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The effect of Al/Si disorder on the bulk moduli of plagioclase feldspars

Published online by Cambridge University Press:  05 July 2018

L. M. Sochalski-Kolbus*
Affiliation:
Virginia Tech Crystallography Laboratory, Department of Geosciences, Virginia Tech, Blacksburg, VA 24060, USA
R. J. Angel
Affiliation:
Virginia Tech Crystallography Laboratory, Department of Geosciences, Virginia Tech, Blacksburg, VA 24060, USA
F. Nestola
Affiliation:
Dipartimento di Geoscienze, Universitá di Padova, Via Giotto 1, I-35137 Padova, Italy

Abstract

The volumes of a disordered An20 (Qod = 0.15), a disordered An78 (Qod = 0.55) and an ordered An78 (Qod = 0.81) were determined up to 9.569(10) GPa, 8.693(5) GPa and 9.765(10) GPa, respectively, using single-crystal X-ray diffraction. The volume variations with pressure for these samples are described with 4th-order Birch Murnaghan equations of state with V0 = 669.88(7) Å3, K0 = 59.7(7) GPa. K′ = 5.7(5), K″ = −0.8(2) GPa−1 for disordered An20, V0 = 1340.48(10) Å3, K0 = 77.6(5) GPa, K0′ = 4.0(3), K″ = -0.59(9) GPa−1 for disordered An78 and V0 = 1339.62(6) A3, K0 = 77.4(6) GPa, K′ = 4.2(4), and K″ = −0.7(1) GPa−1 for ordered An78. Along with data from previous studies (An0 ordered, An0 disordered and An2o ordered), the volumes for the disordered samples were found to be up to ∼0.3% larger than the ordered samples of the same composition. The disordered samples are softer than the ordered samples of the same composition by 4(1)% for An0, 2.5(9)% for An20 and essentially zero for An78. The relationship between volume increase, density decrease, and decreasing bulk modulus with increasing disorder is in accordance with Birch's Law.

Type
Research Article
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 2010

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