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Encyclopedia Britannica



PYRAMIDAL CLASS (Hemimorphic)

This article appears in Volume V07, Page 578 of the Encyclopedia Britannica.

Encyclopedia Britannica - Main :: PRE-PYR
PYRAMIDAL CLASS (Hemimorphic) .
Here there is only one dyad
axis
  in which two planes of symmetry intersect. The crystals are usually so placed that the dyad
axis
  coincides with the
vertical
  crystallographic axis, and the planes of symmetry are also
vertical
 .
The pyramid {hkl} has only four faces at one end or other of the crystal. The macro-prism and the brachy-prism of the last class are here represented by the macro-
dome
  and brachy-
dome
  respectively so called because of the resemblance of the pair of equally sloped faces to the roof of a house. The form {ool }. is a single plane at the top of the crystal, and is called a " pedion "; the parallel 'pedion (ooi} , if present at the
lower
  end of the crystal, constitutes a different form. The prisms {hko} and the macro- and brachy-pinacoids are geometrically the same in this class as in the last. Crystals of this class are therefore differently developed at the two ends and are said to be "
hemimorphic
 ."
Fig. 6o shows a crystal of the
mineral
 
hemimorphite
  (H2Zn2SiO3) which is a combination of the brachy-pinacoid {o101 and a prism,
with the pedion (ooi), two brachy-domes and two macro-domes at the upper end, and a pyramid at the
lower
  end. Examples of other substances belonging to this class are struvite (NH4MgPO4.6H20), bertrandite (H2Be4Si2O3), resorcin, and picric acid.


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