The replacement of anthophyllite by jimthompsonite: a model for hydration reactions in biopyriboles |
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| Authors: | Bernard Henry Grobéty |
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| Institution: | (1) Institut fur Mineralogie und Petrographie, ETH-Zentrum, CH-8092 Zurich, Switzerland, CH |
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| Abstract: | Anthophyllite crystals found in ultramafic lenses of the Lepontine Alps (Switzerland) contain coherent, submicroscopic intergrowths
of ordered and disordered biopyribole polysomes. The chain width distributions of disordered polysomes were analyzed using
high resolution transmission electron microscopy (HRTEM). Chains wider than triple were interpreted as intermediate products
in the transformation of anthophyllite to the triple chain silicate jimthompsonite. The concentration of individual chain
types is strongly correlated with the reaction progress. Based on observed zipper terminations and the transformation rules
given by Veblen and Buseck (1980) a scheme of possible reaction paths leading from anthophyllite to jimthomp sonite is proposed.
The reaction scheme and a simple kinetic model for elementary reactions allow modeling of the observed chain width distributions.
The model suggests that the complex reaction paths involving steps with increasing and decreasing chain width are more important
in the formation of jimthompsonite than the direct transformation from anthophyllite. The wide chains (>triple) occurring
as intermediate products of the multi-step paths are structurally closer to talc than jimthompsonite. The back-transformation
of these wide chains to triple chains is, therefore, a strong argument that jimthompsonite is a stable phase and not only
a metastable intermediate product in the transformation of anthophyllite to talc.
Received: 8 July 1996 / Accepted: 13 December 1996 |
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