, Volume 30, Issue 2, pp 108-124

Crystal chemistry of tremolite–tschermakite solid solutions

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Abstract

 Tremolite–tschermakite solid solutions have been synthesized between 700 and 850 °C and 200 and 2000 MPa. The starting materials were oxide–hydroxide mixtures and an additional 0.1–1.8 molal CaBr2 solution. The run products were characterized using SEM, HRTEM, EMP, XRD and FTIR. The synthesized Al tremolites formed needles and lath-shaped crystals of up to 300 × 20 μm. HRTEM investigations showed that the majority of the amphiboles were well ordered. The EMP analysis revealed that the Al tremolites were solid solutions in the ternary tremolite–tschermakite–cummingtonite. The highest observed Al content was close to the composition of magnesiohornblende (X ts=0.54). Different cummingtonite concentrations (X cum=0.00–0.18) were observed, which generally increased with Al content. Rietveld refinements of the lattice constants showed a linear decrease of the cell parameters a and b with increasing Al content, whereas c and β increased. Small deviations from the linear behaviour were caused by variable amounts of the cummingtonite component. For pure tschermakite lattice parameters of a=9.7438(11) Å, b=17.936(14) Å, c=5.2995(3) Å, β=105.68(9)° and V=891.7 ± 1.4 Å3 were extrapolated by least-squares regression. Using the a and β lattice parameters for tremolite, tschermakite and cummingtonite, it was possible to derive amphibole compositions using powder XRD. IR spectra of the Al tremolites showed a total of 12 individual bands. The FWHMs of all bands increased with increasing Al content. According to their FWHMs, these bands were grouped into three band systems at 3664–3676 cm−1 (I), 3633–3664 cm−1 (II) and 3526–3633 cm−1 (III). Assuming [6]Al substitution at M2 and/or M3 and [4]Al at T1, three principal different configurational groups could be assigned as local environments for the proton. I: only Si4+ at T1 and one or two Al3+ at M2 and/or M3far, II: one Al3+ at T1 and one to three Al3+ at M2 and/or at M3far, III: either Al3+ on M3near and/or two Al3+ on T1 and additional one to four Al3+ at M2. It is assumed that these three configurational groups correspond to the three groups of observed bands. This was quantitativly supported by Monte-Carlo simulations. A model with random distribution at M2 and M3 including Al avoidance at tetrahedral and octahedral sites yielded the best agreement with the spectroscopical results.

Received: 9 November 2002 / Accepted: 11 November 2002
Present address: J. Najorka Corus Research, Development & Technology Ceramics Research Centre Wenckebachstraat 1 1951 JZ Velsen-Noord The Netherlands e-mail: Jens.Najorka@corusgroup.com Tel.: +31 2514 93831 Fax: +31 2514 70489
Acknowledgments J.N. gratefully acknowledges the financial and technical support of the GeoForschungsZentrum Potsdam. The authors acknowledge the support and contributions of W. Heinrich. We thank R. Wirth for assistance with the TEM, O. Appelt and D. Rhede for assistance with the EMP, as well as U. Glenz for assistance with the SEM. We also thank I. Bauer, E. Schemmert, K. Paech for their help in sample preparation and R. Schulz for technical help at the piston-cylinder and hydrothermal apparatus. Reviews by D. Harlov, A. Feenstra and M. Andrut improved earlier versions of this paper.