Crystal chemical investigations on rare zirconium-niobium silicates of the Alfred Lacroix collection of the Muséum National d'Histoire Naturelle (Paris)

The material collected at the end of the 19th century in the former French colonies and preserved in the mineralogical collections of MNHN represents a heritage of relevant scientific and historical interest, an "unicum" deserving a relevant valorization. There are more than 600 samples studied by Lacroix catalogued as nepheline syenites and associated acid differentiates, possibly hosting Zr Ti Nb silicates. Nearly 250 samples come from Madagascar, 100 from Morocco and Algeria, and 250 from the Los Archipelago (Guinea). For several samples it is reported the presence of minerals such eudyalite, lavenite, catapleite. According to Lacroix's study approach, most of these identifications are based on optical studies, with some chemical data from wet analytical techniques. Considering the several easy and common isomorphic substitutions possible between phases within each group, one can easily understand why optical data chemical composition only cannot lead to a sure species identification. The aim of the present study of a series of rare Zr-Nb-Ti disilicates, to fully define their true nature through a combined multidisciplinary approach. The study material was collected during the SYNTHESYS+ FR-TAF_Call3_034, screening 620 specimens selected within the rocks database leading to a final selection of 15 specimens for physical-chemical characterization. We focused on nepheline syenites from Tamazeght mountain range, Maroc; Andevenanaomby, Angalavozy, Madagascar and Los Archipelago, Guinea. The material sampled during the visit was prepared for mineralogical studies currently in progress. Raman spectroscopy and powder X-ray diffraction data were collected, showing interesting results, especially for minerals of the wohlerite and eudialyte groups. Electron microprobe (EMPA) data collection and single crystal intensity data collection, for the determination of the crystalline structure of the studied phases, are currently underway. EMPA and single crystal data collected on samples from Tamazeght mountain range, Maroc allowed to detect the presence of lavenite and normandite, two phases belonging to the wohlerite group. Some new data are available from the Los archipelago samples. Among these, the presence of a Mndominant member, closely related to raite, of the palygorskite-sepiolite group (Guggenheim & Krekeler, 2011), together with ongoing crystal chemical study of eudialyte. Single crystal structural and EMPA chemical data point to the presence of two distinct members of the eudialyte group, related to kentbrooksite and sergevanite (Moreau et al., 1996; Johnsen et al., 2003).