Natural cubic perovskite, Ca(Ti,Si,Cr)O3-δ, a versatile potential host for rock-forming and less-common elements up to Earth's mantle pressure

Perovskite, CaTiO3, originally described as a cubic mineral, is known to have a distorted (orthorhombic) crystal structure. We herein report on the discovery of natural cubic perovskite. This was identified in gehlenite-bearing rocks occurring in a pyrometamorphic complex of the Hatrurim Formation (the Mottled Zone), in the vicinity of the Dead Sea, Negev Desert, Israel. The mineral is associated with native alpha-(Fe,Ni) metal, schreibersite (Fe3P), and Si-rich fluorapatite. The crystals of this perovskite reach 50 mu m in size and contain many micrometer-sized inclusions of melilitic glass. The mineral contains significant amounts of Si substituting for Ti (up to 9.6 wt% SiO2), corresponding to 21 mol% of the davemaoite component (cubic perovskite-type CaSiO3), in addition to up to 6.6 wt% Cr2O3. Incorporation of trivalent elements results in the occurrence of oxygen vacancies in the crystal structure; this is the first example of natural oxygen-vacant ABO(3) perovskite with the chemical formula Ca(Ti,Si,Cr)O3-delta(delta similar to 0.1). Stabilization of cubic symmetry (space group Pm (3) over barm) is achieved via the mechanism not reported so far for CaTiO3, namely displacement of an O atom from its ideal structural position (site splitting). The mineral is stable at atmospheric pressure to 1250 +/- 50 degrees C; above this temperature, its crystals fuse with the embedded melilitic glass, yielding a mixture of titanite and anorthite upon melt solidification. The mineral is stable upon compression to at least 50 GPa. The a lattice parameter exhibits continuous contraction from 3.808(1) angstrom at atmospheric pressure to 3.551(6) angstrom at 50 GPa. The second-order truncation of the Birch-Murnaghan equation of state gives the initial volume V-0 equal to 55.5(2) angstrom(3) and room temperature isothermal bulk modulus K-0 of 153(11) GPa. The discovery of oxygen-deficient single perovskite suggests previously unaccounted ways for incorporation of almost any element into the perovskite framework up to pressures corresponding to those of the Earth's mantle.

Automated summary

This study reports the discovery of natural cubic perovskite in the Hatrurim Formation in the Negev Desert. The mineral contains Si and Cr substitutions, as well as oxygen vacancies. The findings suggest new ways for incorporating elements into the perovskite framework.