Crystal chemistry of synthetic amphiboles along the richterite - ferro-richterite join

In this work we relate the synthesis and crystal-chemical study of amphiboles along the join richterite - ferrorichterite [nominally Na(NaCa)(Mg5-xFe2+ x)Si8O22(OH)2], at 700°C, 1 kbar, under redox conditions imposed by a MW solid buffer. XRPD shows essentially single-phase run products at both endmember compositions, with minor (10-15%) pyroxene and quartz for intermediate compositions. Interestingly, there is a significant evolution of the amphibole morphology across the solid-solution series: Mg end-member richterite is extremely acicular (length up to 15 μm, diameter up to 1 μm) whereas for increasing Fe in the system the amphibole becomes increasingly stubby, with a length:diameter ratio≈ 3:1 for the Fe -rich endmember. Notably, the refined cell parameters show linear variations across the series, suggesting a complete Mg-Fe2+ solid-solution along the join. The OH-stretching FTIR spectra show complex patterns for intermediate compositions with up to eight bands which can be assigned to the various configurations involving Mg/Fe2+ at M(1,3), locally associated with both full and empty A-sites. Combination of Mössbauer and FTIR spectroscopy however shows that the composition of the amphibole is much more complex than expected just from XRPD data: Fe2+ is in fact disordered among all M-sites, and, despite the strongly reducing conditions during syntheses, there is significant Fe3+ in the amphibole; it is fully ordered at M(2). For increasing Fe in the system, the amount of Fetot in the amphibole is systematically lower than expected as is Fe2+ at the M(1-3) octahedra, while M(2)Fe3+ increases toward more ironrich compositions (Fig. 1). These results definitively stress the need for a proper characterization of the experimental run products because these may depart from the expected stoichiometry in a significant way. Direct EMP or single-crystal X-ray refinement is highly desirable, however spectroscopic methods may provide a valuable alternative, particularly when the Fe3+ vs. Fe2+ composition is needed.