Zircon polymorphism

Zircon, complete solid-solution behavior is observed, and a plot of the unit cell volume against x shows that Vdgard s Law is followed. When the end members are not is structural, a systematic change in the solubility range in both structures is found as A is varied, and the data have been systematized in terms of a simple, potentially predictive, structure-field map. The pervasive polymorphism of these ABO4 compounds, involving both reconstructive and displacive transformations and metastable structures produced by different sample preparation methods, indicates that the crystal structural stability of substituted compounds needs to be carefully evaluated as a function of temperature to assess the structural integrity of waste-form materials. 

Cubic stabilized zirconia (CSZ) Pure zirconia (Zr02) is either chemically extracted and purified from the mineral zircon (ZrSi04) or purified from baddeleyite. It occurs as three crystalline polymorphs with monoclinic, tetragonal and cubic structures. The monoclinic form is stable up to 1170°C... 

Zircon-type superstructures apparently occur in actinide arsenates as a result of cation ordering when both monovalent alkali metals and tetravalent actinides occupy polyhedral sites (Table 3). Cation ordering is also a factor in the orthorhombic symmetry of the low-temperature polymorph of CaU(P04)2 [41], shown in Figure 8, and which is closely related to the zircon structure. 

BM(P04). The existence of polymorphic modifications is t5q)ical for Th and U phosphates BM(P04)2. Their crystallization led to formation of monoclinic phases with the monazite structure t5q)e. The minerals brabantite (CaTh(P04)2) and huttonite (ThSi04) belong to the monazite structure t5q)e as well. Tetragonal phases have the zircon (ZrSi04) structure t5q)e [10, 53,65-70,71],... 

Figure 34 The Raman spectra of a zircon microinclusioii ZrSi04 (b) inside an intaglio composed of the chrysoprase variety of a-quartz (a) the extra band close to 500 cm" represents the copresence of minor moganite, which is another polymorph of Si02. (After Ref. 61.)...

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