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152 University of California Publications. [Geology
the standpoint of its chemical composition. Professor Blasdale, who kindly undertook the chemical analysis, reports:
The suggested formula is BaTiSi.,0,,, which yields the following calculated values:
Professor
Blasdale also reports that the mineral is easily 'decomposed by
hydrofluoric acid, but only slowly attacked by molten potassium
pyrosulphate.
Benitoite
is then a very acid titano-silicate of barium, and stands in a class by
itself, both as regards acid silicates and titano-silicates. The
possibility of the titanium acting as a base was considered, but the
summation of the analyses and the fact that the crystals are often
perfectly colorless seem to point definitely to the above
interpretation. The blue color of much of the material may be due to a
small amount of titanium in the sesquioxide condition.
Associated
with benitoite is a black or brownish black prismatic mineral that
also appears to be new. Its most striking characteristic is a very
perfect prismatic cleavage of 80° 10'. Its hardness is between 5 and 6
and it melts easily, at about 1.5, giving a sodium flame and becoming a
lustrous black enamel bead. It appears to be monoclinic, is biaxial,
and gives an extinction angle of about 10 degrees on the cleavage
face. Its cross sections are six-sided, the four cleavage traces being
truncated by a lateral pinacoid. In thick pieces it is opaque, but in
moderately thin ones it is a deep rich red. which changes to a
brownish or ocreous yellow as the thickness decreases. Pleochroism is
prominent. On a cleavage plate the ray vibrating near the prism
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