Rhombohedral carbonates-

The calcium ion is of such a size that it may enter 6-fold coordination to produce the rhombohedral carbonate, calcite, or it may enter 9-fold coordination to form the orthorhombic carbonate, aragonite. Cations larger than Ca2+, e.g., Sr2+, Ba2+, Pb2+, and Ra2 only form orthorhombic carbonates (at earth surface conditions) which are not, of course, isomorphous with calcite. Therefore these cations are incapable of isomorphous substitution in calcite, but may participate in isodimorphous or "forced isomorphous" substitution (21). Isodimorphous substitution occurs when an ion "adapts" to a crystal structure different from its own by occupying the lattice site of the appropriate major ion in that structure. For example, Sr2+ may substitute for Ca2 in the rhombohedral lattice of calcite even though SrC03, strontianite, forms an orthorhombic lattice. Note that the coordination of Sr2 to the carbonate groups in each of these structures is quite different. Very limited miscibility normally characterizes such substitution. 

Figure 3.12. Schematic drawings of Raman modes of vibration in rhombohedral carbonates. Notation and symmetry of the vibration and the range of Raman shift for all rhombohedral carbonates are included for each mode. Hexagonal unit cell axes are shown for orientation. (After White, 1974.)...

Barber, D. J., Wenk, H.-R. (1979). Deformation twinning in calcite, dolomite, and other rhombohedral carbonates. Phys. Chem. Minerals, 5,141-65. 

Brown, N.E., Navrotsky, A., Nord, G.L., Baneijee, S.K. (1993) Hematite (Fe203)-ilmenite (FeTiOs) solid solutions Determinations of FeTi order from magnetic properties. Am Mineral 78 941-951 Burton, B.P. (1987) Theoretical analysis of cation ordering in binary rhombohedral carbonate systems. Am Mineral 72 329-336... 

In contrast to rhombohedral carbon, rBN has been synthesized in significant amounts by several methods and is well investigated [107,108]. In contrast to... 

Fisler DK, Gale JD, Cygan RT (2000) A shell model for the simulation of rhombohedral carbonate minerals and their point defects. Amer Mineral 85 217-224 Florke RWG (1967) Die Modifikationenvon Si02. Fortschr Mineral 44 181-230... 

Duckworth, O. W., Cygan, R. T. Martin, S. T. (2004). Linear fifee energy relationships between dissolution rates and molecular modeling energies of rhombohedral carbonates. Langmuir, 20,2938-2946... 

Reeder, R.J. (1983) Crystal chemistry of the rhombohedral carbonates. In Reeder (ed.) Carbonates Mineralogy and Chemistry. Reviews in Mineralogy. Volume 11 (Washington, DC Mineralogical Society of America), pp. 1—47. 

The plastic deformation of calcite usually involves both slip and twinning both generate dislocations. Shp and twinning in rhombohedral carbonates have a preferred sense, that is, a definite direction of shear. 

Properties. Boron carbide has a rhombohedral stmcture consisting of an array of nearly regular icosahedra, each having twelve boron atoms at the vertices and three carbon atoms ia a linear chain outside the icosahedra (3,4,6,7). Thus a descriptive chemical formula would be [12075-36-4]. 

Each boron atom is bonded to five others ia the icosahedron as well as either to a carbon atom or to a boron atom ia an adjacent icosahedron. The stmcture is similar to that of rhombohedral boron (see Boron, elemental). The theoretical density for B22C2 is 2.52 g/mL. The rigid framework of... 

The sequence of sheets in graphite is also ABAB however, an examination of the atomic positions shows that they are not simply related to those in either kind of diamond. Thus the simple compression of graphite should not be expected to yield diamond. However, we11-crysta11i2ed graphite, in which the ABAB sequence extends for at least hundreds of layers, tends to form wurt2itic carbon. The rare rhombohedral form of graphite has an ABCABC sequence of sheets, but its scarcity has hindered its study as a source for diamond. 

The eommonest erystalline forms of earbon, cubie diamond and hexagonal graphite, are elassical examples of allotropy that are found in every chemistry textbook. Both diamond and graphite also exist in two minor crystallographie forms hexagonal diamond and rhombohedral graphite. To these must be added earbynes and Fullerenes, both of which are crystalline earbon forms. Fullerenes are sometimes referred to as the third allotrope of carbon. However, sinee Fullerenes were diseovered more recently than earbynes, they are... 

Recently, Tanuma and Palnichenko [24] have reported a new form of carbon which they call Carbolite formed by quenching high temperature carbon vapour onto a metal substrate. Hexagonal Carbolite I was formed from an Ar-rich gas a rhombohedral form, Carbolite II, was formed from an Ar-Hj gas mixture. 

Figure 36, P-T phase and reaction diagram of carbon as results from Refs. 509 and 510. Solid lines represent equilibrium phase boundaries. The dashed line is the threshold for conversion of hexagonal diamond and both hexagonal and rhombohedral graphite into cubic diamond.

Fig. 40 Measured (top) and simulated (bottom) XRD patterns of [Nio.7Alo.3(OH)2] (C03)o.i5-yH20 showing ordered (Vl3 x VT3)R 13.90° superlattice of carbonate anions. [The data are given as a plot, where q = 4tt sinO/X], Rhombohedral primitive cells of hydroxyl and carbonate ions are shown as solid and dashed lines respectively in the inset figure. Reprinted with permission from [263]. Copyright Elsevier Science Ltd...

Another zinc-utilizing enzyme is carbonate/dehydratase C (Kannan et al., 1972). Here, the zinc is firmly bound by three histidyl side chains and a water molecule or a hydroxyl ion (Fig. 27). The coordination is that of a distorted tetrahedron. Metals such as Cu(II), Co(Il), and Mn(ll) bind at the same site as zinc. Hg(II) also binds near, but not precisely at, this site (Kannan et al., 1972). Horse liver alcohol dehydrogenase (Schneider et al., 1983) contains two zinc sites, one catalytic and one noncatalytic. X-Ray studies showed that the catalytic Zn(II), bound tetrahedrally to two cysteines, one histidine, and water (or hydroxyl), can be replaced by Co(II) and that the tetrahedral geometry is maintained. This is also true with Ni(Il). Insulin also binds zinc (Adams etai, 1969 Bordas etal., 1983) and forms rhombohedral 2Zn insulin crystals. The coordination of the zinc consists of three symmetry-related histidines (from BIO) and three symmetry-related water molecules. These give an octahedral complex... 

Calcite is a calcium carbonate, CaCOs, with a rhombohedral cell consisting of the large planar CO3 groups, which contain a Ca ion at the center of an... 

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