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Crystal chemistry of K-richterite-richterite-tremolite solid solutions

GeoForschungsZentrum Potsdam, Telegrafenberg, 14473 Potsdam, Germany. e-mail: smelzer{at}gfz-potsdam.de

Solid solutions in the ternary K-richterite-richterite-tremolite (K_xNa_y1-x-y)(Na_x+yCa_2-x-y)₂Mg₅[Si₈O₂₂/(OH)₂] have been synthesized at 200 MPa and 800°C using cold-seal vessels, and at 1800 MPa and 800°C using a piston-cylinder apparatus. The amphiboles were synthesized from oxide and hydroxide mixtures in the presence of a 2-molal aqueous chloridic solution. Solid run products have been investigated by optical, electron scanning and high-resolution transmission electron microscopy, electron microprobe, X-ray powder diffraction and fourier transform infrared spectroscopy.

The synthesized amphiboles are up to 1000 µm x 50 µm in size. The crystals are chemically homogeneous and structurally well-ordered. Complete solid solutions are observed in the ternary Kri-ri-tr, but compositions very close to pure richterite and K-richterite end-member could not be synthesized, always showing small but significant amounts of tremolite component. In addition, small amounts of cummingtonite component are present in most amphiboles. There are no indications for crystal chemical restrictions for complete solid solutions, however.

The lattice parameters of the solid solutions correlate linearly with the cation occupancies on the A-and M4-sites and are therefore a linear combination of the lattice parameters of the end-members tremolite, cummingtonite, richterite and K-richterite.

Two systems of OH-stretching bands are observed. The first band system at wavenumbers between 3669 and 3678 cm^-1 is due to a vacant A-site and the second between 3721 and 3737 cm^-1 due to a filled A-site. The observed fine structure of the bands can be attributed to distinct M4-site occupancies by Ca²⁺, Mg²⁺ and Na⁺. Using pure tremolite as a standard, the vacancy concentration was determined quantitatively from normalized integral absorbances of the band system at 3669-3678 cm^-1. The derived vacancy concentrations are consistent with those derived by electron microprobe. Additional bands at 3659, 3695 and 3710 cm^-1 are probably due to triple-chains or higher chain multiplicity faults in the amphibole. The integral absorbances of these bands may be used to determine, the concentration of chain multiplicity faults.

Key-words: K-richterite, richterite, tremolite, unit-cell data, TR-spectroscopy.

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