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Copper Minerals

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Vol. 5 J
Louderback.—Benitoite.
359
individuals are large—two to four centimeters long—with prism faces 6 to 10 mm. broad, and show a combination of the unit prismand unit pyramid. The faces are somewhat
dull, but they show a very perfect cleavage, yielding lustrous surfaces. A distinct zonal structure is present, appearing to the eye as layers of different degrees of translueency lying parallel to the faces of the external form. These prisms show straight extinction and in a section perpendicular to prism edges an acute positive bisectrix, the axial plane bisecting the cleavage at the obtuse angle, that is, It does not seem probable that
these crystals formed under the same conditions as the natrolite matrix of the titano-silicates. The peculiarity of the prevailing habit may be considered as dependent on the unusual composi­tion of the solutions giving rise to the benitoite and neptunite. No crystals of benitoite or neptunite were found with or near the group showing this prismatic habit. Plate 35, figure 1, is from a photograph of one of these prismatic groups.
A chemical analysis of the pure white benitoite matrix was made by Professor Blasdale showing it to be pure natrolite.
A somewhat careful search failed to show the presence of even traces of titanium, barium, manganese, or potassium.
COPPER MINERALS.
In some parts of the deposit the copper sulphide chalcocite is not infrequent. It occurs enclosed in the natrolite in dark gray or black anhedral grains several millimeters in diameter. Occa­sionally on fractured surface it shows distinct and Characteristic crystal outline, occurring then as a flat table with the trace of a hexagon elongated parallel to one pair of sides. It is to be con­sidered an original mineral of the veins.
In the same part of the deposit is found the copper silicate chrysocolla. It occurs in small quantity as a stain or thin coating
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