Andreozzi, Giovanni B.

Anzolini, Chiara

Lenaz, Davide

Nestola, Fabrizio

Periotto, Benedetta

Princivalle, Francesco

Padua Research Archive

Equation of state of hercynite spinel, FeAl2O4, and high-pressure systematics of Mg- Fe-Cr-Al spinels

Archivio istituzionale della ricerca - Università di Padova

!!!Equation of state of hercynite spinel, FeAl2O4, and high-pressure systematics of Mg-Fe-Cr-Al spinels  Anzolini C.*1, Nestola F.1, Periotto B.1, Andreozzi G.B.2, Lenaz D.3 & Princivalle F.3  1. Dipartimento di Geoscienze, Università di Padova. 2. Dipartimento di Scienze della Terra, Sapienza Università di Roma. 3. Dipartimento di Matematica e Geoscienze, Università di Trieste.  Corresponding email: chiara.anzolini@studenti.unipd.it  Keywords: hercynite, diffraction, pressure, diamond.  Due to their abundance in several geological environments, spinels cover a crucial role in geology. A high number of scientific papers have been published on their stability and behaviour and, more recently, several works focused on their elastic behaviour under extreme conditions of temperature and pressure, which can be used to retrieve the conditions at which spinels crystallize. In detail, spinels of high-pressure mantle origin represent one of the most common inclusions found in diamonds and therefore they can provide strong indications about the diamond formation environment. In this specific case, the most common spinels are represented by the Cr-spinels, mainly by solid solutions constituted by magnesiochromite, MgCr2O4, and chromite, FeCr2O4. However, the hercynite component, FeAl2O4, is not negligible at all in high-pressure spinels with percentage reaching 10-15%, whereas the spinel s.s., MgAl2O4, only represents some unit percentage. However, whereas the high-pressure behaviour of MgCr2O4, FeCr2O4 and MgAl2O4 has been definitively investigated even very recently (i.e. Nestola et al., 2014; Nestola et al., 2007), the elastic behaviour of hercynite remains still to be studied and compared with that of the other end-members in order to understand which is the effect of the Al for Cr substitution under high-pressure conditions on the elasticity of Cr-rich spinels. In our knowledge the only datum on the bulk modulus of hercynite was published 42 years ago by Wang and Simmons (1972) by ultrasonic measurements. Wang and Simmons (1972) reported a Ks value of 210.3 GPa. Here we have investigated a synthetic single crystal of pure hercynite, FeAl2O4, by X-ray diffraction under high-pressure conditions and room temperature using a diamond-anvil cell up to about 7.5 GPa in order to verify if the very old data can be still valid. For purpose of comparison we used the same experimental approach used in Nestola et al. (2014) on MgCr2O4 and FeCr2O4 and Nestola et al. (2007) on MgAl2O4. Our results could be crucial in determining correct stability field calculations, which are based on thermodynamic properties, like the bulk modulus.  Wang E. & Simmons G. 1972. Elasticity of some mantle crystal structures. I. Pleonaste and hercynite spinels. Journal of Geophysical Research, 77, 4379-4392. Nestola F., Periotto B., Andreozzi G.B., Bruschini E. & Bosi F. 2014. Pressure-volume equation of state for chromite and magnesiochromite: a single-crystal X-ray diffraction investigation. American Mineralogist, in press. Nestola F., Boffa Ballaran T., Balic-Zunic T., Princivalle F., Secco L. & Dal Negro A. 2007. Comparative compressibility and structural behavior of spinel MgAl2O4 at high pressures: The independency on the degree of cation order. American Mineralogist, 92, 1838-1843.                     351PLINIUS n. 40, 2014

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Equation of state of hercynite spinel, FeAl2O4, and high-pressure systematics of Mg- Fe-Cr-Al spinels

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