CRYSTAL-CHEMISTRY OF SYNTHETIC AMPHIBOLES ALONG THE RICHTERITEFERRO-RICHTERITE JOIN: A COMBINED SPECTROSCOPIC (FTIR, MOSSBAUER), XRPD, AND MICROCHEMICAL STUDY

摘要

In this paper we report a crystal-chemical study of amphiboles along the nominal join richterite [Na-A(B)(NaCa)(Mg5Si8O22W)-Mg-C-Si-T(OH)(2)] - ferro-richterite [Na-A(B)(NaCa)(Fe52+Si8O22W)-Fe-C-Si-T(OH)(2)], ideally characterized by direct substitution of Mg-C with Fe-C(2+). Six samples were synthesized by hydrothermal techniques: the three Fe-richer compositions at 500 degrees C and the three Mg-richer ones at 600 degrees C, 1 kbar P-(H2O), and internally-buffered f(O2) conditions (Sergent et al. 1996). The experimental powders were studied using a combination of EMPA, FE-SEM, PXRD, and FTIR plus Fe-57 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.