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TETARTOHEDRAL

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Originally appearing in Volume V07, Page 576 of the 1911 Encyclopedia Britannica.
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TETARTOHEDRAL CLASS (See also:

Tetrahedral pentagonal dodecahedral). Here, in addition to four polar triad axes, the only other elements of symmetry are three dyad axes, which coincide with the crystallo- i From a)6ycos, placed sideways, referring to the See also:absence of planes and centre of symmetry. 2 From yipos, a See also:ring or See also:spiral, and tlSos, See also:form.graphic axes. Six of the See also:simple forms, the See also:cube, See also:tetrahedron, rhombic See also:dodecahedron, deltoid dodecahedron, triakis-tetrahedron and pentagonal dodecahedron, are geometrically the same in this class as in either the tetrahedral or pyritohedral classes. The See also:general form is the Tetrahedral pentagonal dodecahedron (fig. 41). This is bounded by twelve irregular pentagons, and is a tetartohedral or See also:quarter-faced form of the hexakis-See also:octahedron. Four such forms may be derived, the indices of which are {hkl), {khll, 1hkl) and (khl); the first pair are enantiomorphous with respect to one another, and so are the last pair. See also:Barium nitrate, See also:lead nitrate, See also:sodium chlorate and sodium bromate crystallize in this class, as also do the minerals ullmannite (NiSbS) and langbeinite (K2Mg2(SO4)2). 2. TETRAGONAL SYSTEl1 (Pyramidal; Quadratic; Dimetric). In this See also:system the three crystallographic axes are all at right angles, but while two are equal in length and interchangeable the third is of a different length.

The unequal See also:

axis is spoken of as the See also:principal axis or morphological axis of the crystal, and it is always placed in a See also:vertical position ; in five of the seven classes of this system it coincides with the single tetrad axis of symmetry. The parameters are a: a: c, where a refers to the two equal hori- zontal axes, and c to the vertical axis; c may be either shorter (as in fig. 42) or longer (fig. 43) than a. The ratio a: c is spoken of as the axial ratio of a crystal, and it is dependent on the angles between the faces. In all crystals of the same substance this ratio is See also:constant, and is characteristic of the substance; for other substances crystallizing in the tetragonal system it will be different. For example, in cassiterite it is given as a : c =I: 0.67232 or simply as c =0.67232, a being unity; and in See also:anatase as c =1.7771.

End of Article: TETARTOHEDRAL

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