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 β 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 α (×10−5 K−1) are: potassic-ferro-richterite: αa = 1.30(6), αb = 0.93(6), αc = 0.12(3), αβ = −0.49(5), αasinβ = 1.34(8), αV = 2.59(2); “oxo-potassic-ferro-richterite”: αa = 1.71 (3), αb = 0.97(1), αc = 0.193(8), αβ = 0.22(1), αasinβ = 1.59(4), αV = 2.74(2).

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