Crystal chemistry of K-richterite-richterite-tremolite solid solutions: a SEM, EMP, XRD, HRTEM and IR study

Solid solutions in the ternary K-richterite-richterite-tremolite (K(x)Na(y) square(1-x-y))(Nax+yCa2-x-y)(2)-Mg-5[Si8O22/(OH)(2)] have been synthesized at 200 MPa and 800 degrees C using cold-seal vessels, and at 1800 MPa and 8 00 degrees C using a piston-cylinder apparatus. The amphiboles were synthes ized 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, ele ctron microprobe, X-ray powder diffraction and fourier transform infrared s pectroscopy. The synthesized amphiboles are up to 1000 mu m x 50 mu m in size. The cryst als are chemically homogeneous and structurally well-ordered. Complete soli d solutions are observed in the ternary Kri-ri-tr, but compositions very cl ose to pure richterite and K-richterite end-member could not be synthesized , always showing small but significant amounts of tremolite component. In a ddition, small amounts of cummingtonite component are present in most amphi boles. There are no indications for crystal chemical restrictions for compl ete solid solutions, however. The lattice parameters of the solid solutions correlate linearly with the c ation occupancies on the A- and M4-sites and are therefore a linear combina tion 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 w avenumbers between 3669 and 3678 cm(-1) is due to a Vacant A-site and the s econd between 3721 and 3737 cm(-1) due to a filled A-site. The observed fin e 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 conc entration was determined quantitatively from normalized integral absorbance s of the band system at 3669-3678 cm(-1). The derived vacancy concentration s are consistent with those derived by electron microprobe. Additional band s 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 th ese bands may be used to determine the concentration of chain multiplicity faults.