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The Synthesis and Stability of Some End-Member Amphiboles

Department of Earth and Space Sciences, University of Washington Seattle Seattle, Washington, 98195-1310, U.S.A., bwevans@u.washington.edu

The first 20% of the full text of this article appears below.

	INTRODUCTION

 
Reliable characterization of the properties of amphibole end-members is a prerequisite for many important things in mineralogy and petrology, especially for the calculation of phase equilibria that involve amphiboles. Laboratory measurements of the thermochemical and thermophysical properties of amphibole end-members, as well as the experimental reversal of critical reactions, require the availability of adequate amounts of appropriately pure samples. More often than not, nature fails to provide material that fulfills the chemical and physical requirements of phase purity. Synthesis of chosen compositions at high pressure and temperature might seem to be the obvious answer to the problem, but it is now well known that amphiboles are difficult customers in this respect (Hawthorne 1983a, 1995; Graham et al. 1989; Raudsepp et al. 1991; Maresch et al. 1994).

In the early days of experimental petrology, in the 1960s and 1970s, phase characterization of products involved optical microscopic measurements and powder XRD, a minimal combination that we now realize was inadequate. Experience has shown that the synthesis of an end-member amphibole commonly yields distinctly less than 100% amphibole, with the result that its composition likely departs significantly from the desired one, often in a reproducible way. In addition, the amphibole may contain unacceptably high concentrations of defects of one sort or another, and, whether or not these drive it off composition, their presence can be energetically detrimental. These deficiencies can be minimized, but not necessarily eliminated, by increasing the pressure, temperature, or duration of synthesis. Heroic efforts have been rewarded in a few cases. For example, by repeated grinding of experimental products, and the use of high pressure and judicious amounts of H₂O, Jenkins and Corona (2006) were able to obtain glaucophane very close to its end-member composition, something that numerous previous workers had failed to . . . [Full Text of this Article]

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