The study of spinels at high pressure (P) provides information relevant to geophysics, as their structure is a model for phases stable at conditions occurring in the mantle of the Earth. In particular, MgAl2O4 is a constituent of the upper mantle in the spinel-type structure ("spinel in sensu strictu"), and it is supposed to stabilize in the CaFe2O4-type structure in subducted oceanic crust.

Notwithstanding the importance of MgAl-spinel, little was known about its behaviour in the high pressure regime (> 10 GPa). Therefore, this studies aim was to investigate the equation of state (EoS) and the structural behaviour of MgAl-spinel from ambient conditions to 30 GPa, at room temperature, by in situ high-pressure powder diffraction using X-rays from a synchrotron source.

The MgAl2O4 sample used in the present investigation was synthesised by a high-temperature solid state reaction of MgO and Al2O3. The high-pressure powder diffraction experiments were carried out on the ID09A beamline using a diamond-anvil cell, with N2 as a pressure-transmitting medium; pressure was estimated by the fluorescence line shift of ruby. The diffraction patterns were treated by the Rietveld method (GSAS-Expgui software), and provided the cell edge (a) (see Figure 57), the oxygen coordinate (u) and the atomic displacement parameters. The elastic parameters (bulk modulus, K0, its first and second derivatives vs P, K'0 and K"0, respectively) were determined by fitting different EoS models to the observed pressures (Birch-Murneghan (BM), Vinet (V) and Poirier-Tarantola (PT)). An comparison between the issues of the different EoS's suggests one to choose the results from the 3rd order BM, yielding the following elastic parameters: K0 = 191(1) GPa, K'0 = 6.7(2) and K''0 = -0.075 GPa-1 (implied value).

 

Fig. 57: Cell edge (Å) as a function of pressure (GPa). The solid line is the third order polynomial fit.

 

Bulk modulus values previously measured by diffraction are in good agreement with ours, whereas those obtained by spectroscopy are systematically higher. The cation replacement in spinels affects significantly the elastic properties; for instance, ZnAl2O4 [1] exhibits K0 = 201.7(±0.9) GPa and K'0 = 7.62(±0.09), hinting that the substitution of Zn with Mg yields a "softening", in keeping with the expectations relying on the cation size.

The structure of spinels is fully described by the one independent coordinate of oxygen, u, displayed in Figure 58 as a function of P. Save statistical oscillations, u exhibits a practically flat trend; if a linear function, u0+u1P, is fitted to the observed u's one obtains u0 = 0.2625(1) and u1 = 0.9(8.9) 10-6 GPa-1. This indicates that the structural behaviour under compression of MgAl-spinel is mainly governed by the shrinking of the cell edge and P acts upon MgAl2O4 producing a scaling of the structure.

 

Fig. 58: u as a function of pressure (GPa). Error bars correspond to 3.

 

References
[1] D. Levy, A. Pavese, A. Sani and V. Pischedda, Phys Chem Miner, 28, 612-618 (2001).

Principal Publication and Authors
D. Levy (a), A. Pavese (b,c), M. Hanfland (d), American Mineralogist, 88, 93­98 (2003).
(a) Università degli Studi di Torino (Italy)
(b) Università degli Studi di Milano (Italy)
(c) CNR (Italy)
(d) ESRF