Helping San Diego, California and beyond since 1997.
Do you need
volunteer, community service, work, military or court hours?
|
|
Helping San Diego, California and beyond since 1997.
|
|
Click here and add this page to your favorites!
|
Encyclopedia Britannica - Main :: HIG-HOR |
|
|
HOLOSYMMETRIC CLASS (Holohedral (ass, whole) ; Hexakis-octahedral). Crystals of this class possess the full number of elements of symmetry already mentioned above for the octahedron
cube
There are seven kinds of simple forms, viz. : Cube
Octahedron
figs
Rhombic dodecahedron (fig. 13). Bounded by twelve rhombshaped faces parallel to the six dodecahedral planes of symmetry. The angles between the normals to adjacent faces are 6o, andbetween other pairs of faces 9o; the indices are {1Io}. Garnet
dodecahedron in combination with the octahedron.In these three simple forms of the cubic system
are fixed and are the same in all crystals; in the four remaining simple forms they are variable. Triakis-octahedron (three-faced octahedron) (fig. 15). This solid is bounded by twenty-four isosceles triangles, and may be considered as an octahedron with a low triangular pyramid on each of its faces. As the inclinations of the faces may vary there is a series of these forms with the indices {221}, {331}, {3321, &c. or in general {hhk}.Icositetrahedron (fig. 17). Bounded by twenty-four trapezoidal faces, and` hence sometimes called a " trapezohedron." ' The indices are 12111, {3.11}, {322}, &c., or in general {hkk). Analcite, leucite and garnet often crystallize in the simple form {211}. Combinations are shown in figs
Tetrakis-hexahedron (four-faced cube) (figs. 21 and 22). Like the triakis-octahedron this solid is also bounded by twenty-four isosceles triangles, but here grouped in fours over the cubic faces. The two figures show how, with different inclinations of the faces, the form may vary, approximating in fig. 21 to the cube and in fig. 22 to the rhombic dodecahedron. The angles over the edges lettered A are different from the angles over the edges lettered C. Each face is parallel to one of the crystallographic axes and intercepts the two FIG. 23.Combination of others in different lengths; the in- Tetrakis-hexahedron and dices are therefore {210}, 131o}, {320}, Cube. &c., in general ihko}. Fluorspar some- times crystallizes in the simple form {310} ; more usually, however, in combination with the cube (fig. 23). Hexakis-octahedron (fig. 24). Here each face of the octahedron is replaced by six scalene triangles. so that altogether there are J (h2+k2+12) (1'+q2+r2) The angles between faces with the same indices are thus the same in all substances which crystallize in the cubic system
forty
Several examples of substances which crystallize in this class have been mentioned above under the different forms; many others might be citedfor instance, the metals iron, copper, silver, gold, platinum , lead, mercury, and the non-metallic elements silicon and phosphorus.End of Article: HOLOSYMMETRIC CLASS (Holohedral (ass, whole) If you wish, you can link directly to this article.
<a href="http://jcsm.org/StudyCenter/Encyclopedia/HIG_HOR/HOLOSYMMETRIC_CLASS_Holohedral.html"> HOLOSYMMETRIC CLASS (Holohedral (ass, whole) </a> |
|
|
(Previous) HOLOSYMMETRIC |
(Next) HOLROYD, SIR CHARLES (1861 ) |
|
Sponsored Advertisements