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On behalf of the journal, AGU, and the scientific community, the editors would like to sincerely thank those who reviewed the manuscripts for Geophysical Research Letters in 2019. The hours reading and commenting on manuscripts not only improve the manuscripts but also increase the scientific rigor of future research in the field. We particularly appreciate the timely reviews in light of the demands imposed by the rapid review process at Geophysical Research Letters. With the revival of the “major revisions” decisions, we appreciate the reviewers' efforts on multiple versions of some manuscripts. With the advent of AGU's data policy, many reviewers have helped immensely to evaluate the accessibility and availability of data associated with the papers they have reviewed, and many have provided insightful comments that helped to improve the data presentation and quality. We greatly appreciate the assistance of the reviewers in advancing open science, which is a key objective of AGU's data policy. Many of those listed below went beyond and reviewed three or more manuscripts for our journal, and those are indicated in italics.

Under the sponsorship of the U.S. National Science Foundation, a workshop on emerging research opportunities in ceramic and glass science was held in September 2016. Reported here are proceedings of the workshop. The report details eight challenges identified through workshop discussions: Ceramic processing: Programmable design and assembly; The defect genome: Understanding, characterizing, and predicting defects across time and length scales; Functionalizing defects for unprecedented properties; Ceramic flatlands: Defining structure-property relations in free-standing, supported, and confined two-dimensional ceramics; Ceramics in the extreme: Discovery and design strategies; Ceramics in the extreme: Behavior of multimaterial systems; Understanding and exploiting glasses and melts under extreme conditions; and Rational design of functional glasses guided by predictive modeling. It is anticipated that these challenges, once met, will promote basic understanding and ultimately enable advancements within multiple sectors, including energy, environment, manufacturing, security, and health care.

Jeffbenite (Mg3Al2Si3O12) is a tetragonal phase found in so far only in superdeep diamonds, and its thermoelastic parameters are a prerequisite for determining entrapment pressures as it is regarded as a potential indicator for superdeep diamonds. In this study, the thermoelastic properties of synthetic Fe3+‐jeffbenite were measured up to 33.7 GPa and 750 K. High‐temperature static compression data were fitted, giving (∂KT0/∂T)P = −0.0107 (4) GPa/K and αT = 3.50 (3) × 10−5 K−1. The thermoelastic properties and phase stability are applied to modeling isomekes, or P‐T paths intersecting possible conditions of entrapment in diamond. We calculate that under ideal exhumation, jeffbenite entrapped at mantle transition zone conditions will exhibit a high remnant pressure at 300 K (Pinc) of ∼5.0 GPa. Elastic geobarometry on future finds of jeffbenite inclusions can use the new equation of state to estimate entrapment pressures for this phase with still highly uncertain stability field in the mantle. Plain Language Summary: Ongoing superdeep diamonds research is providing new insights into the Earth's deep mantle. Natural superdeep diamonds and its inclusions can show compelling evidence for retrograde conversion from the lower mantle or transition zone precursors; along with carbonate melt‐peridotite reactions. Jeffbenite with a composition of Mg3Al2Si3O12, found in so far only in superdeep diamonds can be regarded as a potential indicator mineral for superdeep diamonds. Recent synthesis of Fe3+‐rich jeffbenite provides an opportunity for in‐situ measurements to study the thermodynamic properties of jeffbenite at deep‐mantle conditions. Thus, in this study, we explored the high pressure and temperature stability and thermoelastic properties of Fe‐bearing jeffbenite up to 33.7 GPa and 750 K. The thermoelastic data and phase stability were measured and the results are applied to modeling the isomekes, or P‐T paths intersecting possible conditions of entrapment and along which the pressure on the inclusion is equal to the external pressure on the diamond host. Our finding can be applied to determining entrapment pressures in such diamond inclusions in future finds and its primary or retrograde history is essential in understanding mantle dynamics and the hidden consequences of plate tectonics. Key Points: We measure the phase stability and thermoelastic properties on synthetic Fe‐bearing jeffbeniteThe thermoelastic data are applied to modeling isomekes and the remnant pressure (Pinc) is determinedThe smaller thermal expansivity of jeffbenite gives rise to a broader pressure and temperature stability field in the upper lower mantle [ABSTRACT FROM AUTHOR]

Alkali-rich aluminous high-pressure phases including calcium-ferrite (CF) type NaAlSiO4 are thought to constitute ~20% by volume of subducted mid-ocean ridge basalt (MORB) under lower mantle conditions. As a potentially significant host for incompatible elements in the deep mantle, knowledge of the crystal structure and physical properties of CF-type phases is therefore important to understanding the crystal chemistry of alkali storage and recycling in the Earth's mantle. We determined the evolution of the crystal structure of pure CF-NaAlSiO4 and Fe-bearing CF-NaAlSiO4 at pressures up to ~45 GPa using synchrotron-based, single-crystal X-ray diffraction. Using the high-pressure lattice parameters, we also determined a third-order Birch-Murnaghan equation of state, with V0 = 241.6(1) Å3, KT0 = 220(4) GPa, and K T 0 ′ = 2.6 (3) for Fe-free CF, and V0 = 244.2(2) Å3, KT0 = 211(6) GPa, and ′′ K T 0 ′ = 2.6(3) for Fe-bearing CF. The addition of Fe into CF-NaAlSiO4 resulted in a 10 ± 5% decrease in the stifest direction of linear compressibility along the c-axis, leading to stronger elastic anisotropy compared with the Fe-free CF phase. The NaO8 polyhedra volume is 2.6 times larger and about 60% more compressible than the octahedral (Al,Si)O6 sites, with K 0 N a O 8 = 127 G P a and K 0 (A l , S i) O 6 ∼ 304 G P a. Raman spectra of the pure CF-type NaAlSiO4 sample shows that the pressure coefficient of the mean vibrational mode, 1.60(7) cm–1/GPa, is slightly higher than 1.36(6) cm−1/GPa obtained for the Fe-bearing CF-NaAlSiO4 sample. The ability of CF-type phases to contain incompatible elements such as Na beyond the stability field of jadeite requires larger and less-compressible NaO8 polyhedra. Detailed high-pressure crystallographic information for the CF phases provides knowledge on how large alkali metals are hosted in alumina framework structures with stability well into the lowermost mantle. [ABSTRACT FROM AUTHOR]

Hydroxylation of wadsleyite, β-(Mg,Fe)2SiO4, is associated with divalent cation defects and well known to afect its physical properties. However, an atomic-scale understanding of the defect structure and hydrogen bonding at high pressures is needed to interpret the influence of water on the behavior of wadsleyite in the mantle transition zone. We have determined the pressure evolution of the wadsleyite crystal symmetry and structure, including all O∙∙∙O interatomic distances, up to 32 GPa using single-crystal X-ray difraction on two well-characterized, Fe-bearing (Fo90) samples containing 0.25(4) and 2.0(2) wt% H2O. Both compositions undergo a pressure-dependent monoclinic distortion from orthorhombic symmetry above 9 GPa, with the less hydrous sample showing a larger increase in distortion at increased pressures due to the diference in compressibility of the split M3 site in the monoclinic setting arising from preferred vacancy ordering at the M3B site. Although hydrogen positions cannot be modeled from the X-ray difraction data, the pressure evolution of the longer O1∙∙∙O4 distance in the structure characterizes the primary hydrogen bond length. We observe the hydrogen-bonded O1∙∙∙O4 distance shorten gradually from 3.080(1) Å at ambient pressure to about 2.90(1) Å at 25 GPa, being still much longer than is defined as strong hydrogen bonding (2.5–2.7 Å). Above 25 GPa and up to the maximum pressure of the experiment at 32.5 GPa, the hydrogen-bonded O1∙∙∙O4 distance decreases no further, despite the fact that previous spectroscopic studies have shown that the primary O-H stretching frequencies continuously drop into the regime of strong hydrogen bonding (<3200 cm–1) above ~15 GPa. We propose that the primary O1-H∙∙∙O4 hydrogen bond in wadsleyite becomes highly nonlinear at high pressures based on its deviation from frequency-distance correlations for linear hydrogen bonds. One possible explanation is that the hydrogen position shifts from being nearly on the long O1-O4 edge of the M3 site to a position more above O1 along the c-axis. [ABSTRACT FROM AUTHOR]

MgSi[O.sub.3] akimotoite is stable relative to majoritegarnet under low-temperature geotherms within steeply or rapidly subducting slabs. Two compositions of Mg-akimotoite were synthesized under similar conditions: Z674 (containing about 550 ppm wt [H.sub.2]O) was synthesized at 22 GPa and 1,500°C and SH1101 (nominally anhydrous) was synthesized at 22 GPa and 1,250°C. Crystal structures of both samples differ significantly from previous studies to give slightly smaller Si sites and larger Mg sites. The bulk thermal expansion coefficients of Z674 are (153-839 K) of [a.sub.1] = 20(3) x [10.sup.-9] [K.sup.-2] and [a.sub.0] = 17(2) x [10.sup.-6] [K.sup.-1], with an average of [α.sub.0] = 27.1(6) x [10.sup.-6] [K.sup.-1]. Compressibility at ambient temperature of Z674 was measured up to 34.6 GPa at Sector 13 (GSECARS) at Advanced Photon Source Argonne National Laboratory. The second-order Birch-Murnaghan equation of state (BM2 EoS) fitting yields: [V.sub.0] = 263.7(2) [Å.sup.3], [K.sub.T0] = 217(3) GPa (K fixed at 4). The anisotropies of axial thermal expansivities and compressibilities are similar: [α.sub.a] = 8.2(3) and [α.sub.c] = 10.68(9) ([10.sup.-6][K.sup.-1]); [β.sub.a] = 11.4(3) and [β.sub.c] = 15.9(3) ([10.sup.-4] GPa). Hydration increases both the bulk thermal expansivity and compressibility, but decreases the anisotropy of structural expansion and compression. Complementary Raman and Fourier transform infrared (FTIR) spectroscopy shows multiple structural hydration sites. Low-temperature and high-pressure FTIR spectroscopy (15-300 K and 0-28 GPa) confirms that the multiple sites are structurally unique, with zero-pressure intrinsic anharmonic mode parameters between -1.02 x [10.sup.-5] and +1.7 x [10.sup.-5] [K.sup.-1], indicating both weak hydrogen bonds (O-H ... O) and strong OH bonding due to long O ... O distances. Keywords Akimotoite * Crystal structure * Thermal expansion * Compressibility * Anisotropy * FTIR, Introduction Akimotoite (ilmenite-type MgSi[O.sub.3]) is stable at pressures of 18- 25 GPa and up to about 2,000 K in the MgSi[O.sub.3] system, with wadsleyite plus stishovite stable at lower pressures, [...]

Two samples of hydroxyl-clinohumite, sample SZ0407B with approximate composition Mg8674(14) [Fe0.374.sub.(4)][([Si.sub.0.99(1)][O.sub.4]).sub.4][(OH).sub.2] and sample SZ0411B with composition [Mg.sub.9][(Si[O.sub.4]).sub.4][(OH).sub.2], were synthesized at 12 GPa and 1,250°C coexisting with olivine. Unit-cell parameters determined by single-crystal X-ray diffraction are given as follows: a = 4.7525(4) Å, b = 10.2935(12) Å, c = 13.7077(10) Å, α = 100.645(9)°, V = 659.04(9) [Å.sup.3] for SZ0407B, and a = 4.7518(6) Å, b = 10.2861(12) Å, c = 13.7008(9) Å, α = 100.638(9)°, V = 658.15(9) [Å.sup.3] for SZ0411B. Single-crystal X-ray intensity data were collected for crystal structure refinements of both samples. Relative to the pure-Mg sample, Fe decreases M3-OH bond lengths by ~0.010(3) Å, consistentwithsome ferriciron ordering into M3. Raman spectroscopy shows two strong bands in the lattice-mode region at 650 and 690 [cm.sup.-1] in the Fe-bearing sample, which are not observed in the pure-Mg sample. Spectra in the [H.sub.2]O region show at least five bands, which are deconvolved into seven distinct O-H-stretching modes. Thermal expansion measurements were carried out for both samples from 153 to 787 K by single-crystal X-ray diffraction. The average a-, b-, c-axial and volumetric thermal expansion coefficients ([10.sup.-6] [K.sup.-1]) are 10.5(1), 12.3(2), 12.5(2) and 34.9(5) for SZ0407B, respectively, and 11.1(1), 12.6(3), 13.7(3), 36.8(6) for SZ0411B, respectively. After heating, the unit-cell parameters were refined again for each sample at ambient condition, and no significant changes were observed, indicating no significant oxidation or dehydration during the experiment. For the DHMS phases along the brucite-forsterite join, linear regression gives a systematic linear decrease in expansivity with increasing density. Further, substitution of ferrous iron into these structures decreases thermal expansivity, making the Fe-bearing varieties slightly stiffer. Keywords Clinohumite * Crystal structure * Thermal expansion * Iron effect, Introduction The dense hydrous magnesium silicate (DHMS) minerals along the brucite (Br)-forsterite (Fo) join have been widely studied and proposed as potential carriers of water into the Earth's mantle(McGetchinetal., 1970;Wunder [...]

The electronic spin state of [Fe.sup.2+] in ferropericlase, ([Mg.sub.0.75][Fe.sub.0.25])O, transitions from a high-spin (spin unpaired) to low-spin (spin paired) state within the Earth's mid-lower mantle region. To better understand the local electronic environment of high-spin [Fe.sup.2+] ions in ferropericlase near the transition, we obtained synchrotron Mossbauer spectra (SMS) of ([Mg.sub.0.75],[Fe.sub.0.25])O in externally heated and laser-heated diamond anvil cells at relevant high pressures and temperatures. Results show that the quadrupole splitting (QS) of the dominant high-spin [Fe.sup.2+] site decreases with increasing temperature at static high pressure. The QS values at constant pressure are fitted to a temperature-dependent Boltzmann distribution model, which permits estimation of the crystal-field splitting energy ([[DELTA].sub.3]) between the [d.sub.xy] and [d.sub.xz] or [d.sub.zy] orbitals of the [t.sub.2g] states in a distorted octahedral [Fe.sup.2+] site. The derived [[DELTA].sub.3] increases from approximately 36 meV at 1 GPa to 95 meV at 40 GPa, revealing that both high pressure and high temperature have significant effects on the 3d electronic shells of [Fe.sup.2+] in ferroperielase. The SMS spectra collected from the laser-heated diamond cells within the time window of 146 ns also indicate that QS significantly decreases at very high temperatures. A larger splitting of the energy levels at high temperatures and pressures should broaden the spin crossover in ferropericlase because the degeneracy of energy levels is partially lifted. Our results provide information on the hyperfine parameters and crystal-field splitting energy of high-spin [Fe.sup.2+] in ferropericlase at high pressures and temperatures, relevant to the electronic structure of iron in oxides in the deep lower mantle. Keywords: Ferroperielase, diamond-anvil cell, spin transition, Mossbauer spectroscopy, high pressures, high temperatures

Iron in crustal and mantle minerals adopts several possible oxidation states: this has implications for biogeochemical processes (1), oxygenation of the atmosphere (2) and the oxidation state of the mantle [...]

The pressure-volume equation of state (EoS) of single-crystal MgO has been studied in diamond-anvil cells loaded with helium to 118 GPa and in a non-hydrostatic KC1 pressure medium to 87 GPa using monochromatic synchrotron X-ray diffraction. A third-order Birch-Murnaghan fit to the non-hydrostatic P-V data (KC1 medium) yields typical results for the initial volume, [V.sub.0] = 74.698(7) [[Angstrom].sup.3], bulk modulus, [K.sub.T0] = 164(1) GPa, and pressure derivative, K' = 4.05(4), using the non-hydrostatic ruby pressure gauge of Mao et al. (1978). However, compression of MgO in helium yields [V.sub.0] = 74.697(6) [[Angstrom].sup.3], [K.sub.T0] = 159.6(6) GPa, and K' = 3.74(3) using the quasi-hydrostatic ruby gauge of Mao et al. (1986). In helium, the fitted equation of state of MgO underdetermines the pressure by 8% at 100 GPa when compared with the primary MgO pressure scale of Zha et al. (2000), with [K.sub.T0] = 160.2 GPa and K' = 4.03. The results suggest that either the compression mechanism of MgO changes above 40 GPa (in helium), or the ruby pressure gauge requires adjustment for the softer helium pressure medium. We propose a ruby pressure gauge for helium based on shift of the ruby-[R.sub.1] fluorescence line ([DELTA][LAMBDA]/[[LAMBDA].sub.0]) and the primary MgO pressure scale, with P (GPa) = A/B{[[1 + ([DELTA][LAMBDAP]/[[LAMBDA].sub.0]).sup.B] - 1}, where A is fixed to 1904 GPa and B = 10.32(7). Keywords: MgO, helium pressure medium, static compression, equation of state, ruby fluorescence

A suite of pure magnesian wadsleyite ([beta]-[Mg.sub.2]Si[O.sub.4]) samples containing 0.005, 0.38, 1.18, and 1.66 wt% [H.sub.2]O was studied by single-crystal X-ray diffraction to determine the effects of hydration on cation ordering and crystal symmetry. Separate compressibility experiments were carried out to 9.6 GPa to determine the effects of hydration on isothermal equations of state. Crystal-structure refinements at ambient conditions show cation vacancies order onto the M3 site. The most hydrous sample (1.6 wt% [H.sub.2]O) displayed monoclinic symmetry with [beta] = 90.090(7)[degrees], whereas the samples with lower content were statistically orthorhombic. The density of wadsleyite decreases with increasing water content at STP according to the empirical relation, [rho] = 3.470(2) -0.046(2) [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII.] g/[cm.sup.3] (with [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII.] in wt% [H.sub.2]O). Bulk moduli and pressure derivatives of wadsleyite are [K.sub.T0] = 173(5) GPa, [K.sub.0]' = 4.1(15) for 0.005 wt% [H.sub.2]O; [K.sub.T0] = 161(4) GPa, [K.sub.0]' = 5.4(11) for 0.38 wt% [H.sub.2]O; [K.sub.T0] = 158(4) GPa, [K.sub.0]' = 4.2(9) for 1.18 wt% [H.sub.2]O; and [K.sub.T0] = 154(4) GPa, [K.sub.0]' = 4.9(11) for 1.66 wt% [H.sub.2]O. Variation of the bulk modulus of wadsleyite with water content is non-linear, which may be attributable to softening of the structure by ordering of vacancies onto two non-equivalent M3 sites (M3a and M3b) and an accompanying dilution of orthorhombic symmetry. Keywords: Wadsleyite, bulk modulus, equation of state, nominally anhydrous minerals, mantle Transition Zone

The pressure dependence of the sound velocities, single-crystal elastic constants, and shear and adiabatic bulk moduli of a natural gahnite (Zn[Al.sub.2][O.sub.4]) spinel have been determined to ~9 GPa by gigahertz ultrasonic interferometry in a diamond anvil cell. The elastic constants of gahnite are (in GPa): [C.sub.11] = 290(3), [C.sub.12] = 169(4), and [C.sub.44] = 146(2). The elastic constants [C.sub.11] and [C.sub.12] have similar pressure derivatives of 4.48(10) and 5.0(8), while the pressure derivative of [C.sub.44] is 1.47(3). In contrast to magnetite, gahnite does not exhibit [C.sub.44] mode softening over the experimental pressure range. The adiabatic bulk modulus [K.sub.s0] is 209(5) GPa, with pressure derivative [K'.sub.s] = 4.8(3), and the shear modulus [G.sub.0] = 104(3) GPa, with G'= 0.5(2). Gahnite, along with chromite (Fe[Cr.sub.2][O.sub.4]) and hercynite (Fe[Al.sub.2][O.sub.4]) are the least compressible of the naturally occurring oxide spinels. Evaluation of Birch's Law for isostructural minerals indicates that spinels containing transition metals on both the [[sup.4]]A and [[sup.6]]B sites follow a trend about five times more negative than oxide and silicate-spinel phases without any, or only one transition metal. Keywords: Spinel, sound wave velocities, elastic properties, Bitch's Law