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HOLOHEDRAL

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

Bipyramidal). Here there are three dissimilar dyad axes of symmetry, each coinciding with a crystallographic See also:axis; perpendicular to them are three dissimilar planes of symmetry; there is also a centre of symmetry. There See also:ape seven kinds of See also:simple forms: Bipyramid (See also:figs. 54 and 55). This is the See also:general See also: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 See also: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 See also:combination. See also:Sulphur often crystallizes in simple bipyramids. See also:Prism. This consists of four faces parallel to the See also:vertical axis and intercepting the See also: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 See also: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 See also:

pail of parallel faces perpendicular to the vertical axis; the indices are See also:loot). The macro-pinacoid{See also: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. See also:Hypersthene. Holohedral Orthorhombic Combinations.

Examples of substances crystallizing in this class are extremely numerous; amongst minerals are sulphur, See also:

stibnite, See also:cerussite, See also:chrysoberyl, See also:topaz, See also:olivine, See also:nitre, See also:barytes, See also:columbite and many others; and amongst artificial products See also:iodine, See also:potassium permanganate, potassium sulphate, See also:benzene, See also:barium formate, &c.

End of Article: HOLOHEDRAL

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