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PYRAMIDAL CLASS (Hemimorphic)

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Originally appearing in Volume V07, Page 578 of the 1911 Encyclopedia Britannica.
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PYRAMIDAL CLASS (See also:

Hemimorphic) . Here there is only one dyad See also:axis in which two planes of symmetry intersect. The crystals are usually so placed that the dyad axis coincides with the See also:vertical crystallographic axis, and the planes of symmetry are also vertical. The See also:pyramid {hkl} has only four faces at one end or other of the crystal. The macro-See also:prism and the brachy-prism of the last class are here represented by the macro-See also:dome and brachy-dome respectively so called because of the resemblance of the pair of equally sloped faces to the roof of a See also:house. The See also:form {ool }. is a single See also:plane at the See also:top of the crystal, and is called a " pedion "; the parallel 'pedion (ooi} , if See also:present at the See also: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 See also:developed at the two ends and are said to be " hemimorphic." Fig. 6o shows a crystal of the See also:mineral See also:hemimorphite (H2Zn2SiO3) which is a See also: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), See also:resorcin, and picric See also:acid.

End of Article: PYRAMIDAL CLASS (Hemimorphic)

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