Aragonite, structure

Aragonite structure KNOi 0.70 CaC03 0.67 SrCO,. 75 BaCO,. 87 LaBOj 0.79... 

EuBOz. — Rare earth borates of the type MBOs, where M = Sm—Lu and Y, are isostructural with vaterite ( x—CaCOs). The lattice constants [325] of EuBOs are a = 3.842 and c = 8.937 A. Lanthanum borate, however, crystallizes in the aragonite structure [306]. Europium borate and the above mentioned borates can be obtained by the reaction of the metal oxide and B2O3 in an equimolar ratio at 1200—1400° C in air. 

An equally important carbonate mineral is aragonite, the common polymorph of calcite. Polymorphism of minerals implies the same chemical composition but distinct crystal structure. The aragonite structure also has alternate layers of carbonate groups and cations with the... 

Some substitution of strontium (up to 14 mol.%), of lead (2 mol.% reported) but no barium has been reported in aragonite, although investigations at elevated temperatures and pressures show almost complete miscibility of these elements in the structure (Gaines et al., 1997, p. 442), and SrCOs (strontionite), BaCOs (witherite), and PbCOs (cerussite) are common minerals. A calculated plot (Figure 3(b)) for cations in ninefold coordination shows that this coordination theoretically allows trivalent rare earth elements and quadravalent and many other elements to be substituents in the structure. Ytterbium, europium, samarium, and radium carbonates with aragonite structure have been synthesized (Spear, 1983). 

The calcium carbonate can precipitate either with a calcite or an aragonite structure and can be obtained with a pure white color. The average particle size is in the range 0.01 to 1 jm with specific surface areas (BET) in the range 5 to 100 m /g. The surface treatment of calcium carbonate with organic compounds is also becoming increasingly important. 

Aragonite structure KNO3 CaC03, SrC03 LaB03... 

A further example of a compound in which the geometrical conditions are favourable for dimorphism is calcium carbonate. This compound occurs naturally as both calcite and aragonite, two distinct minerals with quite different structures (see 10.10 and 10.11). The calcite structure is common to a considerable number of carbonates (and nitrates) in which the radius of the cation is less than about i-o A, but when the radius exceeds this value the aragonite structure is found. Calcium carbonate itself is dimorphous because the radius of the calcium ion (0 99 A) is close to this critical value, but dimorphism is not found in those carbonates in which the cation is either appreciably smaller or appreciably larger. The closely analogous case of the dimorphism of CdTi03 has already been described ( 8.52). 

Table 10.02. Some compounds with the calcite and aragonite structures...

The decomposition of CaC03 and of solid solutions of CaCOs and SrC03 has been investigated, using vapour-pressure techniques. The results show that the solid solutions exhibit positive deviation from ideality and adopt the aragonite structure. 

The lower limits for cationic radii are rA > 0.09 nm, and rB > 0.051 nm in the case of oxides. Goldschmidt (79), on the basis of geometric considerations, defined the tolerance limits of the size of ions through a tolerance factor t = (rA + rx)l 2(rB + rx), where rA,rB, and rx are the radii of the respective ions t would be equal to one for the ideal cubic structure (Fig. 2). In fact, the perovskite structure exists in oxides only between the limits 0.75 < t < 1.0 with t between 0.8 and 0.9 in most cases. For t > 1 the calcite and aragonite structures are prevalent, whereas for t < 0.75 the stable structure is ilmenite. Roth (20) has classified the limits of the existence of these competing structures according to the ionic radii values. 

Figure 68 A view of the aragonite structure down the c-axis.

The type structure depends on the size of the cation smaller cations lead to lower coordination numbers (as we have seen before). MgCOs has the calcite structure, whereas SrCOs has the aragonite structure. 

In crystals where B is a complex group of atoms, we see that in the sulfates and probably also in the nitrates, the ionicity is not as high. By the way, note the weak difference between the structures of calcite and aragonite. For a given metal (e.g., Ca) the charge seems less in the aragonite structure. 

If cold limewater is used the original calcium carbonate precipitated has a calcite stmc-ture hot limewater yields an aragonite structure. 

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