Third polymorph

A third polymorph of CaCOs, vaterite, may form as an inorganic precipitate, but is most likely to be encountered as the mineral forming in the nacre of mollusks (Section 8.04.3.4.6). The mineral crystallizes in the hexagonal/rhombohedral space group P63/mmc, and the calcium is in sixfold coordination (Gaines et al., 1997, p. 440). It is unstable and often reverts to calcite over time. Elevated temperatures or water high in NaCl will accelerate the transition in dry environments, vaterite will slowly revert to calcite. 

Another case of non-interconverting polymorph is provided by crystalline [Fe(7] -CsH4COOH)2], which is known in three polymorphic forms, a monoclinic (form I) and a triclinic (form II) crystal form determined decades ago." A third polymorphic form III has been obtained recently.All three crystal forms are based on dimers [Fe(7] -CsH4COOH)2] 2 of doubly hydrogen-bonded carboxylic rings. The simplest way to compare the three crystal... 

A third polymorph, silk III, has also been described based on the interfacial behavior of the silkworm silk fibroin and the partitioning behavior at an air-water interface [44, 45]. Silk III, a structure stabilized at interfaces optimizes the surfactancy of the silk in the core repeats of glycine, alanine, and serine. 

Structurally, the cubic form of BN resembles diamond (Figure 6.19) and the two materials are almost equally hard. Crystalline cubic BN is called borazon and is used as an abrasive. A third polymorph of boron nitride with a wurtzite-type structure is formed by compression of the layered form at 12 kbar. 

A third polymorph, known as the y-phase, has been found for PPL crystallized by rapid cooling from the melt". From combined electron and x-ray diffraction data, the crystal structure of this phase was determined to be an orthorhombic lattice containing antiparallel 2j helices of the same chirality s. 

Polymorphism is possible for any compound, but Ihe conditions under which these polymorphs are stable may be difficult to know a priori. McCrone stated almost half a century ago that the number of forms known for a given compound is proportional to the time and money spent in research on that componnd and this statement is turning out to be truer than ever. Componnds formerly believed to exist in a single form are yielding a second or even a third polymorph. For example, novel polymorphs of aspirin, maleic acid, and 1,3,5-trinitrobenzene, have been fonnd after several decades of their first crystal structure being reported. Polymorphism is not as widespread in terms... 

Second and third polymorphs discovered after blind test... 

In a-Al203 the oxide ions form a hexagonal close-packed array, and the aluminum ions occupy octahedral holes. y-Al203 is a defect spinel structure in which there are not enough cations to occupy the usual fraction of tetrahedral and octahedral holes. (See Section 7.6, pp. 188-189 for more on spinel structures.) The AI2O3 formed on the surface of aluminum metal is a third polymorph based on the rock salt (NaCl) structure, with every third aluminum ion missing in the cubic close-packed array of oxides. (See Chapter 7 for more details on these crystal structures.)... 

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