Amphibole synthesis at low pressure: what grows and what doesn't
Raudsepp, Mati; Turnock, Allan C.; Hawthorne, Frank C.
European Journal of Mineralogy Volume 3 Number 6 (1991), p. 983 - 1004
published: Dec 19, 1991
manuscript accepted: Jul 10, 1991
manuscript received: Apr 26, 1991
ArtNo. ESP147050306008, Price: 29.00 €
Abstract Despite a considerable number of experimental studies on amphibole stability and end-member synthesis, until recently few synthetic amphiboles have been adequately characterized by modern methods. This study is a reconnaissance of iron-free, monoclinic amphibole synthesis possibilities over a wide range of compositions, to see what can and what cannot be synthesized at low pressures. Synthesis results of end-member and intermediate amphibole compositions from the calcic, sodic-calcic and sodic groups are described. Selected run products were characterized by optical and scanning electron microscopy, electron microprobe analysis and X-ray diffraction. The composition and cation ordering of high-yield > 80 % amphibole) run products were characterized in detail using infrared spectroscopy, and/or Rietveld structure analysis and magic-angle spinning nuclear magnetic resonance spectroscopy. The results of amphibole syntheses are sensitive to the exact nature of the starting materials used, especially the source of SiO2. Furthermore, the treatment of the mix prior to use (e.g. decarbonation of CaCO3 after rather than before mixture of the components) is important for growing high yields of amphibole in the run product. In general, the results from dry-oxide mixes and gels were similar. Amphibole compositions that seem to grow fairly easily, that is syntheses giving high yields of amphibole (presumably close to nominal composition), are few. These include pargasite, richterite, potassium-richterite and sodian magnesio-cummingtonite. Fluorine endmembers include luor-tremolite, luor-edenite, luor-richterite, scandium- and indium-eckermannites, and scandium- luor-nyboite. Compositions that gave low to nil amphibole yields under low pressure conditions include edenite, tschermakite, manesio-hornblendes, all sodic-calcic amphiboles, and eckermannite. Much of the previous amphibole synthesis and stability work has been predicated on the assumption that end-member compositions will be stable under some set of P and T conditions. It seems that there is no intrinsic reason why this should be so, and this work, together with a consideration of previous results, suggests that this may not generally be the case. Further progress requires that proper characterization of run products must be a significant priority in future end-member synthesis and phase studies.