SYNTHETIC POTASSIC-FERRO-RICHTERITE: 1. COMPOSITION, CRYSTAL STRUCTURE REFINEMENT, AND H T 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 stnicture refinement (SKIT) 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 Redhanuner & Roth (2002). In operand HT measurements up to 1073 K showed quite anomalous behavior with respect to pargasitelkaersutites, specifically a much lower T fru- the onset of the deprotonation process (around 500 K), and strongly anomalous behavior of the beta angle, which shows inverse slopes fru- 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 alpha(X10(-5) K-1) are: potassic-femi-richterite: alpha(a) = 1.30(6), alpha(b), =0.93(6), alpha(c)= 0.12(3), alpha(beta) = 0.49(5), alpha(sin beta)= 1.34(8), alpha(nu) = 2.59(2); "oxo-potassic-ferro-richterite" : alpha(a) = 1.71 (3), alpha(b) = 0.97(1), alpha(c), = 0.193(8), alpha(beta) = 0.22(1), alpha(asin beta)= 1.59(4), alpha(nu) = 2.74(2).