Electron microscopy investigation of a flower-like mineralization on Bi-sulfosalts from Alfenza (Crodo). A new Bi hydrated sulfate?

A flower-like mineralization of white, micrometre-sized crystallites, occurs on Bi sulfosalts from the Alfenza Mine (Crodo). With a standard optical stereomicroscope each single crystal is hardly resolvable. At the scanning electron microscope (SEM), the crystallites show perfect, pseudo-hexagonal, randomly oriented platelets forming a hemispheric aggregate up to several tens of microns in radius. The EDS spectra show distinct S, Bi, O peaks, pointing to a Bi sulfate (or Bi hydrated sulfate). As far as we know, only two natural phases, cannonite [Bi2O(OH)SO4] and riomarinaite [Bi(OH) SO4•H2O], have been reported with a composition that qualitatively match the observed one. However, available physical properties for these minerals (habit, color, cleavage…) do not match with those observed in our case. In order to solve the puzzle, few mineral aggregates were peaked up with a knife tip, dispersed in ethanol, ultrasonicated, and pipetted on a holey-carbon Cu grid for TEM analyses. Preliminary results show a Pmm projected symmetry down the pole of the pseudo-hexagonal platelets consistent with a monoclinic or orthorhombic system, 2D cell parameters a = 17.2(4) and b = 15.9(3) Å, (gamma = 90°), and systematic absences consistent with a 21 screw along the b-axis. A comparison of the observed electron diffractions with those simulated for (synthetic) cannonite and riomarinaite reveals substantial differences, excluding these two phases as possible and opening to the possibility for a new mineral. Deeper investigations are in progress.