Crystal-Chemistry of Synthetic Amphiboles Along the Richterite–Ferro-Richterite Join: A Combined Spectroscopic (FTIR, Mössbauer), XRPD, and Microchemical Study

Abstract

In this paper we report a crystal-chemical study of amphiboles along the nominal join richterite [ANaB(NaCa)CMg5TSi8O22W(OH)2] – ferro-richterite [ANaB(NaCa)CFe2+5TSi8O22W(OH)2], ideally characterized by direct substitution of CMg with CFe2+. Six samples were synthesized by hydrothermal techniques: the three Fe-richer compositions at 500 °C and the three Mg-richer ones at 600 °C, 1 kbar P(H2O), and internally-buffered fO2 conditions (Sergent et al. 1996). The experimental powders were studied using a combination of EMPA, FE-SEM, PXRD, and FTIR plus 57Fe Mossbauer spectroscopy. Due to crystal-size limitations, only the Fe endmember could be analyzed by EMP-WDS, while two additional Fe-rich samples could be semi-quantitatively analyzed by FE-SEM-EDS. Rietveld refinement of X-ray powder diffraction patterns show that, although a linear trend in the amphibole cell-parameters is observed, there is a significant decrease in the amphibole yield counterbalanced by an increase in clinopyroxene (cpx), up to 27 wt.%, for the Fe endmember. 57Fe Mossbauer spectra could be fitted on the basis of known spectral parameters of similar amphiboles in the literature and corrected for the spectral contribution of cpx. The results show that Fe3+ is completely ordered at M(2), while Fe2+ is disordered over all available C sites; no ferrous iron is resolved at M(4). The OH-stretching FTIR spectra show complex patterns for intermediate compositions with up to eight bands; these can be assigned to the various configurations involving Mg/Fe2+ at M(1,3), locally associated with both full and empty A sites. Details of the crystal chemistry for all amphiboles in the series were obtained with the spectroscopic methods, and the resulting data were in reasonable agreement with those derived by WDS-EDS analyses for the three Fe-richer samples. For increasing Fe in the system, the amphibole composition deviates toward a sodic endmember following the exchange vector M2Fe2+M4Ca M2Fe3+–1M4Na–1, possibly due to the instability of endmember ferro-richterite for stereochemical reasons. Interestingly, along the studied series there is a general increase in the amphibole crystal size and an evolution of the crystal morphologies, from extremely acicular at the Mg endmember composition to prismatic and stubby for the Fe2+-richer compositions.