Experimental delineation of the Cl = /1 transformation in intermediate plagioclase feldspars

Content Provider	Semantic Scholar
Author	No, J. Desuo
Copyright Year	1984
Abstract	The Cl = 11 AySi order/disorder transformation in intermediate plagioclase feldspars has been experimentally bracketed by dry and hydrothermal annealing of natural samples in the composition range An56An6s. It can be represented on the albite-anorthite binary phase diagram by a steep line passing through the points -Anss, lfi)0"C and -An77, 1440"C (where it meets the one atmosphere solidus). The transformation appears to be characterized by a change from long range order (sharp D reflections in electron diffraction patterns) to short range order (diffuse D reflections) over a fairly narrow temperature interval. No sign of any two phase region separating the Cl and 1l fields was found on an electron optical scale and the observations are thus consistent with (but do not prove) non-first order properties for the transformation. Assuming the transformation to be of higher than first order leads to some reappraisal of existing thermodynamic data for the high plagioclase solid solution. At the simplest level such data are consistent with approximately ideal mixing of albite and anorthite in both the CT and 1T structures but with a continuous, non-ideal transition region between them. The apparent deviations from ideality can be ascribed to AVSi ordering in anorthite rich compositions. The order/disorder transformation line extrapolates at lower temperatures into the exact composition range of B/ggild exsolution, and must, therefore, be implicated in that miscibility gap.
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Alternate Webpage(s)	http://rruff.info/doclib/am/vol69/AM69_112.pdf
Language	English
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Content Type	Text
Resource Type	Article