A multi-methodological study of syntactic intergrowths and polysomatism in Ca-REE fluorcarbonate minerals from Mount Malosa (Malawi)

Ca-REE fluorcarbonates play a crucial role for at least two (apparently) distant reasons, one as critical raw materials closely linked to the industrial progress, to the development of modern technologies and environment-friendly green technologies. The other tied to fundamental research, since they form a polysomatic series and commonly occur as polycrystals characterized by microscale syntactic (crystallographically oriented) intergrowths of different polysomes/polytypes; often with stacking faults at the nanoscale. Their identification and study is therefore not straightforward. We evaluated the potentialities of non-destructive techniques, such as Electron BackScattered Diffraction (EBSD) and Raman spectroscopy, in the characterization of Ca-REE fluorcarbonates at a microscopic scale in order to provide a “road map” for further investigation with destructive techniques, such as Transmission Electron Microscopy (TEM). We investigated well characterized Ca-REE fluorcarbonates from Mount Malosa (Malawi) (Guastoni et al., 2009; Capitani, 2019), which belong to the bastnäsite-synchysite series. EBSD was effective to establish the sample orientation, to setup the oriented cuts and to ascertain the effective syntactic relationship among all the detected Ca-REE fluorcarbonate phases, but failed to distinguish among different polysomes. The latter can be easily understood taking into account that electron diffraction (alike X-ray diffraction) is dominated by heavy atoms, and that the Ca-REE fluorcarbonates studied, although monoclinic, have an identical hexagonal stacking of heavy atoms (Ca and REE) as bastnäsite, the only effective hexagonal term. Micro-Raman spectroscopy, performed on samples with different orientations and characterized by SEM-EDS, allows the distinction of polysomes based on the differences of intensity and position of the symmetric stretching vibration (v1) of the carbonate group (CO32-), in the region around 1080-1099 cm-1. However, as evidenced by TEM-EDS, what appears as a homogeneous polysome in BSE images, actually may be a disordered intergrowth of compositional faults with a bulk composition matching only by accident that of a real polysome. Actually, there are two different types of CO3 layers in the Ca-REE fluorcarbonates structure: those in contact with two CeF-layers (or e-layers), and those in contact with one CeF- and one Ca-layer (or g-layers; Donnay & Donnay, 1953). We believed that the Raman signal is sensitive to different abundances of e- and g-layers in the structure, whose proportions vary with composition, and therefore also with the polysome, but not to the order of the layers within the analysed volume, making the distinction by Raman spectroscopy of ordered and disordered intergrowths with similar composition unfeasible.