individuals are large—two to four centimeters long—with prism faces 6 to 10 mm. broad, and show a combination of the unit prism
and 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
