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Encyclopedia Britannica



HOLOHEDRAL

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

Encyclopedia Britannica - Main :: HIG-HOR
HOLOHEDRAL CLASS
(Holohedral;
Bipyramidal
 ).
Here there are three dissimilar dyad axes of symmetry, each coinciding with a crystallographic
axis
 ; perpendicular to them are three dissimilar planes of symmetry; there is also a centre of symmetry. There ape seven kinds of simple forms:
Bipyramid (
figs
 . 54 and 55). This is the general form and is bounded by eight scalene triangles ; the indices are { 111 } , 1211),
{221} 11121, {3211, 11231, &c., or in general {hkl}. The crystallographic axes join opposite corners of these pyramids and in the fundamental bipyramid 111 1} the parametral plane has the intercepts a: b: c. This is the only closed form in this class; the others are open forms and can exist only in combination.
Sulphur
  often crystallizes in simple bipyramids.
Prism. This consists of four faces parallel to the
vertical
 
axis
  and intercepting the
horizontal
  axes in the lengths a and b or in any multiples of these; the indices are therefore {110}, 12101, 11201 or {hko}.
Macro-prism. This consists of four faces parallel to the macro-
axis, and has the indices poll, {201} , . Or {hon.
Brachy-prism. This consists of four faces parallel to the brachyaxis, and has the indices loll), 10211 . {okl}. The macro- and brachy-prisms are often called " domes."
Basal pinacoid, consisting of a
pail
  of parallel faces perpendicular to the
vertical
  axis; the indices are
loot
 ). The macro-pinacoid{loo} and the brachy-pinacoid {oio} each consist of a pair of parallel faces respectively parallel to the macro- and the brachy-axis.
Figs
 . 56-58 show combinations of these six open forms, and fig. 59 a combination of the macro-pinacoid (a), brachy-pinacoid (b), a prism (m), a macro-prism (d), a brachy-prism (k),and a bipyramid (u).
Pinacoid. Hypersthene. Holohedral Orthorhombic Combinations.
Examples of substances crystallizing in this class are extremely numerous; amongst minerals are
sulphur
 , stibnite, cerussite, chrysoberyl,
topaz
 , olivine, nitre,
barytes
 , columbite and many others; and amongst artificial products iodine, potassium permanganate, potassium sulphate, benzene, barium formate, &c.


End of Article: HOLOHEDRAL


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