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Experimental delineation of the cl = /1 transformation in intermediate plagioclase feldspars.
| Content Provider | CiteSeerX |
|---|---|
| Author | Npbnter, Mrcheer A. Ca. Mcconxbn, J. Desuono C. |
| 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 character-ized 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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| Access Restriction | Open |
| Subject Keyword | Intermediate Plagioclase Feldspar Experimental Delineation Range Order Exact Composition Range Sharp Reflection Non-ideal Transition Region Hydrothermal Annealing Ggild Exsolution Thermodynamic Data Atmosphere Solidus Level Data Order Disorder Transformation Line Narrow Temperature Interval Steep Line Composition Range An56an6s Albite-anorthite Binary Phase Diagram First Order Ideal Mixing Phase Region Apparent Deviation Aysi Order Disorder Transformation Electron Optical Scale Long Range Order Non-first Order Property Anorthite Rich Composition Natural Sample Electron Diffraction Pattern High Plagioclase Solid Solution Miscibility Gap |
| Content Type | Text |
