Synthetic potassic-ferro-richterite: 1. Composition, crystal structure refinement and HT behavior by in operando single-crystal X-ray diffraction.

The high temperature behavior of synthetic potassic-ferro-richterite was addressed to obtain data relevant to understand petrogenetic processes as well as to test complementarity and mutual calibration of single-crystal X-ray diffraction (XRD) analysis + structure refinement (SREF) with single-crystal FTIR spectroscopy. This experimental approach aims at: i) better quantifying the onset of deprotonation, its development and the amount, if any, of relict OH at the end of the process; ii) verifying whether or not the process is homogeneous within the crystal; iii) evaluating local changes in cation environments close to the OH dipole. In this first part of a series of two papers, we report on the crystal-chemical characterization of potassic-ferro-richterite and on the single-crystal XRD study at high T. Detailed analysis of the available data allowed us to obtain a full characterization of the bulk and crystal-chemistry of the studied crystal, hence improving the unit-formula suggested by Redhammer & Roth (2002). In operando HT measurements up to 1073 K showed a quite anomalous behavior with respect to pargasite/kaersutites, specifically a much lower T for the onset of the deprotonation process (around 500 K), and a strongly anomalous behavior of the ? angle, which shows inverse slopes for protonated and deprotonated phases. "Oxo-potassic-ferro-richterite" is formed upon deprotonation and remains stable at least up to 1073 K under the conditions of this study. Structure refinements from data collected at different temperatures allowed us to detect changes in the crystal structure geometry, and in turn to decipher the way in which amphiboles with such a peculiar composition respond to increasing T and to deprotonation. The thermal expansivity coefficients ? (×10-5 K-1) are: potassic-ferro-richterite: ?a = 1.30 (6), ?b = 0.93(6), ?c = 0.12(3), ?? = -0.49(5), ?asin? = 1.34(8), ?V = 2.59(2); "oxo-potassic-ferro-richterite": ?a = 1.71 (3), ?b = 0.97(1), ?c = 0.193(8), ?? = 0.22(1), ?asin? = 1.59(4), ?V = 2.74(2).