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Encyclopedia Britannica



SCALENOHEDRAL CLASS (Bisphenoidal-hemihedral)

This article appears in Volume V07, Page 577 of the Encyclopedia Britannica.

Encyclopedia Britannica - Main :: SAR-SCY
SCALENOHEDRAL CLASS (Bisphenoidal-hemihedral) .
Here there are only three dyad axes and two planes of symmetry, the former coinciding with the crystallographic axes and the latter bisecting the angles between the
horizontal
  pair. The dyad
axis
  of symmetry, which in this class coincides with the
principal
 
axis
  of the crystal, has certain of the characters of a tetrad axis, and is sometimes called a tetrad axis of " alternating symmetry "; a face on the upper half of the crystal if rotated through 90 about this axis and reflected across the equatorial plane falls into the position of a face on the
lower
  half of the crystal. This
kind
  of symmetry, with simultaneous rotation about an axis and reflection across a plane, is also called " composite symmetry."
In this"class all except two of the simple forms are geometrically the same as in the holosymmetric class.
Bisphenoid (orpirv, a wedge) (fig. 50). This is a
double
  wedge-shaped solid bounded by four equal isosceles triangles; it has the indices lilt), 12111, 11121, &c., or in general 1hhll. By suppressing either one or other set of alternate faces of the tetragonal bipyramid of the first order (fig. 42) two bisphenoids are derived, in the
Fig. 53 shows a combination of a tetragonal prism of the first order with a tetragonal bipyramid of the third order and the basal pinacoid, and represents a crystal of fergusonite.
Scheelite
  (q.v.), scapolite (q.v.), and
erythrite
  (C4Hio04) also crystallize in this class.


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