bipyramid

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bipyramid

[¦bī¦pir·ə‚mid]
(crystallography)
A crystal having the form of two pyramids that meet at a plane of symmetry. Also known as dipyramid.
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This conjecture is best possible in the following sense: The k-fold direct sum of skew bipyramids over [P.sub.6] yields a smooth Fano polytope of dimension d = 3k with 8k = 3d - k vertices, but it has no copy of P6 as a direct summand.
The powellite crystals are loose, complete floaters, with white sprigs of scolecite protruding a millimeter or so here and there, and they are razor-sharp, textbook examples of tetragonal bipyramids, with c axes about 1.25 times as long as a and b.
Guyot-Sionnest, "Mechanism of silver(I)-assisted growth of gold nanorods and bipyramids," Journal of Physical Chemistry B, vol.
I also purchased several sapphire crystals from this locality; they are medium to pale blue, very slightly stream-rounded, hexagonal bipyramids. The rubies are also generally hexagonal bipyramids, although I purchased one very sharp, red, flattened tabular crystal.
These include mainly silver and gold nanocubes, gold nanostars, silver nanotriangles, gold bipyramids and gold nanocrescents.
The most common forms are dominant {111} and {[bar.1]11} bipyramids, modified by minor {001}, {010} and {100} pinacoids.
Pale blue-green crystals can be found on euhedral microcline where they form trigonal prisms and bipyramids. Aggregates of pale yellow phenakite crystals up to 10 cm in diameter have been found where beryl has been noticeably leached, providing the source of Be (Ramdohr, 1940a).
Less familiar and much much less common are the exceptional specimens of anatase from the same locality--Herb Obodda (who should know, and who had the best anatase specimens from the place) gave Taftan (a town), near Dallbandi, Baluchistan, as the locality's name, though the term given on many other dealers' labels for the brookite is "Kharan." I'll try to get this cleared up, but meanwhile I invite you to imagine the wondrous anatase crystals in a handful of specimens at Herb Obodda's stand at the Main Show: they are highly lustrous, lightly horizontally striated bipyramids with small basal faces on both ends.
Forms include a first-order and second-order prism, two first-order bipyramids, one second order bipyramid and the basal pinacoid.
The glittering white drusy quartz crusts do not disguise the sharp forms of the former tetragonal bipyramids of apophyllite; as before, the clusters reach cabinet size, with individual pseudocrystals to 4 cm.
Mirror-faced and brilliantly lustrous, smoky blue to maroonish gray scheelite bipyramids rest lightly on a pristine white matrix of drusy dolomite.
These include well-crystallized rods with a hexagonal cross section (elongated {110} dodecahedra), pseudohexagonal bipyramids and pseudoscalenohedra (the faces of both correspond to peculiarly developed {210} tetrahexahedra).