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High‐pressure silica polymorphs may contribute to lithologies in Earth's lower mantle. Stishovite, the tetragonal rutile‐type polymorph of SiO2, distorts to an orthorhombic CaCl2‐type structure at pressures and temperatures of the lower mantle. We compressed sintered polycrystalline stishovite and determined the unit cell parameters as a function of pressure using synchrotron X‐ray diffraction. The compression behavior of sintered polycrystalline stishovite deviates systematically from previous results on stishovite powder and single crystals. Following an initial stiffening, the bulk modulus of sintered polycrystalline stishovite drops at the ferroelastic phase transition. We analyzed the observed spontaneous strains using Landau theory to predict the complete elastic behavior of sintered polycrystalline silica across the phase transition. The reduction in bulk modulus at the phase transition as derived here from the compression curve of sintered polycrystalline silica may have implications for the seismic detection of silica‐rich materials in Earth's lower mantle.

Cubic CaSiO3 perovskite is a major phase in subducted oceanic crust, where it forms at a depth of about 550 km from majoritic garnet 1,2 . We measured the plastic strength of cubic CaSiO3 perovskite at pressure and temperature conditions typical for a subducting slab up to a depth of about 1200 km. Contrary to tetragonal CaSiO3 previously investigated at room temperature 3,4 , we find that cubic CaSiO3 perovskite is a comparably weak phase at temperatures of the lower mantle. We find its viscosity to be substantially lower as compared to bridgmanite and ferropericlase, possibly making cubic CaSiO3 perovskite the weakest lower mantle phase. Our findings suggest that cubic CaSiO3 perovskite will govern the dynamics of subducting slabs. It further provides a mechanism to separate subducted oceanic crust from the underlying mantle. Depending on the depth of the separation, basaltic crust could accumulate at the boundary between the upper and lower mantle, where cubic CaSiO3 2 perovskite may contribute to the seismically observed regions of low shear wave velocities in the uppermost lower mantle 5,6 , or sink to the core-mantle boundary and explain the seismic anomalies associated with Large Low Shear Velocity Provinces LLSVPs beneath Africa and the Pacific 7-9 .CaSiO3 perovskite is expected to be the third most abundant phase in Earth's transition zone and lower mantle, where it may account for up to 25 Vol.% of subducted basaltic crust and up to 10 Vol.% of a pyrolitic mantle 1,2 . Direct evidence for its existence has recently been reported from analysis of inclusions in super-deep diamonds 10 . At room temperature (T) and high pressure (P), CaSiO3 perovskite has a tetragonal crystal structure, but undergoes a phase transition to a cubic structure with increasing temperature, stabilizing the cubic phase along a typical mantle geotherm 6,7,11,12 . It has been inferred that the elastic properties of cubic CaSiO3 perovskite might explain regions of reduced shear wave velocities in the uppermost lower mantle 6 as well as some of the unique seismic properties of the Large Low Shear Velocity Provinces (LLSVP) observed by seismic tomography in the deep lower mantle [7][8][9] .While recent works succeeded to measure elastic wave velocities of cubic CaSiO3 perovskite at high-pressure/-temperature 6,7 , experimental information on its rheology is absent. Previous studies of the deformation behavior of CaSiO3 are limited to its tetragonal polymorph at room T (refs. 3,4) due to experimental difficulties preventing access to mantle-like P-T-conditions. These works suggest that CaSiO3 perovskite is characterized by a high plastic strength, exceeding that of MgSiO3 bridgmanite, the dominant phase in Earth's lower mantle. Early computational work, however, predicted that the lattice friction, i.e. the Peierls stress, of the <110>{1-10} slip system

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11We present the first equation of state and structure refinements at high-pressure of single-crystal 12 Phase Egg, AlSiO3OH. Single-crystal synchrotron X-ray diffraction was performed up to 23 GPa. 13We observe the b axis to be the most compressible direction and the β angle to decrease up to 16 14GPa and then to remain constant at a value of ~ 97.8° up to the maximum experimental pressure. 15Structure refinements performed at low pressures reveal a distorted octahedron around the silicon 16 atom due to one of the six Si-O bond lengths being significantly larger than the other five. The 17 length of this specific Si-O4 bond rapidly decreases with increasing pressure leading to a more 18 regular octahedron at pressures above 16 GPa. We identified the shortening of the Si-O4 bond and 19 the contraction of the vacant space between octahedral units where the hydrogen atoms are assumed 20 to lie as the major components of the compression mechanism of AlSiO3OH Phase Egg. The unit-21 cell volume decrease with pressure can be described by a third order Birch-Murnaghan equation of 22 state with the following parameters: V0 = 214.1(2) ų, K0 = 153(8) GPa and K0' = 8.6(1.3). 23 24

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