348 University of California Publications. [Geology
Basal sections show a perfect uniaxial cross which gives a distinct positive reaction with the mica plate.
Color,
most commonly pale to deep blue, generally with a slightly violet tint;
transparent. Colorless crystals occur, but are more rare. The variation
in color frequently occurs on the same crystal, and the writer has many
in which part of the crystal is blue and part colorless. The transition
is sometimes gradual and irregular; sometimes it is sharp, the zones
being separated by crystallographic planes. The writer has a slab cut
parallel to the vertical axis to exhibit the pleochroism in which the
line separating the blue from the colorless portion is parallel to that
crystallographic axis. On another crystal a colorless layer 2 mm. thick
lies at the top, and is separated from the bulk of the crystal which is
blue by a plane parallel to its base. The physical properties of the
colorless material are, except for those dependent on color, the same
as those of the most highly colored ones.
An
effort has been made to determine the source of the blue color of most
of the material, but the results have so far been negative. Some of the
colorless material was carefully separated and submitted to Professor
Blasdale for chemical analysis, but it shows but slight variation (if
any) from that of the blue, as may be seen by referring to the analyses
given farther on. He also made a careful qualitative examination of a
two-gram portion, but failed to detect any appreciable amounts of any
element that might be reasonably supposed to influence the color of the
minerals. That practically all of the titanium is in the highest state
of oxidation was also shown by dissolving the material in hydrofluoric
acid in an atmosphere of carbon dioxide. A colorless solution was
obtained which failed to reduce potassium permanganate. In the
preliminary report the writer suggested that the color might be due to
a small amount of titanium in the reduced condition in solid solution
in the benitoite molecule. A comparison of the analyses of colorless
and blue samples shows that the Ti02 of the former is a half
per cent, or more less than in the latter, and if this can be accepted
as an essential difference it at least is in consonance with that view.
The violet-tinted blue of the extraordinary ray
