Abstract
An alternative description of zirconolite, zirkelite, pyrochlore and polymignyte as modular structures is proposed. This approach not only emphasizes their crystallographic similarities but is also readily extended to include “defects” which were observed by high resolution electron microscopy. Each module consists of two layers of octahedrally coordinated cations (Ti, Ta, Nb) arranged as an hexagonal tungsten bronze (HTB) type motif. The interstice formed by the six-membered octahedral ring is occupied by a REE/ACT atom in 8-fold coordination or a transition metal atom accommodated statistically in either tetrahedral or trigonal bipyramidal sites. Interlayer cations are in cubic (Ca, Na, REE, ACT), monocapped octahedral (Zr) or octahedral (Ti, Ta, Nb) coordination. Every HTB layer is displaced with respect to those immediately adjacent by an interlayer stacking vector whose magnitude and direction may be varied to derive the various structures. Thus the structures may be regarded as polytypic; the simplest member (i.e., the aristotype) is zirconolite. Because the HTB layers possess (pseudo) hexagonal symmetry subsequent modules are related to each other by —nπ/3 rotations. The description is conveniently simplified by considering the cation net of each HTB layer, the nodes of which correspond to the kagomé net. Polymignyte is composed of slightly different modular units in which the layer cations are arranged as intergrowths of the kagomé net and a distorted triangular net.
The justification for this approach was confirmed by examining zirconolite, doped with Mg, Al, REE, ACT and various transition metal elements, by high resolution electron microscopy and analytical electron microscopy. At low concentrations—the exact level was dependent upon the element(s)—the dopants were accommodated as dilute solid solutions; i.e., isomorphic substitutions. Incorporation of higher dopant concentrations led to polysynthetic twinning on [130], [130] and [010] twin axes; on occasion the twinning was completely regular and new polytypes resulted. At sufficiently high concentrations of REE/ ACT pyrochlore was stabilized in favor of zirconolite. Coupled ACT/Mg, Al, Fe substitutions yielded zirkelite and polymignyte. Since the minerals have stability fields which are composition dependent they cannot strictly be regarded as polytypic and are best considered as pseudotypes.
