Ulexite has been known since about 1840, but only as the fine fibers known as “cotton-ball,” or more recently as massive prismatic aggregates, but never in definite crystals. Accordingly, little has been known of its crystal character. Hintze1 states that it is possibly monoclinic, but says that nothing definite is known of its crystallography. The suggestion of monoclinic symmetry is based on the work of Buttgenbach,2 who studied minute flakes under the microscope. The flakes were sometimes bounded by edges making angles of 45° and 70° with their elongation. Assuming these edges to represent faces normal to the flakes, he suggested that the mineral may occur in crystals which are platy parallel to (010), and bounded by a monoclinic combination of (100) (101) (103). Other flakes were nearly rectangular in outline, and suggested orthorhombic symmetry. He determined extinction as always parallel to the elongation, which was positive. This agrees with Des Cloizeaux,3 who considered the extinction as nearly or exactly parallel to elongation. Larsen and Berman4 made the following determinations of optical properties: Biaxial positive; indices, α= 1.491, β= 1.504, γ=1.520; X = b; Y∧c = 23°—0°; 2V moderate. They give the specific gravity as 1.65 and hardness = 1, (erroneous, because determined on “cotton-ball”). Foshag5 found massive fibrous material at Lang, California, and gives an analysis of it. Schaller,6 with similar material from the Kramer district of California, determined indices in close agreement with Larsen and Berman. He found Y to be approximately parallel to the elongation, the angle Y∧c variable, and about 20°. Elongation was usually negative, though occasionally positive. His material showed universal and intimate polysynthetic twinning parallel to the elongation. Specific gravity of this massive material is 1.963, a much more accurate value than could be obtained from the “cotton-ball.” Many reliable chemical analyses have fixed the chemical composition as NaCaB5O9 · 8H2O.

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