Carbon, polymorphs

Note Electronic conductivity, and even superconductivity at ca. 10 K, was observed with a new carbon polymorph, buckminsterfullerene, synthesized in 1990. Electrochemical and photoelectrochemical properties of a semiconducting diamond electrode were studied by Pleskov.)... 

Wada, N., Yamashita, K. and Umegaki, T. (1995) Effects of divalent cations upon nucleation, growth and transformation of calcium carbonate polymorphs under conditions of double diffusion. Journal of Crystal Growth, 148, 297-304. 

Equilibria are normally treated by considering calcite as the only carbonate polymorph present in the system. Actually, as already mentioned, crystal-... 

Concerning calcium carbonate polymorphs calcite and aragonite, the experiments of Romanek et al. (1992) at low T (10 to 40 °C) seem to indicate that the enrichment factor of in the solid phase for CaC03-C02 partitioning is reduced by about two units in the aragonite polymorph with respect to the calcite counterpart, r being equal (e.g., at T = 10 °C, 1000 In a 13 for calcite-C02 and 11 for... 

POLYMORPHISM. 1, A phenomenon in which a substance exhibits different forms. Dimorphic substances appear in two solid forms, whereas trimorphic exist in three, as sulfur., carbon, tin, silver iodide, and calcium carbonate. Polymorphism is usually restricted to the solid state, Polymorphs yield identical solutions and vapors (if vaporizable). The relation between them has been termed physical isomerism. See Allotropes under Chemical Elements, See also Mineralogy,... 

This chapter describes the preparation and examination of ceramic matrix composites realized by the addition of different carbon polymorphs (carbon black nanograins, graphite micrograins, carbon fibers and carbon nanotubes) to silicon nitride matrices. In the following sections, structural, morphological and mechanical characteristics of carbon-containing silicon nitride ceramics are presented. 

Wilkinson B.H., Owen R.M. and Carroll A.R. (1985) Submarine hydrothermal weathering, global eustasy and carbonate polymorphism in Phanerozoic marine oolites. J. Sediment. Petrol. 55, 171-183. 

Winland H.D. (1969) Stability of calcium carbonate polymorphs in warm, shallow seawater. J. Sediment. Petrol. 39, 1579-1587. 

Another important point about calcium carbonate biomineralization is that more than one of the three polymorphs may be present in close proximity within an individual sample and each may be a different calcium carbonate polymorph (mollusks provide such an example), or the mineral may change over time. In many species the larval stage is aragonitic, while the adult biomineral is calcite. The different minerals in biomineralized tissues play specific roles in the proper functioning of the organism. 

Carbon atoms crystallize in several forms. Graphite and diamond are well known carbon polymorphs. Fullerenes, which were discovered in the 1980 s, have also been well characterized. Carbon materials show a variety of different physical and chemical properties. Because of this the electronic structure of carbon materials has been investigated using a number of different experimental techniques, for example, XPS, UPS and XANES. Theoretical studies of carbon materials have been also performed. However, experimentally observed spectra are not always consistent with theoretical predictions. Recently, in order to understand the various kinds of observed electronic spectra, DV-Xa calculations have been performed on a small cluster model. [1] In the present paper, we report results of DV-Xa calculations performed on the carbon materials graphite, alkali graphite intercalation compounds (GIC), fullerene, and fluorinated fullerenes. 

Yuen et al. determined atomic site charges on carbonate anion by fitting the crystal structures of calcium carbonate polymorphs calcite and aragonite. They obtained c = 0-95(26) and qo = -0.98(9). The 6-31G PD values are = 1-210(11) and qo = - 1.070(4), which are narrowly within the confidence limits of the crystal structure derived charges. Hirshfeld and Mirsky found qQ= - 0.410 e from the observed quadrupole moment of carbon dioxide, in good agreement with the PD value. 

Table 5. Calcium carbonate polymorphs infrared bands...

Corrales LR (1999) Dissociative model of water clusters. J Chemical Physics 110 9071-9080 Curtiss LA, Halley JW, Hautman J, Rahman A (1987) Nonadditivity of ab-initio pair potentials for molecular dynamics of multivalent transition metal ions in water. J Chem Phys 86 2319-2327 de Leeuw NH, Parker SC, Catlow CRA, Price GD (2000) Proton-containing defects at forsterite (010) tilt grain boundaries and stepped surfaces. Am Min 85 1143-1154 de Leeuw NH, Parker SC (1998) Surface stracture and morphology of calcium carbonate polymorphs calcite, aragonite, andvaterite An atomistic approach. JPhys ChemB 102 2914-2922... 

Katzke H, Bismayer U, Toledano P (2006) Reconstructive phase transitions between carbon polymorphs limit states emd periodic order-parameters. J Phys Condens Matter 18 5129-5134... 

The element carbon is detected in abundance in the universe, in the sun, stars, comets, and in the atmosphere of the planets. It is the fourth most abundant element in the solar system, after hydrogen, helium, and oxygen, and is found mostly in the form of hydrocarbons and other compounds. The spontaneous generation of fullerene molecules may also play an important role in the process of stellar dust formation. l Carbon polymorphs, such as microscopic diamond and /onsda/e/te, a form similarto diamond, have been discovered in some meteorites (see Ch. 11).t i... 

While organic chemistry is not a subject of this book, it cannot be overlooked since organic compounds play a major part in the processing of carbon polymorphs. Some examples of organic precursors are shown in Table 2.1.PI... 

The characteristics and properties of the single carbon atom were described in the preceding sections. This section is a review of the ways carbon atoms bond together to form solids, such as diamond, graphite, and other carbon polymorphs. 

The band positions shift with environment and are in different positions in calcium carbonate (polymorphs calcite and aragonite), dolomite (calcium magnesium carbonate), sodium carbonate, and so on, alone or in aqueous solutions. 

But see T. Ferroir, L. Dubrovinsky, A. El Goresy, A. Srmiono-vici, T. Nakamura and P. Gillet (2010) Earth Plant. Sc. Lett., vol. 290, p. 150 - Carbon polymorphism in shocked meteorites Evidence for new natural ultrahard phases . 

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