Mechanisms and conditions of Ti and Cr incorporation in mantle phlogopite: the results of atomistic simulation

Abstract

Modeling of eight mechanisms for the incorporation of Ti4+ and Cr3+ impurity components into phlogopite was carried out by a semi-empirical method using the GULP (General Utility Lattice Program) software. The calculation of thermodynamic mixing properties in the range of 1–7 GPa and 373–1573 K and the analysis of the structure geometry for the simulated solid solutions provided the following energy-preferred schemes of isomorphic substitution: VI(Mg2+) + 2IV(Si4+) = VI(Ti4+) + 2IV(Al3+) and VI(Mg2+) + 2IV(Al3+) = VI(□) + 2IV(Ti4+), VI(Mg2+) + IV(Si4+) = VI(Cr3+) + IV(Al3+), and 3VI(Mg2+) = VI(Al3+) + VI(Cr3+) + VI(□). It is shown the scheme 2VI(Mg2+) = VI(Ti4+) + VI(□) illustrating entrance of Ti with the formation of a vacancy is realized in the case of microconcentrations of Ti only. Accumulation of high-Ti contents associates with the formation of a vacancy in the octahedral site. This provides incorporation of Ti via the schemes VI(Mg2+) + 2IV(Al3+) = VI(□) + 2IV(Ti4+) and (Mg, Fe2+) + 2OH− = Ti4+ + 2O2− only. It is shown that incorporation of high-Cr concentrations (> 5.5 wt % Cr2O3) is accompanied by an increase in the number of vacancies in the octahedral site with an increase in the proportion of the dioctahedral component K(Al, Cr, □)2AlSi3O10(OH)2.