HRTEM investigation of bastnäsite–parisite intergrowths from Mount Malosa (Malawi): ordered sequences, polysomatic faults, polytypic disorder, and a new parisite-(Ce) polymorph

This high-resolution transmission electron microscopy (HRTEM) investigation of (Ca, REE)-fluorcarbonates from Mount Malosa (Malawi), previously characterized by bulk techniques, is aimed at elucidating the pervasive syntactic intergrowths that commonly affect these minerals. The studied samples are dominated by bastnäsite-(Ce)–parisite-(Ce) intergrowth with rhythmic repetitions along the c axis from the microscale to the nanoscale. Whereas bastnäsite is always structurally and compositionally homogeneous, parisite-(Ce) may be either ordered or disordered, in the latter case showing polysomatic faults (compositional faults) that locally change the composition towards bastnäsite-(Ce). The HRTEM image contrast of ordered and disordered intergrowths is interpreted and discussed in terms of layer shifts on (001) of ~2.37 Å of the bastnäsite portions (B) above the Ca-layers (V) along stacking vectors rotated by n × 60° (n = odd) with respect to each other. In a few cases, sequential faults (conjugate polysomatic faults that locally alter the layer sequence without altering the overall composition) occur periodically, leading to a new parisite-(Ce) polymorph, described in terms of V and B stacking sequences. HRTEM images and associated simulations show that the new parisite-(Ce) polymorph has C1 symmetry and cell parameters similar to those of normal parisite. The detected nanostructure is consistent with a primary growth mechanism in which bastnäsite and parisite crystallize directly from a fluid whose thermodynamic (close to) equilibrium conditions quickly and repeatedly crossed their stability boundary. Polysomatic disorder in the compositional interval between parisite and bastnäsite, as occurred in the studied samples, is indicative of growth rates faster than those leading to the formation of long-range polysomes detected in other occurrences. Finally, the present study demonstrates how HRTEM techniques can elucidate polysomatic and polytypic disorder in a comprehensive way, since the contrast is sensitive to the different stacking of CO₃ layers that appears with different slants in HR images, revealing the actual crystal system symmetry and any polytypic disorder.