Calcium carbonate minerals

Calcium carbonate exists in three crystal modifications, aragonite, calcite and vaterite, but only the calcite form is of real importance. Because of calcite s perfect rhombohedral cleavage, it is a soft mineral with a Mohs hardness of 3.0. It has a specific gravity of 2.7 and is birefringent having refractive indices of 1.65 and 1.48. 

Commercially viable deposits of calcite occur throughout the world. These deposits differ considerably not only in purity, but in size and genesis, and the variations affect the nature of a filler produced from them. Over the years various terms have been used to describe these different materials but the ones in general use today are chalk, limestone and marble. 

However, there are many deposits around the world of sufficient purity and close enough to the surface that products suitable for use in a wide range of applications can be obtained by a relatively simple dry-mining operation followed by grinding. Grinding can be carried out dry or in aqueous suspension depending on the exact nature of the impurities present. 

Marble deposits can be processed in the same way as chalk but, because marble contains different sized calcite crystals, or grains, a wider range of particle sized products, with wider size distributions are obtained, especially as there may be a need to fragment large crystals. Actual particle sizes will depend on the extent of processing, but typical products will have a mean value of 3-5 0,m, with 20-30 wt% finer than 2 pm and a specific surface area around 1 m g  

There are also a large number of products produced from heavily compacted limestone, which are known as ground limestones. Their average particle-size distributions will be similar to ground-marble products, while colours can range from fairly pure white to grey. All of these products, because of the rhombohedral cleavage pattern of calcite crystals, consist of (mostly) uniform calcite crystals with a fairly symmetrical aspect ratio of 3 1. 

Calcium carbonate occurs in nature in different fonns — colourless, white to coloured materials and physical state being anything from a soft amorphous mass to hard rocks  

These minerals are used for a variety of purposes—building, whitening, manufacture of lime, portland and natural cements, fertiliser, soft glass, refractory material (dolomite), as a flux in various metallurgical processes, source of CO2, railway ballast, macadam in lithography, cement concrete and asphalt concrete, in agriculture, as a solid diluent carrier in pesticides (dolomite). 

The value and suitability of tlie mineral for any particular commercial purpose depends on the physical state of the mineral and to a large extent on the amount of CaC03, MgC03 and the amounts and nature of other impurities, for which a complete analysis of the mineral will be ideal. For routine work, however, the analysis of limestone and dolomite includes the following determinations  

Impure silica or acid insoluble matter Combined oxides (AI2O3 + Fc203) 

A known weight of the powdered sample is decomposed by beating with syrupy phosphoric acid. Using a current of CO2 -free air, the CO2 evolved from the sample is led through a water absorbent (anhydrous CaS04) into weighed bulbs containing KOH solution. 

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The choice of selected raw materials is very wide, but they must provide calcium oxide (lime), iron oxide [1309-37-1/, siHca, and aluminum oxide (alumina). Examples of the calcereous (calcium oxide) sources are calcium carbonate minerals (aragonite [14791-73-2] calcite [13397-26-7] limestone [1317-65-3] or mad), seasheUs, or shale. Examples of argillaceous (siHca and alumina) sources are clays, fly ash, mad, shale, and sand. The iron oxide commonly comes from iron ore, clays, or mill scale. Some raw matedals supply more than one ingredient, and the mixture of raw matedals is a function of their chemical composition, as deterrnined by cost and availabiHty. 

Keywords. Calcium carbonate. Mineralization, Hyperbranched polymer, Supramolecular assembly. Anionic dendrimer... 

Liu Z, Dreybrodt W. Dissolution kinetics of calcium carbonate minerals in H20-C02 solutions in turbulent flow—the role of the diffusion boundary layer and the slow reaction H20 + CO2 <-> H+ + HC03. Geochim Cosmochim Acta 1997 61(14) 2879-2889. 

Arnorsson, S. 1989. Deposition of calcium carbonate minerals from geothermal waters - theoretical considerations. Geothermics, 18, 33-39. 

The only significant production process today is by carbonation of lime slurry, itself obtained by calcination and hydration of a calcium carbonate mineral. The process is set out below and it can be seen that this is another example of using a chemical method to achieve a particle size reduction ... 

Aragonite A calcium carbonate mineral (CaCCF) that is a polymorph of calcite. 

The basic properties of calcium carbonate mineral surfaces have not been extensively studied. The work of Somasundaran and Agar (1967) on the zero point of charge (ZPC) on calcite certainly has at least qualitative application to other... 

The most common cation in reservoir water is sodium (Na+). Both sodium sulfide and sodium carbonate are soluble in water and thus do not provide a mechanism for sequestering the acid gas. Perhaps the next most common cation in the reservoir is calcium (Ca2+). Carbon dioxide can react with the calcium ion and form one of many calcium carbonate minerals including calcite (CaC03). Calcium sulfide is not a very stable compound and readily decomposes and thus is not common on the earth. However, H2S can react with other cations in the reservoir water and produce several sulfide minerals including pyrite. 

Among all the minerals used by organisms, three categories are widely dominant silica, calcium phosphates, and calcium carbonates. Calcium phosphate minerals are usually associated with vertebrates , where they form bone and the dentine and enamel of teeth. However, phosphate minerals are present in some invertebrate shells, such as Brachiopods, whereas vertebrates used calcium carbonate minerals in eggshells and otoliths. Calcium minerals represent about 50% of all biogenic minerals. 

Calcium carbonate minerals (CaCOs) are ubiquitous in biomineralizing systems, since they are in the skeletons of organisms ranging from bacteria and algae to humans, most commonly as calcite (rhomboedral) or aragonite (orthorhombic). Thus, their structures and compositions are highly diversified. Here, only some examples are discussed. 

Selective extractions, chemical equilibria calculations, and crystallization measurements presented here imply that the hydrous iron oxides, even in the carbonate dominated Genesee River, play a major part in inorganic phosphorus transport by sediments in the fluvial system. Saturation levels of inorganic phosphate and calcium carbonate minerals in the Genesee River... 

The uptake and incorporation of other elements, magnesium or strontium, in the calcite have been investigated and shown the bioavailability of these elements. In modem ocean waters, the content and range of magnesium in biotic and abiotic deposited minerals were virtually identical in studies by Carpenter and Lohmann (1992). However, there was a more rapid strontium uptake in the calcium-rich biomineral. The composition of the calcium carbonate mineral deposited is a function of kinetics and related to the metabolism of the organism. 

Calcium carbonate minerals are deposited in a wide variety of bacteria, protozoa, algae, higher plants, and invertebrates (Table I). They are also formed, although not as frequently, in vertebrates. The major structural polymorphs identified in biological systems are calcite, aragonite, and vaterite, although there is also evidence for monohydrate, amorphous, and a range of Ca/Mg carbonate phases. 

Hostetler, P. B. 1964. The degree of saturation of magnesium and calcium carbonate minerals in natural waters. I.A.S.H. Comm Subterranean Waters 64 34-49. 

Carbonate minerals form from dissolved HCO3 with only a minor fractionation that is relatively insensitive to temperature. The two most important calcium carbonate minerals—calcite and aragonite— typically have values that are 0.9%o and 2.7%o enriched in 5 C relative to seawater bicarbonate at 25°C (Rubinson and Clayton 1969). The assumption that fractionation factors between seawater HCO3 and calcium carbonate minerals are invariant through time permits the use of stratigraphic variations in carbonate S C values as a proxy for secular changes in seawater 5 X values. 

With the exception of calcium carbonate, mineral fillers are inert across the papermaking pH range. Neither PCC nor GCC may be used in systans exhibiting a pH lower than 6.5, and special attention must be paid to systems in the neutral papermaking range (6.5-V.5) to ensure that the carbonate filler does not dissolve and create both foaming and calcium deposit issues. 

Calcite is primarily calcium carbonate (minerals usually contain small amounts of other compounds). Calcium carbonate has a percentage composition of 40.0% Ca, 12.0% C, and 48.0% O. Determine the weight of carbon in a 12 g sample of calcite. 

Organized to lies in protecting the inner organs from swelling in sea water, the exo- and endo-cuticle are composed of crystalline-alfa-nanoflbrils chitin (CN) associated with various proteins and hardened by a considerable amount of calcium carbonate minerals. CN is thus an insoluble linear polymer easily available in nature, organized by polysaccharide chains arranged in different planes (Eigure 37.1). 

With growing concern of environment and sustainability, Imerys has taken an initiative to evaluate the sustainability aspects of the calcium carbonate mineral. lifecycle inventory (LCI) is a key part of the International Environmental Management System standards 14044 2006. This tool is used by companies in the plastics supply chain to understand the environmental impact of raw materials critical for plastics production. LCI analysis of treated ground calcium carbonate compared to base resins was carried out since treated GCC is used to replace some of the resin volume in plastics production. The production of treated GCC was found to have a lower impact on the environment than the production of various resin systems, including PVC, PE, and PP. Raw material and energy use inputs were included in the study, as were environmental emissions to land, air, and water [11]. 

As shown in Table 8.1 more than 80% of the filler used in thermoplastic is based on calcium carbonate minerals. Most is used in PVC, with major sectors being cables, flooring, hose, plastisols, pipe, profiles and fittings. The main reason for this is firstly due to the fact that PVC has to be compounded in order for it to be used. The incorporation of stabilisers is an essential prerequisite for its successful use and therefore fillers can be... 

The mechanical properties of polymers can be considerably improved by the incorporation in their formulation of reinforcing agents or fillers. These have been used to improve or alter the mechanical properties of polymers. These include glass fiber, glass beads, calcium carbonate, minerals, mica, talc, clay, carbon fiber, carbon nanotubes, aluminum or other metal powders, silica and silicones, and others [1-16]. 

The above studies show that although there has been some work on calcium carbonate mineralization at the air-water interface, we are far from a quantitative and robust model of how interfaces control (bio)mineral formation. The situation is even more difficult for other minerals. Calcium phosphate is an equally important biomineral, and a number of studies have been done on the synthesis, structure, and properties of bio-inspired polymer/calcium phosphate materials [154]. However, there are even less data on the effect of surfaces and interfaces on calcium phosphate nucleation and growth than for calcium carbonate. 

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