Calcite crystalline structure

The problem with limited selectivity includes some of the minerals which are problems for XRD illite, muscovite, smectites and mixed-layer clays. Poor crystallinity creates problems with both XRD and FTIR. The IR spectrum of an amorphous material lacks sharp distinguishing features but retains spectral intensity in the regions typical of its composition. The X-ray diffraction pattern shows low intensity relative to well-defined crystalline structures. The major problem for IR is selectivity for XRD it is sensitivity. In an interlaboratory FTIR comparison (7), two laboratories gave similar results for kaolinite, calcite, and illite, but substantially different results for montmorillonite and quartz. 

Polymorph Two or more solid crystalline substances with the same chemistry, but different crystalline structures. Calcite (rhombohedral CaCCE) and aragonite (orthorhombic CaCCE) are polymorphs. 

Figure 2(C) is the result of a calculation that illustrates which cations might fit into sixfold coordination position in the calcite group structures. It is an interesting insight as both light and heavy rare earth elements are possible substitutes for calcium in the calcium carbonate structure, i.e., they plot within 15% of the calcium ionic size. However, some of the end-members incorporate elements into this crystal structure and are outside this deviation but within 30%, an expression of the potential physical expansion for this layered crystal structure. These are ionic charge differences important in whether a stable crystalline structure can be produced. Trace amounts of all these ions can be incorporated in calcite and may dictate the morphology of the crystallites. Therefore, the presence and amount of any ions in the environment in which carbonate crystallization occurs may possibly be recorded. However, in spite of the predominance of sodium and potassium in the solutions where...

Aragonite is one of the less abundant crystalline (rhombic) forms of calcium carbonate. It slowly recrystallises to calcite (hexagonal structure) in the presence of water. 

Crystalline structure of calcite (Iceland spar, CaCOf). Contrary to simple Lewis structures, all three C-0 bond lengths in COf are equal. (Adapted from J. 

Nontreated calcite powder, but having a special crystalline structure, is produced by Dixie Lite and Stone Co. The company s bulletin mentions that this is a better filler than treated calcite powder for PVC pipes. 

Fig. 16 SEM image of the convex-concave calcite (a) and its proposed formation mechanisms (b). The primary blocks assemble to give flat, pseudo-symmetric mesocrystal structures. When a certain size is exceeded, not only primary platelets, but also amorphous intermediates (spheres) are attracted. By recrystallization of those species, bent crystalline structures without translational order can develop. Reprinted with permission from Wiley-VCH. ...

The packing arrangement of atoms or molecules in a crystalline solid phase is generally not unique, and for organic molecules in particular, it is common for two or more crystalline forms of the same substance to exist. The most familiar example in elemental terms is Graphite and Diamond. Both are composed entirely of the element Carbon, however their ciystal structures are very different, and so too are their physical properties. Calcium Carbonate is another common example with three polymorphic forms Calcite, Aragonite and Vaterite. 

Cleavage, occurs when a crystalline substance breaks along planes that are parallel to crystal faces or structural planes (diamond, fluorite, calcite). 

The octahedron also is adopted in the known carbonates249 (Table 22). MnC03, although thermodynamically unstable in 02 or air, decomposes only slowly, and the pink, almost white, compound occurs naturally as the mineral rhodochrosite. Only one crystalline form is known which has the calcite structure and both CaMn(C03)2 and BaMn(C03)2 have the dolomite structure. The carbonate mineral sidorenkite (Na3MnC03P04) has a bidentate carbonate group, which is somewhat unusual in inorganic structures. 

An additional possible reason for differences in behaviour is that CaCOj exists in three crystalline forms. Calcite is the most stable and aragonite and vaterite undergo transitions to calcite at 728 K and between 623 and 673 K, respectively. Consequently, at decomposition temperatures (above 900 K) the reactant CaCOj would normally be in the calcite structure [2]. Salvador et al. [3] concluded that the... 

Table 2.4b lists the vibrational frequencies of planar X03-type compounds. Figure 2.13 shows the Raman spectra of KNO3 in the crystalline state and in aqueous solution. As discussed in Sec 1.26, the spectra of calcite and aragonite are markedly different because of the difference in crystal structure. More recent normal coordinate calculations [713] on the CO3 radical indicate a trans-Cs... 

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