Rhombohedron

Rauten-flach, n., -flachner, m. rhombohedron. -flache, /. rhombus. 

The crystal structures of hematite and corundum have been determined through the use of Taue and spectral photographs, interpreted with the aid of the theory of space groups. The unit of structure is a rhombohedron with a = 55° 17 and a = 5.420 = = 0.010 A. for hematite, and with a = 55° 17 and a = 5.120 = = 0.010 A. for corundum. The space group underlying the atomic arrangement is D. ... 

ACs. Moreover, the glue atoms beyond triacontahedra, in fact, center prolate rhombohedron (a cube elongated along a threefold axis), the recognized interstitial cavity among primitive cubic packed triacontahedra (also shown in Fig. 18). 

Rhombic prism lattice, 8 114t Rhombohedral structure, of ferroelectric crystals, 11 95, 96 Rhombohedron lattice, 8 114t Rhomboidal symmetry, 8 114t Rhone-Poulenc process, 24 482, 485 Rhovanil extra pure vanillin, 25 548t, 549-550... 

Fig. 77. Large (32-molecule) unit rhombohedron of calcite based on cleavage rhomb. The true unit cell is the small (2-molecule) steep rhombohedron shown inside.

Suppose we have rhombohedral indices hR kR lR and wish to transform them to hexagonal indices hH IcH iHlH. The c axis of the hexagonal cell is 00 (through the middle of the rhombohedron). Pass from O to O, first of all directly the phase-difference between waves from O and those from O js In. Now go from O to O by way of rhombohedral axial directions-—for instance, via OD, DK, and KO. Waves from D are hR wavelengths ahead of those from 0, those from K are k.R wavelengths ahead of those from D, and those from O are lR wavelengths ahead of those from K. The total is h R- rk R -l r- Thus lH = hRA-kRA lR ... 

Certain trigonal crystals may also be referred to rhombohedral axes, the unit cell beaig a rhombohedron defined by cell edge a and irteraxial angle a (j 90°)... 

ALUNITE. The mineral alunite, KAl3(S04)2(0H)t , is a basic hydrous sulfate of aluminum and potassium a variety called natroalunite is rich in soda. Alunite crystallizes in the hexagonal system and forms rhombohedrons with small angles, hence resembling cubes. It may be in fibrous or tabular forms, or massive. Hardness, 3.5-4 sp gr, 2.58-2.75 luster, vitreous to pearly streak white transparent to opaque brittle color, white to grayish or reddish,... 

QUARTZ, [CAS 14808-60-7]. The mineral quartz, oxide of the nonmctallic element silicon, is the commonest of minerals, and appears in a greater number of forms than any other. Its formula is SiC>2. Quartz commonly occurs in prismatic hexagonal crystals terminated by a pyramid. This pyramid is due to the equal development of two rhombohedrons, and may be observed in cases where one rhombohedron predominates. Cleavage is not observed the fracture is typically conchoidal, hardness is 7 specific gravity, 2.65 luster, vitreous to greasy or dull colorless to white, pink, purple, yellow, blue, green, smoky brown to nearly black transparent to opaque. 

On the (111) crystal face of an fee metal the (2x2) unit cell is a rhombohedron while the c(4x2) primitive unit cell is a rectangle. 

Figure 7.19. A packing drawing showing part of the unit rhombohedron of [Ni(H20)6][SnCl6]. Sn atoms are black and three of them are shown as [SnClg]2 octahedra. The [Ni(H20)6]2+ octahedron is at the center (H atoms are not shown).

The Seven Systems of Crystals are shown in Figure 2.2. The relationship between the trigonal and rhombohedral systems is shown in Figure B.la. The possibilities of body-centered and base-centered cells give the 14 Bravais Lattices, also shown in Figure 2.2. A face-centered cubic (fee) cell can be represented as a 60° rhombohedron, as shown in Figure B.lb. The fee cell is used because it shows the high symmetry of the cube. 

Oblate and prolate rhombohedrons that can be combined to form three-dimensional tiling necessary for a quasicrystal. 

For the discrete molecule [Ag8(C2)(CF3C02)6(L1)6] (L1 = 4-hydroxyquinoline) of 3 symmetry displayed in Fig. 20.4.26(c), the encapsulated acetylenediide dianion in therhombohedral Ags core is disordered about a crystallographic threefold axis that bisects the C=C bond and passes through two opposite corners of the rhombohedron. Apart from the 2-O, O trifluoroacetate... 

Figure 1.1 Top Quartz crystal exhibiting the true symmetry of the crystal class to which quartz belongs. Bottom The forms comprising such a quartz crystal. From left to right, the hexagonal prism, trigonal dipyramid, rhombohedron, and trigonal trapezohedron.

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