Calibration of Al/Si order variations in anorthite |
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| Authors: | M A Carpenter R J Angel L W Finger |
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| Institution: | (1) Department of Earth Sciences, University of Cambridge, Downing Street, CB2 3EQ Cambridge, England;(2) Department of Geological Sciences, University College London, Gower Street, WC1E 6BT London, England;(3) Department of Crystallography, Birkbeck College, Malet Street, WC1E 7HX London, England;(4) Geophysical Laboratory, 2801 Upton Street, NW, 20008 Washington DC, USA |
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| Abstract: | New single crystal diffraction data for natural and heat-treated anorthite crystals (Angel et al. 1990) allow the determination of their states of Al/Si order in terms of a macroscopic order parameter,Q
OD
, for the transition. Numerical values ofQ
OD
obtained from estimates of site occupancies are shown to vary with the scalar spontaneous strain, 
s
, as
s
 Q
OD
2
, and with the ratio of the sums of typeb (superlattice) reflections and typea (sublattice) reflections as I
b/I
a
Q
OD
2
. An empirical calibration for pure anorthite is obtained giving
varies between  0.92 and 0.87 in samples equilibrated at T 1300° C, but then falls off relatively rapidly with increasing temperature, reaching 0.7 near the melting point ( 1557° C). The observed temperature dependence does not conform to the predictions of the simplest single order parameter models; coupling ofQ
OD
withQ of the transition is suspeeted. |
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