Crystallization calcite

While CaC03 crystals (calcite and aragonite) predominantly appear in egg shells and in biomineralisates from invertebrates, calcium phosphates are predominantly involved in processes which play an inportant role in medicine. They will be described here in detail the knowledge of their structures are most relevant for the understanding of the cellular and molecular processes in bones and teeth. 

Not all isomorphous substances form mixed crystals. Calcite (CaC03) and sodium nitrate (NaN03) form similar atomic arrangements, their unit cells are both rhombohedra of very similar dimensions, and also the corresponding ions are closely similar in size but they do not form... 

Figure 1.13. Dark-field TEM images of single-crystal calcite shocked to 85 GPa, displaying (a) the crossing of multiple twins, a large number of perfect dislocations and (b) numerous partial dislocations decorating the twin planes, (c) Secondary electron image of compacted calcite powder shocked to 85 GPa. The recovered specimen is composed of numerous foamy aggregates containing bubbles, voids, and crater-shaped objects.

Using the techniques described above, we have performed systematic shock experiments on single-crystal calcite as well as on chemically pure calcite powder in the pressure range from 12.5 to 100 GPa. The shock front propagated parallel to the (1014) plane of single-crystal calcite. The calcite powder was compacted to pellets with a porosity on the order of 5 %, leading to higher shock and postshock temperatures than with the otherwise identical experimental setup. 

Figure 8.2 Scanning electron micrographs of the base-plates in the sea-urchin cidnris The images (at various magnifications) reveal cubic symmetry within the (single crystal) calcite network. The turmels in these images are of the order of 1 pm in diameter. Note the loss of meso-crysttdlinity towards the surface of the shell (top left image). (These pictures have been kindly provided by Hans-Udde Nissen.)...

FIGURE 5.10. Some crystals, each with their optical indicatrices drawn within them, (a) A uniaxial negative crystal (calcite) showing orientation of the indicatrix. (b) A uniaxial positive crystal (quartz) showing orientation of the indicatrix. 

In line with the other shock experiments, compact single-crystal calcite shocked to 85 GPa contains basically two types of lattice defects numerous twins and curved dislocations (Fig. 1.13a). The activated twins possess r-type... 

Fig. 13.1 Examples of nrai-polar, piezoelectric and pyroelectric crystals calcite (a), ZnS (b) and tourmalme (c). An arrow shows the direction of the polar axis in tourmaline...

Temperature variations of the reduced hyperfine tensor components of the AsO molecule-ion in single-crystal calcite [69Ma3]. 

It has also been reported that the second derivative DTA is more useful to examine the products formed during the autoclaving of cement-quartz-metakaolin mixtures. Klimesch and Rayt ] subjected a mixture of quartz (38.5%) and cement (61.5%) containing different amounts of metakaolin and autoclaved them for 8 hrs at 180°C. It was found that the second derivative differential thermal curve provided a more detailed information, particularly in temperatures of 800-1000°C. In Fig. 35, DTA and second derivative curves for cement-quartz-metakaolin pastes are compared. The exotherms occur at 840, 903, and 960°C due to the formation of wollastonite from C-S-H, aluminum-substituted tobermorite, and anorthite from the hydrogamet residue respectively. The small endot-herm at 828°C preceding the first exotherm is probably caused by well crystallized calcite. 

Texture. All limestones are crystalline, but there is tremendous variance in the size, uniformity, and arrangement of their crystal lattices. The crystals of the minerals calcite, magnesite, and dolomite are rhombohedral those of aragonite are orthorhombic. The crystals of chalk and of most quick and hydrated limes are so minute that these products appear amorphous, but high powered microscopy proves them to be cryptocrystalline. Hydrated lime is invariably a white, fluffy powder of micrometer and submicrometer particle size. Commercial quicklime is used in lump, pebble, ground, and pulverized forms. 

A significant advantage of the PLM is in the differentiation and recognition of various forms of the same chemical substance polymorphic forms, eg, brookite, mtile, and anatase, three forms of titanium dioxide calcite, aragonite and vaterite, all forms of calcium carbonate Eorms I, II, III, and IV of HMX (a high explosive), etc. This is an important appHcation because most elements and compounds possess different crystal forms with very different physical properties. PLM is the only instmment mandated by the U.S. Environmental Protection Agency (EPA) for the detection and identification of the six forms of asbestos (qv) and other fibers in bulk samples. 

The commercial grades of calcium carbonate from natural sources are either calcite, aragonite, or sedimentary chalk. In most precipitated grades aragonite is the predominant crystal stmcture. The essential properties of the two common crystal stmctures are shown in Table 1. 

Nancollas, G.H. and Reddy, M.M., 1971. The crystallization of calcium caronate. II Calcite growth mechanism. Journal of Colloid and Interfacial Science, 37, 824-833. 

Spanos, N. and Koutsoukos, P.G., 1998. The transformation of vaterite to calcite effect of the conditions of the solutions in contact with the mineral phase. Journal of Crystal Growth, 191, 783-790. 

We say then that a crystal is satisfactory for purposes of chemical analysis if the beam it reflects is monochromatic within the limits established by the collimating system. As theory shows,15 some broadening is to be expected on Bragg reflection even from perfect crystals, but this broadening is so small (not over 0.001°) that we need not consider it. Relatively few crystals, notably some diamonds and calcites, approach perfection. Sodium chloride, more useful in x-ray spectrog-raphy, broadens monochromatic x-rays appreciably, but the. total broadening can be smaller than 0.30°,16 the collimator a perture. See Figure 4-9. 

Catalytic devices These nonmagnetic devices use a perforated non-ferrous tube to encourage small calcite seed crystals to form and reduce the risk of bulk water scaling. They are promoted for use in hard waters under conditions where supersaturation can easily occur. 

Additionally, for previously scaled systems, it is claimed that as new calcite seed crystals flow away from scaled sites, a shift in CaC03 equilibrium occurs, resulting in a continuous descaling action. 

The tannins influence calcium salt crystal structures to produce the easily dispersible calcite (cubic structure) rather than the scaleforming aragonite (rhombic structure). 

Calcium salt crystal structures producing calcite 406... 

Basu and Searcy [736] have applied the torsion—effusion and torsion— Langmuir techniques, referred to above for calcite decomposition [121], to the comparable reaction of BaC03, which had not been studied previously. The reaction rate at the (001) faces of single crystals was constant up to a product layer thickness of 1 mm. The magnitude of E (225.9 kJ mole-1) was appreciably less than the enthalpy of the reaction (252.1 kJ mole-1). This observation, unique for carbonates, led to the conclusion that the slowest step in BaC03 vacuum decomposition at 1160—1210 K is diffusion of one of the reaction components in a condensed phase or a surface reaction of C02 prior to desorption. 

Even higher temperatures are required for calcite dissociation. As f>co2 is increased to 760 Torr, the reaction temperature rises to 1170 K and the extent of dissociation is diminished [29]. The rate of decomposition of dolomite in vacuum [734] was intermediate between those for magnesite and calcite. Ranges of study were magnesite 810—870 K, dolomite 910— 990 K, and calcite 990—1050 K. Values of E were in the different sequence, magnesite < calcite < dolomite. Magnesite, which would decompose very rapidly at the temperature of dolomite dissociation, is, therefore, relatively stabilized, whereas the reactivity of calcite is enhanced in the mixed crystal. 

A unit cell for the calcite structure can be found, on the Web site for this book. From this structure, determine (a) the crystal system and (b) the number of formula units present in the unit cell. 

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