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 understanding petrogenetic processes as well as to testing complementarity and mutual calibration of single-crystal X-ray diffraction (XRD) analysis plus structure refinement (SREF) with single-crystal FTIR spectroscopy. This experimental approach aims at: (1) better quantifying the onset of deprotonation, its development and the amount, if any, of relict OH at the end of the process; (2) verifying whether or not the process is homogeneous within the crystal; and (3) 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 a 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 quite anomalous behavior with respect to pargasite/kaersutites, specifically a much lower T for the onset of the deprotonation process (around 500 K), and strongly anomalous behavior of the b 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 deprotonation. The thermal expansivity coefficients a(X10∼5 K-1) are: potassic-ferro-richterite: aa= 1.30(6), ab = 0.93(6), ac = 0.12(3), a =-0.49(5), aasinp= 1.34(8), aV = 2.59(2); oxo-potassic-ferro-richterite: aa = 1.71 (3), ab = 0.97(1), ac = 0.193(8), a.p = 0.22(1), aasinii = 1.59(4), aV = 2.74(2).