Calcolo ab initio delle proprietà termodinamiche della maohokite: una fase minerale che potrebbe essere presente nel mantello inferiore terrestre. Implicazioni per la geochimica del mantello.

Naturally impacted terrestrial rocks and meteorites afford a unique window to look into the mineral constituents of the mantle rocks in fact, they provide a suite of natural high-pressure minerals similar to those believed to make up the deep Earth and provide an alternative source of information on deep-mantle mineralogy; this because although the deep mantle contains abundant high-pressure minerals, most of them cannot be preserved during the long journey to the surface. In shocked gneiss coming from the Xiuyan crater in China a new mineral was found: maohokite, a high-pressure post-spinel polymorph of MgFe2O4. This mineralogical phase might be present in the lower Earth mantle and it could have significant implications regarding the oxidation state of the Earth mantle due to the fact that it incorporates trivalent iron within its crystalline structure. Starting from this discovery, the aim of this thesis has been to carry out a thermodynamic modeling of this mineralogical phase. In order to achieve it, it has been necessary to calculate thermodynamic properties of maohokite starting from ab initio quantum mechanical calculations of static energy and frequencies of vibrational normal modes, computed at different values of the unit cell volume of the crystal. Once thermodynamic properties were computed, the maohokite stability field has been investigated in order to infer at which pressures and temperatures this mineralogical phase is stable and therefore to identify the portion of the Earth mantle and the range of depth at which maohokite might be present. As the stability field of maohokite is assessed, it has been possible to observe its role as possible carriers of trivalent iron in the Earth mantle and therefore to better understand the implications to the possible oxidation state of the Earth mantle.