Aragonite crystalline structure

The devices are further claimed to ensure that magnetically treated water keeps minerals in a soft amorphous powder form instead of the usual hard crystalline structure. This amorphous powder deposits a thin film of aragonite talc on the inside of pipes and the waterside of condensers which prevents free oxygen in the water from attacking the metal surfaces. ... 

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

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. 

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... 

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. 

Quite original is the attempt to obtain porous materials, for example, from crystalline calcium carbonate (aragonite) similar to the natural material chalk of a certain porosity [192]. Another attempt was made to synthesize macro-porous aragonite with a structure similar to the cocco-spheres of certain marine algae [295]. For this purpose, oil-water-surfactant microemulsions supersaturated with calcium bicarbonate were obtained. The pore size was determined by the water and oil concentration ratio. Microemulsions were applied on the substrate of micrometersized polystyrene beads. Hollow spherical shells of finished structure were produced as a result of a rapid mineralisation. The authors suggest that such materials could gain widespread use in materitils chemistry. 

Figure 2.14. Different crystalline forms of calcium carbonate. Courtesy of Omya/Pliiss-Staufer AG (micrographs of crystals), Solvay, GmbH, Rheinberg, Germany (crystal structure and micrographs of Socal trigonal-scalenohedral calcite),and ECC International Ltd., St, Amtell, UK (rhombohedral calcite and aragonite).

The many forms of calcium carbonate described clockwise from top). An abalone shell is composed of thin overlapping layers of calcium carbonate, interspersed with a protein (a class of macromolecules described in Chapter 22). Limestone is a mixture of calcium carbonate and other compounds found in sedimentary rocks. Aragonite is one of the crystalline solid forms of calcium carbonate, characterized by the arrangement of carbonate ions in two planes that point in opposite directions. Blackboard chalk is often incorrectly believed to be made from the mineral chalk, which is a form of calcium carbonate, but, in fact, it is made from calcium sulfate. Iceland spar is the clear form of the mineral calcite, which is calcium carbonate arranged in a different crystal structure from that of aragonite. 

A phase transition occurs when a pure component changes from one phase to another. Table 6.1 lists the different types of phase transitions, most of which should already be familiar to you. There are also phase transitions between different solid forms of a chemical component, which is a characteristic called polymorphism. For example, elemental carbon exists as graphite or diamond, and the conditions for phase transitions between the two forms are well known. Solid H2O can actually exist as at least six structurally different solids, depending on the temperature and pressure. We say that water has at least six polymorphs. (In application to elements, we use the word allotrope instead of polymorph. Graphite and diamond are two allotropes of the element carbon.) In mineral form, calcium carbonate exists either as aragonite or calcite, depending on the crystalline form of the solid. 

A similar structure of biogenic origin is nacre, which forms the inner shells of some molluscs. Nacre is a composite material of aragonite, a crystalline modification... 

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