be considerably broader than the latter, even when its corresponding pyramid p is smaller than the negative pyramid ¼, as in figure 2.

The form r (1012), has been found on considerably less than half of the crystals in which p does not reach the basal plane, and then always as a narrow truncation of the ¼ edges. This is shown in figure 4, plate 37, together with x (101910) which is here represented as complete for the crystal, although actually it is not found in all the sectants, and occurs only on a few crystals. In a certain number of crystals the place of r is then taken by a strip of horizontal striations or narrow planes due to oscillatory growth. The striations are shown in plate 38, figure 3, and in figure 4 is represented a set of coarser oscillations showing the origin of the striations as alternations of c and p.

The prism of the second order a (1121) was observed in

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