Laueite/stewartite epitaxy – a single-crystal diffraction study

Abstract Laueite/stewartite epitaxy was studied using single-crystal diffraction applied to a composite crystal from Hagendorf-Süd, Bavaria. The orientation relationships between the crystals of the two minerals was facilitated by using a non-conventional B $\bar {1}$ space group setting for stewartite, giving unit cells with parallel axes and with a s = 2a l , b s = b l and c s = 2 c l . Face indexing of the crystals of the two minerals confirmed the epitaxial relationship, with the {100} and {010} faces parallel. The plane of epitaxy is {010}. Refinement of laueite and stewartite datasets extracted from the composite-crystal data collection showed a significant decrease in the mean Mn-site bond distances in laueite, consistent with chemical analyses of the crystals that gave site compositions of Mn 0.92 Fe 3+ 0.08 for stewartite and Mn 0.66 Mg 0.17 Fe 3+ 0.17 for laueite. The epitaxial growth of laueite on {010} planes of stewartite appears to have been initiated by a change in solution chemistry. Possible paragenesis of the secondary phosphate minerals from primary triphylite is discussed.