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Post-print of: Physics of the Earth and Planetary Interiors volumes 190191, January 2012, pages 8794

Aluminum incorporation in -PbO2 type TiO2 at pressures up to 20 GPa

Alberto Escudero, Falko Langenhorst

Bayerisches Geoinstitut, Universitt Bayreuth, D-95440 Bayreuth, Germany

Abstract

Aluminum incorporation into the high pressure polymorph of TiO2 with the structure of -PbO2 has been studied from 10 to 20 GPa and 1300 C by XRD, high-resolution 27Al MAS-NMR and TEM. Al-doped -PbO2 type TiO2 can be recovered at atmospheric pressure. Al2O3 solubility in -PbO2 type TiO2 increases with increasing the synthesis pressure. The -PbO2 type TiO2 polymorph is able to incorporate up to 35 wt.% Al2O3 at 13.6 GPa and 1300 C, being the substitution of Ti4+ by Al3+ on normal octahedral sites the mechanism of solubility. The transition to the higher pressure TiO2 polymorph with the ZrO2 baddeleyite structure, Akaogiite, has not been observed in the quenched samples at room pressure. The microstructure of the recovered sample synthesized at 16 GPa and 1300 C points to the existence of a non-quenchable aluminum titanium oxide phase at these conditions.

Keywords

TiO2-II; Alumina; High-pressure; TEM; 27Al MAS-NMR; Non-quenchable phases; Solid solutions; Stacking faults

1. Introduction

Titanium dioxide TiO2 has been intensively studied for a few decades due to both basic and applied interests in geology and material science. Rutile is a common minor mineral in metamorphic and plutonic igneous rocks. Rutile transforms to a high pressure TiO2 polymorph with the structure of -PbO2 at about 9 GPa and 1300 C, despite experiments on the phase boundary between rutile and the -PbO2 type TiO2 polymorph yielded quite controversial results (Akaogi et al., 1992, Olsen et al., 1999 and Withers et al., 2003). This last polymorph has been used as a geobarometer to indicate ultra high pressure metamorphism (UHPM) (Hwang et al., 2000 and Wu et al., 2005), despite the suitableness of using nano-structured -PbO2 type TiO2 as an indicator of ultra high pressure is still under discussion (Chen and Fu, 2006, Escudero et al., 2012 and Wu et al., 2006). At about 17 GPa and 1300 C TiO2 transforms into a polymorph with the structure of ZrO2 baddeleyite (Sato et al., 1991). This mineral has been recently encountered in heavily shocked garnet-cordierite-sillimanite gneiss in the Suevite breccia of the Ries meteorite impact crater in Germany and has been named as Akaogiite (El Goresy et al., 2010). Higher pressure TiO2 polymorphs have also been reported in the literature (Dubrovinskaia et al., 2001).

Aluminum is one of the trace elements in rutile that has been suggested to provide information on the P-T conditions of the rutile-bearing metamorphic rocks. In fact, exsolved corundum lamellae have been reported in diamondiferous eclogites from South Africa (Sobolev and Yefimova, 2000). Recent studies have shown that Al solubility in TiO2 rutile increases drastically with pressure, with two different mechanism of solubility. Enhanced aluminum concentration in TiO2 rutile as well as (1 1 0) twinned CaCl2-type structure TiO2 grains are thus a clear indication of high-pressure conditions (Escudero et al., 2011b). However, in order to evaluate the possibility of using the Al incorporation into TiO2 phases as a possible indicator of high pressure conditions, the effect of the nature of the different TiO2 polymorphs at higher pressure on the Al incorporation into TiO2 phases should be determined. We present in this paper a chemical and microstructural study of the Al incorporation into TiO2 phases at pressures between 7 and 20 GPa, where the high pressure polymorphs of TiO2 are expected to be stable.

2. Experiments and methods

2.1. Synthesis

High-pressure experiments were performed from commercial Al2TiO5 (SigmaAldrich nanopowder,