Elasticity of tetragonal end-member majorite and solid solutions in the system Mg4Si4O12-Mg3Al2Si3O12 |
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Authors: | Stanislav V Sinogeikin Jay D Bass Bridget O'Neill Tibor Gasparik |
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Institution: | (1) Department of Geology, University of Illinois, Urbana IL 61801; USA. stashercules.geology.uiuc.edu, fax: (217) 244–4996, US;(2) Department of Geology and Geophysics, University of California, Berkeley, CA 94720, USA, US;(3) CHiPR and Department of Earth and Space Sciences, State University of New York, Stony Brook, NY 11794, USA, US |
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Abstract: | The adiabatic elastic moduli of polycrystalline En50Py50 and En80Py20 majorite-garnet solid solutions (where En=4MgSiO3, Py=Mg3Al2Si3O12) and the end-member En100 tetragonal majorite were determined at ambient conditions using Brillouin spectroscopy. The adiabatic bulk modulus, K, and
shear modulus, μ, of En50Py50 were found to be K=173.1 (20) and μ=92.3 (10) GPa, and are indistinguishable from those of the end-member pyrope, Py100. The moduli of the more majorite-rich samples are significantly lower and are virtually identical to each other (K=162.6(11)
and μ=85.7(7) for En80Py20; K=166(5) and μ=88(2) for En100). In combination with previously reported moduli for this system, we conclude that both K and μ are constant over the compositional
range from Py100 to a majorite content of about 70–80%, whereupon the moduli decrease substantially. For compositions ranging from En80Py20 to the end-member majorite, the moduli are also approximately independent of composition, but at a lower value. An alternative
model with a continuous decrease in moduli with increasing majorite content cannot be excluded, within the uncertainties of
existing measurements. The contrast in moduli between aluminous pyrope garnet and Al-free majorite are small compared with
the modulus changes accompanying the pyroxene – majorite phase transformation.
Received August 16, 1995 /Revised, accepted January 12, 1996 |
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