The
following consideration decided the writer in the choice of the
positive unit form and seems to him to indicate its more fundamental
character than the complementary pyramid of the same parameters. In
growth the positive pyramid produces more perfect planes and more
brilliant faces, the negative pyramid showing most commonly uneven,
curved, influenced and otherwise less perfect forms even when it is
areally about equal to the positive form. In the attack of corrosive
agents, the positive pyramid is much more resistant than the negative.
In concentrated hydrofluoric acid the negative faces immediately
become dull and are rapidly corroded, the positive planes remain bright
and show the production of small well-formed etch figures. The positive
faces must eventually be attacked over their whole surface but this
was not observed during the progress of the experiment which lasted at
least two hundred times and more as long as it took to entirely destroy
the original surface of the negative pyramids. That this same relative
resistance of the positive planes exists under very varying conditions
is shown by the facts that it was observed (1) in the natural
weathering process, (2) in hydrofluoric acid, both hot and cold,
concentrated and dilute, and (3) in fused caustic potash.
Goldschmidt and Wright26
in their work on that form-rich mineral, calcite, found that the more
fundamental planes give the best etch figures, and suggest it as a
possible general method for their determination. In benitoite there is
a very marked superiority in this respect of the form selected as
positive by the writer over the corresponding negative form.
Altogether, then, the different lines of evidence are consistent and
definite and indicate the writer's positive unit form as the more
fundamental.
Refractive index of benitoite. Hlawatsch has also determined the refractive index of benitoite by the prism method (loc. cit., p. 301) with results almost identical with the writer's.
m Neues Jahrb. für Mineral., etc. (1903), Beilage-Band 17. p. 365.
