# rhombohedron

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Related to rhombohedrons: Scalenohedron

## rhombohedron

a six-sided prism whose sides are parallelograms

## Rhombohedron

a parallelepiped (usually oblique) whose faces are equal rhombuses. For at least two vertices all the adjacent angles must be equal. A parallelepiped is a rhombohedron if and only if it has an axis of three-fold symmetry.

## rhombohedron

[¦räm·bō¦hē·drən]
(crystallography)
A trigonal crystal form that is a parallelepiped, the six identical faces being rhombs. Also known as rhomb.
(mathematics)
A prism with six parallelogram faces.
References in periodicals archive ?
The model in Figure 19 illustrates the general features of the calcite rhombohedron of six sides and eight corners, shown here truncated with small planar surfaces and with the c axis vertical, in the plane of the page.
Similarly, the degree of transparency is never as high in Calcite-II as in Calcite-I; however, the larger rhombohedrons of Calcite-II (more than 2 cm) may be transparent.
Crystal habits range from elongated hexagonal prisms with low-angle rhombohedron terminations to prism and scalenohedron combinations in varying proportions.
According to Kostov (1968) and Moroshkin and Frishman (2001), the temperature of crystallization of various calcite forms proceeds from the {10[bar.1]1} rhombohedron at high temperature to the {[bar.1][bar.1]20} prism, {21[bar.3]1} scalenohedron, and lastly the {02[bar.1]1} steep rhombohedron at low temperature.
Not only could Hauy cleave a calcite scalenohedron down until the rhombohedron emerged, but he could also derive the scalenohedral shape by stacking up tiny rhombohedrons in parallel (Hauy, 1801).
The distinctive thing is that starry microcrystals of pyrite form perfectly straight bands, some wide, some narrow, along the outside edges of the calcite rhombohedrons, or following striations across their faces.
Smaller, deep red crystals display hexagonal prisms with low-angle rhombohedrons; larger crystals are commonly rounded and blackish.
Most commonly calcite forms tightly intergrown coatings of flat rhombohedrons, in many cases twinned, covering vast surfaces of sulfides (sphalerite, galena, pyrrhotite, pyrite) which are thus nearly totally hidden.
In most specimens of "normal" quartz which collectors are used to seeing, the prismatic crystals (whether single or twinned) rise from the matrix with the long crystallographic c axis more or less vertical, and are terminated by two alternating but usually unequal rhombohedrons. Gwindels, by contrast, are aggregate forms in which a bottom or "root" crystal lies horizontally on its prism edge (i.e.
Calcite is encountered very frequently in the Fengjiashan mine, a first generation occurring as squat rhombohedrons and a second generation as steep scalenohedrons.
The calcite crystals found in association with fluorite in the Xianghuapu mine are highly flattened rhombohedrons to 15 cm across.

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