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CHEMISTRY.-Crystals of barium disilicate in optical glass. N. L. BOWEN, Geophysical Laboratory. (Communicated by J. C. Hostetter.)

In optical glass of the variety known as "barium crown,' and especially in those types rich in barium, there frequently form in the melting furnace numerous six-sided crystal plates

FIG. 1. Fragment of barium crown glass showing crystals of BaSiOs with "frayed" edges. (Natural size.)

upwards of 3 mm. in diameter. These crystals are colorless and transparent in their central portions but are surrounded by white opaque rims that render them very conspicuous (see photograph, fig. 1). A piece of glass containing these crystals, with their

marked symmetry of outline and their common arrangement with greater dimensions parallel to flow lines in the glass, constitutes a specimen of much beauty and perhaps of some interest to the mineralogist, though nothing could be more ruinous to the glass for optical purposes than this incipient crystallization. Even glass which comes from the melting furnace free from this defect may devitrify during subsequent heat treatment with formation of crystals of the same nature, though in this case of much smaller dimensions. One step in an investigation designed to discover the best conditions for avoiding the formation of these crystals involved a determination of their nature.

X

Z

FIG. 2. BaSiOs showing optical orientation.

in

Under the microscope the larger crystal plates that form in the melting furnace are found to be about 0.03 mm. thick in the transparent central portion, which is a single crystalline unit of uniform orientation. Around the edges of the larger crystal, however, numerable tiny crystals, each identical in nature and in habit with the larger crystal, have sprouted out in all directions. These tiny crystals with their interstitial glass, giving diffusion of light, constitute the white opaque rims of the larger crystals. The arrangement recalls the feldspar microlites with fibrous edges sometimes seen in rocks.' The crystal plates have the shape of an elongated hexagon as shown in figure 2, and are about 3 mm. long and 2 mm. wide. The terminal angles as measured under the microscope are approximately 100° and the lateral angles 130°. As shown by their symmetry, taken together with their optical properties, the crystals are orthorhombic. There is a good cleavage parallel to the elongation. The elongation is always negative. The plane of the optic axes is parallel to the platy development and the optical character negative with 2V = 70° approximately. The refractive indices are: y = 1.613 and a 1.595, as measured in immersion liquids under the microscope.

=

1 Figured by J. P. Iddings in Rock Minerals, p. 215, 1911.