The Significance of the Reaction Path in Synthesizing Single-Phase Amphibole of Defined Composition

Abstract

Amphiboles are common and very complex rock-forming minerals. Knowledge of their physical, chemical and thermodynamic properties allows us to understand rock-forming and geodynamic processes. Amphiboles also possess distinctive physical properties that make them potentially attractive in applied fields. Certainly, millions of dollars, rubles, etc. were spent 30–40 years ago to optimize techniques of amphibole synthesis, and, ironically, comparable sums have been or are still being spent to remediate asbestos danger. In short, for whatever purpose, be it basic research or applied mineralogy, the task of synthesizing amphiboles of defined composition and crystal-chemical nature has been a challenge for a very long time. However, even a cursory review of the literature on amphiboles converges on one, often recognized and much discussed dilemma (e.g., Hawthorne 1983b, 1995; Maresch and Czank 1983b; Graham et al. 1989; Raudsepp et al. 1991). Compositions of amphibole products most often depart from the nominal experimental target. Is it then possible to synthesize single-phase amphibole products, and, if not, why not? Ideally, synthesizing single-phase hydroxyl-amphibole from a given bulk composition would be the most accurate way to proceed to a well characterized amphibole product. Simply in terms of chemical composition, a bulk starting material can be prepared with greater accuracy than a multi-grain product can be characterized analytically. What goes in must come out! However, reality is more sobering. Various states of disorder are possible at constant bulk composition (e.g., Oberti et al. 2007; Hawthorne and Della Ventura 2007). Above all, the necessity to introduce H2O as a fluid, both as an intrinsic component of the structure and as a means of transmitting pressure in the experiment, introduces new and difficult to control degrees of freedom. Changes in valence states become possible. The target stoichiometry of the amphibole may be changed …