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Structural refinements of magnesite at very high pressure
-
Guillaume Fiquet
,
François Guyot
Veröffentlicht/Copyright:
28. März 2015
Abstract
Unit-cell parameters of magnesite were measured between ambient pressure and 80 GPa using angle dispersive powder X-ray diffraction. The isothermal bulk modulus determined from a third order Birch Murnaghan equation of state is KT = 108(3) GPa with K'T = 5.0(2), and V0 = 279.2(2) Å3, in agreement with previously reported values. Combining this result with previous measurements, we show that magnesite with R3̄c structure is stable compared to the assemblage periclase + carbon dioxide at pressures and temperatures corresponding to the core-mantle boundary. Crystal structure refinements have also been carried out up to 80 GPa. The main structural change is a strong compression of the MgO6 octahedra with increasing pressure, largely reflected in the anisotropic compression of the c axis. This compression, however, tends to level off at around 50-60 GPa. On the other hand, the CO3 groups do not remain invariant since they undergo first a slight expansion and then a compression above the same threshold pressure of 60 GPa above which Mg-O bonds cannot compress further. Thus, in this structure-type, the energy gain due to a drastic volume reduction of the MgO6 octahedron compensates in a given pressure range for the energy cost of the small expansion of the CO3 carbonate unit.
Received: 2001-12-11
Accepted: 2002-4-18
Published Online: 2015-3-28
Published in Print: 2002-8-1
© 2015 by Walter de Gruyter Berlin/Boston
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Artikel in diesem Heft
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- Experimental and modeled solubilities of chlorine in aluminosilicate melts, consequences of magma evolution, and implications for exsolution of hydrous chloride melt at Mt. Somma-Vesuvius
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