Calcite properties

Hardness. The hardness (qv), or related property abrasiveness, is an important filler property. Hardness is determined by comparison to materials of known hardness on the Mohs scale. On this nonlinear scale, diamond is rated 10, quartz 7, calcite 3, and talc 1. The abrasiveness of a filler is also dependent on psd and the presence of impurities, eg, ka olin clay (Mohs hardness of 3) can be quite abrasive because of the presence of quartz impurities. 

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. 

Efflorescence. The solvent properties of water also causes efflorescence, a phenomenon whereby soluble or slightly soluble substances migrate from the interior of porous solids to the surface, where they precipitate. Efflorescence is an important factor in the decay and disintegration of many rocks, and of human-made porous materials such as ceramics, and even of some types of glass. On archaeological objects, efflorescence generally occurs mostly as a white, powdery, but sometimes consolidated accretion on the surface of the objects. Calcite, a form of calcium carbonate, is one of the most common substances to effloresce on archaeological ceramics. 

In arid and semi-arid soils, calcite, dolomite, leonhardite (Ca2Al4Si8024.7H20) and lawsonite (CaAl2Si208.2H20) can be possible minerals. Calcium carbonate strongly influences soil properties in arid and semi-arid soils. Most calcareous soils have soil a pH in the range of 7.3-8 5. When sodium is predominant in soils, soil pH is above 8.5. In most arid and semi-arid soils, calcium carbonates (calcite and dolomite) generally accumulate and are most likely to control the Ca2+ and Mg2+ solubility in these soils (Lindsay, 1979). 

In fact, the choice of CO2 fugacity has little effect on the mineralogical results of the mixing calculation. In the model, the critical property of the Fountain fluid is that it is undersaturated with respect to calcite, so that calcite dissolves when the fluid mixes into the Lyons. Because we assume equilibrium with dolomite and magnesite, the saturation index (log Q/K) of calcite is fixed by the reaction... 

The treatment process and flotation properties of pyrochlore are very much dependent on the gangue composition of the ore. The selective flotation of pyrochlore from carbonatite ore is not possible since calcite and dolomite have similar flotation properties as pyrochlore. In addition, in the presence of carbonates, the stable pH required for flotation of pyrochlore (i.e. 5.0-5.5) cannot be maintained. 

From disseminated ores contained in mineral lenses, the recovery of bastnaesite and monazite is accomplished using flotation. The flotation properties of bastnaesite and monazite are similar to the gangue minerals contained in the bastnaesite and monazite, such as calcite, barite, apatite, tourmaline, pyrochlore and others, which represent difficulties in selective flotation. However, in recent years, significant progress has been made in the flotation of both monazite and bastnaesite [2,3]. 

A large portion of the REOs are produced from monazite- and bastnaesite-containing ores. In the majority of cases, bastnaesite and monazite ores are relatively complex and contain gangue minerals (calcite, barite, fluorite and apatite) with similar flotation properties as the monazite and bastnaesite. 

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. 

Many processes involving carbonates - ubiquitous minerals in natural systems -are controlled by their surface properties. In particular, flotation studies on calcite have revealed the presence of a pH-variable charge and of a point of zero charge (Somasundaran and Agar, 1967). Furthermore, electrokinetic measurements have shown that Ca2+ is a charge (potential) determining cation of calcite. (Thompson and Pownall, 1989). 

Dolomite is one of the most abundant sedimentary carbonate minerals but its mode of formation and its surface properties are less well known than for most other carbonate minerals. As we have mentioned, the nucleation of dolomites and its structural ordering is extremely hindered. There is a general trend for the "ideality" of dolomite to increase with the age of dolomite over geological time (Morse and Mackenzie, 1990). Most dolomites that are currently forming in surfacial sediments and that have been synthesized in the laboratory are calcium-rich and far from perfectly ordered. Such dolomites are commonly referred to as "protodolomites . Morse and Mackenzie (1990) have reviewed extensively the geochemistry (including the surface chemistry of dolomites and Mg-calcites. 

Modifications of surface layers due to lattice substitution or adsorption of other ions present in solution may change the course of the reactions taking place at the solid/liquid interface even though the uptake may be undetectable by normal solution analytical techniques. Thus it has been shown by electrophoretic mobility measurements, (f>,7) that suspension of synthetic HAP in a solution saturated with respect to calcite displaces the isoelectric point almost 3 pH units to the value (pH = 10) found for calcite crystallites. In practice, therefore, the presence of "inert" ions may markedly influence the behavior of precipitated minerals with respect to their rates of crystallization, adsorption of foreign ions, and electrokinetic properties. 

A single substance may crystallise in more than one of seven crystal systems, all of which differ in their lattice arrangement, and exhibit not only different basic shapes but also different physical properties. A substance capable of forming more than one different crystal is said to exhibit polymorphism, and the different forms are called polymorphs. Calcium carbonate, for example, has three polymorphs — calcite (hexagonal),... 

For crystals with molecule-like constituents, like the BO, " and BO4 " groups in some borates, semi-quantitative models of the molecular component as a gas-phase entity have been proposed (Oi et al. 1989). This is conceptually similar to the approximation made for species in solution, although in practice most studies of crystals consider additional frequencies that reflect inter-molecular vibrations. The spectroscopic data on these vibrations (which typically have lower frequencies than the intra-molecular vibrations) are often available, at least approximately, from infrared and Raman spectroscopy and elastic properties. This type of hybrid molecule-in-crystal model has been applied to many minerals in theoretical studies of carbon and oxygen isotope fractionation, the most noteworthy being studies of calcite (Bottinga 1968 Chacko et al. 1991) and sihcates (Kieffer 1982). Because specfroscopic dafa are always incomplete (especially for subsfances substifufed wifh rare isolopes), some amounl of vibralional modeling is necessary. 

Gillet P, McMillan P, Schott J, Badro J, Grzechnik A (1996) Thermodynamic properties and isotopic fractionation of calcite from vibrational spectroscopy of 0-substituted calcite. Geochim Cosmochim... 

Dolomite is an alternative mineral that is used in some regions in place of cal-cite for certain applications. Dolomite is a calcium magnesium carbonate (CaCOj.MgCOj) and occurs widely in nature. Although generally similar to cal-cite in properties, it is shghtly harder (3.5), denser (2.85) and more acid resistant. Production is similar to that for calcite,but miUing is more costly and it tends to only be available at the coarser end of the size spectrum. 

The optical properties of crystals are usually quite reliable criteria for identification but occasionally crystals have sifbmicroscopic cracks and cavities, and although appearing quite normal, give refractive indices lower than those of an entirely solid crystal. This phenomenon, which is obviously very misleading, is fortunately very rare, but has been observed in anhydrite (calcium sulphate) and calcite (calcium carbonate) prepared in the laboratory. In cases erf doubt, X-ray powder photographs should be taken—see Chapter V. 

ANISOTROPIC MEDIUM. An anisotropic medium has different optical or other physical properties in different directions. Wood and calcite crystals are anisotropic, while fully annealed glass and, in general, fluids at rest are isotropic. 

Although calcium is more metallic in character than magnesium, compounds of the two elements share some similar properties. Calcium carbonate, CaC03, occurs naturally as chalk and limestone. Marble is a dense form of calcium carbonate that can be given a high polish it is often colored by impurities, most commonly iron cations (Fig. 14.26). The two most common forms of pure calcium carbonate are calcite and aragonite. All these carbonates are the fossilized remains of marine life. Calcium carbonate decomposes to calcium oxide, CaO, or quicklime, when heated ... 

---