Crystal calcite

Haul, RA.W. and Stein, L.H. 1955 Diffusion in calcite crystals on the basis of isotope exchange with carbon dioxide. Faraday Society Transactions 51 1280-1290. 

Spherical vaterite crystals were obtained with 4-mercaptobenzoic acid protected gold nanoparticles as the nucleation template by the carbonate diffusion method [51]. The crystallization of calcium carbonate in the absence of the 4-MBA capped gold nanoparticles resulted in calcite crystals. This indicates that the polymorphs of CaCOj were controlled by the acid-terminated gold nanoparticles. This result indicates that the rigid carboxylic acid structures can play a role in initiating the nucleation of vaterite as in the case of the G4.5 PAMAM dendrimer described above. 

Figure 10. SEM photographs of polished, etched thin sections of fossil Acropora palmata coral (after Edwards 1988). The scale bar in a is 10 microns, a depicts sample AFS-12, a last interglacial coral from Barbados. The crystal morphology in this well-preserved sample is indistingnishable from that of a modem sample (see Fig. 9b). The scale bar in b is 100 microns, b depicts sample PB-5B, a fossil coral collected from North Point Shelf on Barbados. The crystal morphology of this sample shows clear evidence of alteration, inclnding a large calcite crystal filling in a macroscopic pore (dark area in npper right portion of photograph).

Kendall AC, Broughton PL (1978) Origin of fabrics in speleothems composed of columnar calcite crystals. J Sediment Petrol 48 519-538... 

Sonawane S, Shirsath S, Khanna P, Pawar S, Mahajan C, Paithankar V, Shinde V, Kapadnis C (2008) An innovative method for effective micro mixing of CO2 gas during synthesis of nano calcite crystal using sonochemical carbonization. Chem Eng J 143(1—3) 308—313... 

Uranium is distributed uniformly within individual growth layers, but varies greatly in concentration between successive layers (A, B, and C). The distribution of tracks suggests that U is distributed homogeneously in single calcite crystals, and is not concentrated at grain boundaries or in inclusions. 

The last two assumptions may not be true under actual well conditions, where instantaneous boiling and turbulent flow in the pipe occur. However, these conditions do not allow certain calcite crystals to deposit immediately and fractions may be removed by the fluid. 

The improvement of enzyme like MIP is currently another area of intense research. Beside the use of the MIP themselves as catalysts, they may also be applied as enhancer of product yield in bio-transformation processes. In an exemplary condensation of Z-L-aspartic acid with L-phenylalanine methyl ester to Z-aspartame, the enzyme thermolysin was used as catalyst. In order to shift the equilibrium towards product formation, a product imprinted MIP was added. By adsorbing specifically the freshly generated product from the reaction mixture, the MIP helped to increase product formation by 40% [130]. MIP can also be used to support a physical process. Copolymers of 6-methacrylamidohexanoic acid and DVB generated in the presence of calcite were investigated with respect to promotion of the nucleation of calcite. Figure 19 (left) shows the polymer surface with imprints from the calcite crystals. When employing these polymers in an aqueous solution of Ca2+ and CO2 the enhanced formation of rhombohedral calcite crystals was observed see Fig. 19 (right) [131]. 

Figure 17. Plot of Li isotopic composition vs. temperature of growth for synthetic calcite crystallized from a solution containing Li from L-SVEC (Marriott et al. 2004). The results are most consistent with temperature not being a significant control on mass fractionation of Li during crystallization from aqueous solution, thus essentially eliminating Li isotopes as a paleotemperature proxy in marine carbonates.

Let us equilibrate some pure calcite crystals with an aqueous phase at T = 25 °C and P = bar. Calcite partially dissolves to reach the solubility product (eq. 8.77). 

Calcite crystals grow at the expense of aragonite Crystals grow Other minerals nucleate and grow Bubbles grow volume of bubbly magma expands rapidly... 

Calcite crystals coarsen Solid state reactions and coarsening of crystals Bubbles coalesce or fragment into explosive emption... 

The [hkil] faces, which determine the dog-tooth (scalenohedral) Habitus of calcite crystals, and the prismatic [hkil] face of tourmaline crystals show striations only, never step patterns. These faces are S faces, by PBC analysis, and they appear due to a pile up of steps developing on the neighboring F faces. Yet they develop as large as those that determine the Habitus. 

Figure 7.10. (a) Sketches and (b) photograph of parallel growth due to preferential nucleation at the corners and along the edges of existing calcite crystals (ref. [3], Chapter 11). Note the Habitus change between earlier and later formed crystals. 

Spherulite formation by geometrical selection may rarely be seen on crystals with isotropic Habitus. Native arsenic. As, occurs in a confeito-like form, and is a type of spherulite grown through the geometrical selection of rhombohedral crystals. Spherical aggregation of calcite crystals with nail-head Habitus is also observed. Semi-spherical aggregates of platy barite crystals known as desert rose are shown in Fig. 8.6. 

Figure 11.4 shows the various Habitus of calcite crystals observed in nature. To demonstrate Habitus changes at different orders of crystallization, or those depending on crystallization temperatures, three examples of intergrowths of two different Habitus of calcite are shown in Fig. 11.5 [3], see also Fig. 7.10. Figure 11.5(a) shows that later-grown calcite crystals on earlier-formed prismatic crystals of calcite show a different prismatic to a dog-tooth Habitus from the host crystal. 

Figure 11.4. Various Habitus observed for natural calcite crystals C 0001, M 1010, ...

Figure 11.5(b) shows that the later-formed calcite crystal has a different Habitus from the earlier-formed prismatic crystal around which it grows, and Fig. 11.5(c) is an example of the growth of a crystal with dog-tooth Habitus on an earlier-formed crystal with rhombohedral Habitus. (See also Fig. 7.10.) By compiling these relations, it is possible to trace systematically how the Habitus of calcite changes from earlier to later stages or as the temperature decreases in the case of contact metasomatism [2], [3]. The Habitus variation with decreasing temperature summarized above is a general trend based on data of this type.

Although calcite crystals exhibit a great variety of Habitus change and many crystal faces are known, the crystal faces maybe grouped as follows. 

Figure 11.5. Three examples showing different Habitus of later-grown calcite crystals preferentially nucleated on the edges and corners of earlier-formed calcite crystal. Changes in Habitus depending on crystallization stages or growth temperatures are indicated [3]. (a) Earlier-formed hexagonal prism (A) and later-formed scalenohedral crystal (B). (b) Earlier-formed hexagonal prism (b) and later-formed thicker crystal (a), (c) The shaded area shows an earlier-formed rhombohedral crystal, and the remaining area represents later-formed scalenohedral crystals.

Coccolith, an exo-skeleton of coccolithophores, consists of calcite crystals of uniform size showing a most unusual morphology, which resembles a trug. The structure of the exo-skeleton consists of about thirty calcite crystals of equal size, which are regularly aligned and conjugated. This unusual form of calcite crystals (shown in Fig. 14.5) has stimulated particular interest, and many studies have been conducted on this structure [8]. 

Calcite crystals with this unusual morphology are formed in the cell called the Golgi complex, and are expelled continuously from the cell to form a circular... 

---