dipyramid


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dipyramid

[dī′pir·ə‚mid]
(crystallography)
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.
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The habit consists of the tetragonal dipyramid capped by the basal pinacoid, and ranges from rather flattened (dominant c face) to steep dipyramids, all with excellent luster and sharpness.
The milarite crystal is a simple, lightly striated hexagonal prism with a basal pinacoid termination and very small faces of a single dipyramid; besides being stunningly large, the crystal is stunningly beautiful: sharp, highly lustrous, and translucent pale yellow.
Faces of the trigonal dipyramid s{1121} are often present, but faces of trigonal trapezohedra such as x{5161} have not been observed.
The mineral, at first thought to be a form of barytocalcite, is trimorphous with bary-tocalcite and paralstonite, and forms white to colorless or faintly pink acute pseudohexagonal pyramids or dipyramids up to 6 mm long.
The mineral is trimorphous with barytocalcite and paralstonite, and forms white to colorless or faintly pink acute pseudohexagonal pyramids or dipyramids up to 6 mm long.
The other 40% of the zircon grains are stubby to slightly elongate, euhedral, clear, colourless multifaceted dipyramids with an average length to breadth ratio of 2.
The specimens show green and yellow color-zoned, discoidal crystals (low-angle hexagonal dipyramids) to 3 cm pleasingly set on feldspar matrix.
By new-new I mean that the wulfenite crystals are simple, flat-topped tetragonal prisms with a length-to-width ratio of around 2:1, although there is also a minority of the spiky dipyramids. The very sharp, silkily lustrous, butterscotch-orange wulfenite crystals from the new-new find reach 2 cm, and occur scattered thickly on the dark brown rusty matrix.
His summary of the typical habit is as follows: For rhyolitic topaz, a fairly large basal pedion {001} with two equally developed prisms (third order, {110} and {120}, two dipyramids typically {h11} and {112},two first-order prisms typically {021} and {hkl} and one second-order prism).
Tabular crystals show the following forms, in descending order of dominance: {001} and {010} pinacoids, {101} and {011} prisms, and small {322} dipyramids. Prismatic crystals show major {001} and minor {010} pinacoids, prominent {011} and {012} prisms, terminated by {21l} dipyramids (usually etched).
Thin-tabular, platy hematite crystals are common at many localities; these crystals generally have large, wide {0001} faces rimmed by positive and negative rhombohedrons {1010} and {0111}, in some cases also by first-order and second-order prisms {1010} and {1120} and/or dipyramids {1123}--the little "rims" outboard from the basal expanses, when wide enough to show faces at all, are commonly quite complex.
Morphology: no forms were identified, but equant "rhombic dipyramids" are mentioned (see Comments).