Natural calcite

Analyses of natural calcites, formed at low temperatures, show MgC03 contents of up to 30 mol %. 

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

Di Benedetto, F., Costagliola, P., Benvenuti, M. et al. (2006) Arsenic incorporation in natural calcite lattice evidence from electron spin echo spectroscopy. Earth and Planetary Science Letters, 246(3-4), 458-65. 

Ishikawa and Ichikuni (1984) also conducted a study of the coprecipitation of sodium with calcite. They suggested that Na+ is located at interstitial sites in the calcite lattice, and good agreement was observed between their predicted Na concentrations and those in natural calcites for waters up to about a salinity of 10. At higher salinities the natural samples generally had higher Na concentrations than... 

In this equation, the rate constant k and the exponent were observed to vary for different samples of natural calcite. Here, fi = exp(AG/R7) and the equation can be rewritten as... 

MgC03 (magnesite), FeC03 (siderite). The basic similarity of structure allows interchangeability of cations between end-member minerals. For example, most natural calcite has a measurable amount of both Mg2+ and Fe2+ substituted for some of the Ca2+. The amount of isomorphous substitution is shown by the following notation, (Ca0.85Mgo.iFe0.o5)C03. In other words, 85% of the Ca2+ sites are occupied by Ca2+, 10% of the Ca2+ sites are occupied by Mg2+, and 5% of the Ca2+ sites are occupied by Fe2+. The complex chemistry of freshwater and... 

For powders of rock samples quartz offers itself as a natural standard, especially because quartz is a rather pure compound with approximately constant lattice constants (Table 4.10). Natural calcite often contains Mg in solid solution with decreased lattice constants and is less suited as standard. Furthermore, calcite exhibits a negative temperature coefficient in the u-direction. 

Mishra studied effects of addition of calcium and carbonates and phosphates (concentrations on the order of 10" mol dm ) on the C(pH) curves for natural calcite and apatite [246]. Usually the presence of calcium induced a shift in to more positive and presence of carbonates and phosphates to more negative value, i.e. according to expectations, but over certain pH ranges there was no effect or even the effect was opposite to the expected one. The electrokinetic behavior of apatite in calcite supernatant was similar to that of calcite in water, and the electrokinetic behavior of calcite in apatite supernatant was similar to that of apatite in water [247]. The potentials of calcite and apatite were positive and rather pH insensitive in the presence of >10" mol dm" of calcium, and negative and rather pH insensitive in the presence of >10 mol dm of phosphate, and the effect of carbonates on potentials of apatite was negligible. 

Johnston, Merwin, and WiQiamson Amer.J, Sci. 1916 [4], 16, 504) have pointed out that whereas natural calcite usually occurs in a very pure state, natural aragonite is contaminated by various impurities, such as the carbonates of lead, strontium, etc. 

Dickson JAD (1991) Disequilibrium carbon and oxygen isotope variations in natural calcite. Nature 353 842-844... 

Svensson, U. and Dreybrodt, W., 1992. Dissolution kinetics of natural calcite minerals in C02-water systems approaching calcite equilibrium. Chemical Geology, 100 129-145. 

Kelly S, Newville M. Cheng L, Kenmer K, Sutton S, Fenter P, Sturchio NC (2002) Uranyl incorporation in natural calcite. Environmental Science and Technology (in review)... 

A rather curious observation was made in 1670 by Barthohnus, who discovered that when a beam of ordinary light passes through certain crystals, such as natural calcite or Icelandic spar, each ray splits into two. When he passed the two rays through a second crystal, he observed that the effect depends on the orientation... 

Jacobs, G.K., Kerrick, D.M., and Krupka, K.M. (1981) The high- temperature heat capacity of natural calcite (CaCOs), Phys. Chem. Minerals 7, 55-59. 

If natural calcite crystals are used, then a decomposition curve of the type shown in Fig. 8-13 is observed. The details of the curve will depend upon the deviation of the CO2 partial pressure from the equilibrium value. Such a decomposition curve with a reproducible incubation period indicates that the rate is controlled by the nucleation and growth of oxide particles. 

Chemical diffusion of the rare-earth element was measured in natural calcite under anhydrous conditions, using rare-earth carbonate powders as the sources. Experiments were run in sealed silica capsules together with finely ground calcite to ensure stability of the single-crystal samples during diffusion anneals. Rutherford back-scattering spectroscopy was used to measure diffusion profiles. The Arrhenius relationship at 600 to 850C was ... 

Chemical diffusion of the rare-earth element was measured in natural calcite... 

Chemical diffusion of Sr was measured in natural calcite under anhydrous conditions. The source of diffusants for the diffusion experiments was SrC03 powder. 

It is important to understand that plane-polarized light can be produced both naturally (calcite and quartz crystals) and synthetically (with aligned polymers [Chapter 6]), and when such polarized light passes through chiral ( handed ) materials (or solutions of chiral materials), the plane of polarization is rotated. Historically, this property has been called optical rotation, and the chiral material was said to be optically active. Indeed, experimentally, the rotation of the plane of plane-polarized light demonstrates optical activity and thus, that the material is chiral. 

UV-phot./ Natural calcite (CaCOj) crystal EPR/ 77 2.0027 ) H 50.26 70Ko3... 

Matveeva 1987). The natural calcite in our study consisted of 30 samples from a... 

BenstockE, BuseckP, Steele 1 (1997) Cathodoluminescence of meteoritic and synthetic forsterite at 296 and 77 K using TEM. Am Mineral 82 310-315 Blasse G (1980) Luminescence and energy transfer. Struct Bond 42 1 1 Blasse G, Aguilar M (1984) Luminescence of natural calcite (CaC03). J Lumin 29 239-241 Bokii G, Bezrykov G, Khyev Yu et al (1986) Natural and synthetic diamonds. Nauka, Moscow (in Russian)... 

Gaft M, Nagli L, Panczer G et al (2008a) The nature of unusual luminescence of natural calcite CaCOj. Am Miner 93 158-167... 

Unfavorable combinations and/or choice of chalk (natural calcite, precipitated cal-cite/aragonite) may cause discoloration in the form of yellow spots in chalk-filled PVC [685]. The formation of yellow spots is attributed to oxalic acid (salts) detected by FTIR-spectroscopy. The creation of oxalic acid (salts) depends on the presence of calcium carbonate. The type of filler and stabilization determine the rate of calcium oxalate creation [685]. 

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