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We report on the occurrence of a new high-pressure Ca-Al-silicate in localized shock melt pockets found in the feldspatic lunar meteorite Oued Awlitis 001 and discuss the implications of our discovery. The new mineral crystallized as tiny, micrometer-sized, acicular grains in shock melt pockets of roughly anorthitic bulk composition. Transmission electron microscopy based three-dimensional electron diffraction (3D ED) reveals that the CaAl4Si2O11crystals are identical to the calcium aluminum silicate (CAS) phase first reported from static pressure experiments. The new mineral has a hexagonal structure, with a space group of P63/mmcand lattice parameters of a= 5.42(1) Å; c= 12.70(3) Å; V= 323(4) Å3; Z= 2. This is the first time 3D ED was applied to structure determination of an extraterrestrial mineral. The International Mineralogical Association (IMA) has approved this naturally formed CAS phase as the new mineral “donwilhelmsite” [CaAl4Si2O11], honoring the U. S. lunar geologist Don E. Wilhelms. On the Moon, donwilhelmsite can form from the primordial feldspathic crust during impact cratering events. In the feldspatic lunar meteorite Oued Awlitis 001, needles of donwilhelmsite crystallized in ~200 mm sized shock melt pockets of anorthositic-like chemical composition. These melt pockets quenched within milliseconds during declining shock pressures. Shock melt pockets in meteorites serve as natural crucibles mimicking the conditions expected in the Earth’s mantle. Donwilhelmsite forms in the Earth’s mantle during deep recycling of aluminous crustal materials, and is a key host for Al and Ca of subducted sediments in most of the transition zone and the uppermost lower mantle (460–700 km). Donwilhelmsite bridges the gap between kyanite and the Ca-component of clinopyroxene at low pressures and the Al-rich Ca-ferrite phase and Ca-perovskite at high-pressures. In ascending buoyant mantle plumes, at about 460 km depth, donwilhelmsite is expected to break down into minerals such as garnet, kyanite, and clinopyroxene. This process may trigger minor partial melting, releasing a range of incompatible minor and trace elements and contributing to the enriched mantle (EM1 and EM2) components associated with subducted sedimentary lithologies.

The subsolidus phase diagram of silica in the 80–220 GPa pressure range was determined by density functional theory (DFT). The transition pressures calculated using the generalized gradient approximation (GGA) in the static limit (at 0 K, without zero point vibrational energy) for the β-stishovite (CaCl2- structure) to seifertite and the seifertite to pyrite-type transitions are 95 and 213 GPa, respectively. These are in good agreement with those calculated using hybrid functionals, giving transition pressures of 96 and 215 GPa. This indicates that previous local density approximation (LDA) results underestimate the transition pressure by 10–15 GPa. Density functional perturbation theory calculations, carried out using GGA within the quasi-harmonic approximations, give Clapeyron slopes of 5.4 and –2.8 MPa/K for the β-stishovite to seifertite and seifertite to pyrite-type transitions, respectively. This suggests that the seifertite-forming transition occurs at 109 GPa (470 km above the core-mantle boundary, CMB) at an ambient mantle geotherm, whereas the pyrite-type transition occurs at 200 GPa (620 km below the CMB) at 4700 K, which is close to the core adiabat. We also calculate the equation of state and show that the stability of seifertite in the lowermost mantle contributes negative buoyancy to recycled oceanic crust, although not as much as in some previous studies. Nevertheless, the increased density of seifertite over β-stishovite may lead to layers with elevated proportions of basaltic material within the large low S-wave velocity provinces. The seifertite to pyrite-type silica transition in the outer core will affect the silica liquidus surface in the system Fe-Si-O and forms a basis for further investigations of silica crystallization in the protocore.

We investigated the properties of Al-bearing SiO2(with 4 or 6 wt% Al2O3) at pressures and temperatures corresponding to the lowermost mantle, using laser-heated diamond-anvil cell coupled with synchrotron-based in situ X-ray diffraction. The phase transition from CaCl2-structured to α-PbO2- structured (seifertite) polymorphs occurs between 113 and 119 GPa at 2500 K. The range of pressure where the two phases coexist is small. There is a slight decrease of the transition pressure with increasing Al-content. We propose a tentative phase diagram reporting the minerals composition as a function of pressure in the SiO2-Al2O3system.

Unit-cell lattice parameters have been measured to ~8 GPa in a diamond anvil cell for two single crystals of CaIrO3: one with the perovskite (Pbnm) and the other with the post-perovskite (Cmcm) structure. The CaIrO3post-perovskite structure is more compressible than the perovskite. A third-order Birch Murnaghan equation of state has been used to fit the measured P-V data with the following refined parameters: V0= 229.463(8) Å3, K0= 198(3) GPa, K’ = 1.2(8); and V0= 226.38(1) Å3, K0= 181(3) GPa, K’ = 2.3(8) for CaIrO3perovskite and post-perovskite, respectively. The compressibility of the unit-cell axes of the perovskite structure is highly anisotropic with βa>> βc>> βb. In contrast, the b axis is the most compressible in the post-perovskite structure, whereas the a and c axes have similar compressibilities (with c slightly less compressible than a) and are much stiffer. A comparison between the compressibility of CaIrO3perovskite and post-perovskite with the isostructural MgSiO3phases, reveals a similar general behavior, although in detail CaIrO3perovskite is more and the postperovskite less anisotropic than the corresponding MgSiO3compounds.