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FORM OF MINERALS.
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43
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but
on several grounds it is found better to designate it by R or mR, and
its complimentary figure by — mR. When the prism or pinacoid arise as
its limiting forms, they are designated by
from the limiting forms
scalenohedron
is properly the hemihedric form of the dihexagonal pyramid, but is
better derived from the inscribed rhombohedron ?«R. If the halves of
the principal axis of this are multiplied by a definite number n,
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and
then planes drawn from the extremities of this enlarged axis to the
lateral edges of the rhombohedron, as in figure 42, the scalenohedron
is constructed. Hence it is designated by
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