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Experiments at high pressures and temperatures reveal the stability of a [Fe.sub.4][O.sub.5]-type structured phase in several simple chemical systems. On the one hand, the [Fe.sub.4][O.sub.5] end-member is stable in the presence of Si[O.sub.2]-rich phases, including stishovite, but contains ≤ 0.01 Si cations per formula unit. This indicates that Si is essentially excluded from this phase. On the other hand, the [Fe.sub.4][O.sub.5] phase can form solid solutions with Mg and Cr and can coexist with silicate phases at the high P-T conditions expected in the transition zone of the mantle (i.e. >~9 GPa). It can coexist with both wadsleyite and Mg-rich ringwoodite and can contain at least 25 mol% [Mg.sub.2][Fe.sub.2][O.sub.5] component. The [Fe.sub.4][O.sub.5] phase always contains the least amount of Mg in any given mineral assemblage. Cr-bearing [Fe.sub.4][O.sub.5] has been synthesised with up to 46 mol% [Fe.sub.2][Cr.sub.2][O.sub.5] component and can coexist with spinel and/or hematite-eskolatite solid solutions. Substitution of Mg and Cr for [Fe.sup.2+] and [Fe.sup.3+], respectively, leads to variations in [Fe.sup.3+]/ ΣFe from the ideal value of 0.5 for the [Fe.sub.4][O.sub.5] end-member composition, which can influence its redox stability. These cations also have contrasting effects on the unit-cell parameters, which indicate that they substitute into different sites. This initial study suggests that [Fe.sub.4][O.sub.5]-type structured phases may be stable over a range of P-T-f[O.sub.2] conditions and bulk compositions, and can be important in understanding the post-spinel phase relations in a number of chemical systems relevant to the Earth's transition zone. Thus, the presence of even small amounts of [Fe.sup.3+] could alter the expected phase relations in peridotitic bulk compositions by stabilising this additional phase. Keywords [Fe.sub.4][O.sub.5] * Solid solution * Transition zone * High pressure * Fe-oxides * Oxidation state, Introduction A new iron oxide phase with [Fe.sub.4][O.sub.5] stoichiometry has been recently discovered to be stable at high pressures and temperatures (Lavina et al. 2011). More recently, Woodland et al. [...]

The role of the oxo-component on the compressibility of amphibole was studied by means of high-pressure in situ single-crystal X-ray diffraction on two natural kaersutite megacrysts (samples DL5 and FR12) from alkaline basalts. The oxo-component varies significantly (1.1 and 1.9 apfu in DL5 and FR12, respectively), whereas the cation composition is very similar, apart from the [Fe.sup.3+]/([Fe.sup.2+][Fe.sup.3+]), which is 0.33 in DL5 and ~1 in FR12. The larger oxo-component of FR12 is attributed to the [Fe.sup.2+] + O[H.sup.-] = [Fe.sup.3+] + [O.sup.2-] + 1/2[H.sub.2] substitution. Unit-cell parameters were collected at different pressures up to about 8 GPa. Structural refinements of both samples were performed with data collected at different P up to 6 GPa. Fitting the P-V data to a third-order Birch Murnaghan EoS yielded the following parameters: [K.sub.0] = 94(1) GPa, K' = 6.3(4), and [V.sub.0] = 903.6(2) [[Angstrom].sup.3] for FR12 and [K.sub.0] = 91(2) GPa, K' = 6.2(4), and [V.sub.0] = 914.1(2) [[Angstrom].sup.3] for DL5. The axial moduli of the two amphibole samples were: [K.sub.0a] = 86(3) GPa, [K'.sub.a] = 7(1), and [a.sub.0] = 9.815(2) [Angstrom]; [K.sub.0b] = 115(3) GPa, [K'.sub.b] = 4.8(8), and [b.sub.0] = 18.012(2) [Angstrom]; [K.sub.0c] = 112(5) GPa, [K'.sub.c] = 7(1), and [c.sub.0] = 5.300(1) [Angstrom], for sample FR12 and [K.sub.0a] = 85(3) GPa, [K'.sub.a] = 5(1), and [a.sub.0] = 9.8660(9) [Angstrom]; [K.sub.0b] = 113(2), [K'.sub.b] = 4.4(6), and [b.sub.0] = 18.0548(6) [Angstrom]; [K.sub.0c] = 107(3) GPa, [K'.sub.c] = 7(1), and [c.sub.0] = 5.3185(5) [Angstrom], for sample DL5. This suggests that the compressibility of kaersutite decreases with increasing oxo-component. Structural refinements show that the polyhedral compressibility follows the order A = M4 > M2 > M3 > M1 for DL5 andA = M4 > M2 > M1 > M3 for FR12. The most evident geometrical effect induced by P is the decrease in the bending of the double tetrahedral chain, when adjacent I-beams are pushed against each other. This effect is largest for DL5, which has a larger concavity of the A site, (O7-O7' changes from 3.03 to 2.82 [Angstrom]) compared to the one of FR12, (O7-O7' changes from 2.92 to 2.79 [Angstrom]). This mechanism is confirmed by the evolution of T1-O7-T1 angle (from 135.4[degrees] to 132.5[degrees] in FR12 and from 136.6[degrees] to 132.2[degrees] in DL5). Keywords: Oxo-amphiboles, kaersutite, compressibility, equation of state, high-pressure structure, amphibole DOI: 10.2138/am.2010.3429

Garnet solid solutions have been synthesized across the skiagite-Fe-knorringite [Fe.sup.2+.sub.3][Fe.sup.3+.sub.2] [Si.sub.3][O.sub.12]-[Fe.sub.3][Cr.sub.2][Si.sub.3][O.sub.12] binary join. Such compositions reflect a simple Cr-[Fe.sup.3+] exchange on the octahedral sites, with [Fe.sup.2+] occupying the adjacent dodecahedral sites. Solid solution is complete across the join and the correct stoichiometry was verified by Mossbauer spectroscopy. A symmetric fit to the molar volume data yields 119.77(2) [cm.sup.3]/mol [unit-cell parameter [a.sub.o] =11.6736(7) [Angstrom]] for the Fe-knorringite end-member and a small negative excess volume, [W.sub.r] = -0.76(15) [cm.sup.3]/mol. Combining this result with literature data reveals that binary joins involving octahedral site substitutions exhibit significantly different behavior than those where substitution occurs on the dodecahedral sites. In the former case, [W.sub.v] is usually negative, whereas the latter joins have positive deviations from ideal behavior. Therefore, we conclude that the garnet structure responds in a fundamentally different way when accommodating different cations on the dodecahedral or octahedral sites. Mossbauer spectra of skiagite-Fe-knorringite garnets do not exhibit any significant asymmetry in the [sup.[8]] [Fe.sup.2+] doublet. [Fe.sup.3+]/[summation]Fe values determined at room temperature and 80 K confirm the general applicability of the recoil-free fraction correction factors reported by Woodland and Ross (1994) for mixed-valence garnets. Coexisting spinels in some samples are either binary [Fe.sub.3][O.sub.4]-[Fe.sub.2]Si[O.sub.4] or [Fe.sub.3][O.sub.4]-Fe[Cr.sub.2][O.sub.4] solid solutions. Very little mutual solubility is apparent suggesting a significant solvus may exist between the silicate and Cr-bearing spinel series. Keywords: Fe-knorringite, Fe-Cr garnet, skiagite, molar volume, Mossbauer spectroscopy, excess thermodynamic properties, Fe-Cr spinel

Four synthetic crystals belonging to the jadeite (Jd, NaAl[Si.sub.2][O.sub.6])-hedenbergite (Hd, CaFe[Si.sub.2][O.sub.6]) solid solution were investigated by X-ray diffraction in situ at high pressure using a diamond anvil cell to [P.sub.max] = 10.6 GPa. The samples exhibited space group symmetry C2/c throughout the investigated pressure range and did not show any phase transformations. [V.sub.0], [K.sub.T0], and K' were simultaneously refined by fitting a third-order Birch-Murnaghan equation of state to pressure-volume data for all samples. The following relationship between bulk modulus and molar fraction of jadeite is observed: [K.sub.T0] = 108.7(2) (GPa) + 0.191(9) x [% molar Jd] + 0.0006(1) x [[% molar Jd].sup.2] The bulk modulus of hedenbergite is 19% lower than jadeite with a strong axial anisotropy that increases with increasing the Hd content. In particular, the compressibility along the b axis (the most compressible in pyroxenes) increases by about 35% going from Jd to Hd while along the c axis the increase in compressibility is about 24%. The a axis does not show any variation in the deformation rate along the join. The analysis of the crystal structure behavior with pressure for all samples clearly indicates that the main cause of the strong anisotropy on the b-c plane is related to the narrowing of the M 1 octahedral chain and to anion-anion interactions increasing the packing efficiency of the anion skeletons of the crystals going from Jd to Hd. Keywords: High pressure, solid solution, crystal structure, X-ray

A single crystal of a natural orthopyroxene with composition [sup.M2][[[Fe.sup.2+.sub.818][Mg.sub.0.156][Ca.sub.0.010] [Mn.sub.0.016]].sup.Ml][[Fe.sup.2+.sub.0.081][Mg.sub.0.0767] [Al.sub.0.84][Fe.sup.3+.sub.0.068]sup.TA][Si][sup.TA] [Si.sub.0.848][Al.sub.0.152][O.sub.6] and space group Pbca(sampleS95) was investigated a thigh pressure by X-ray diffraction using a diamond-anvil cell up to 9.56 GPa. No phase transitions were detected in the pressure range investigated. The unit-cell parameters, a, b, and c, decrease non-linearly with pressure and show an axial compression anisotropy with a ratio [[beta].sub.a]:[[beta].sub.b]: [[beta].sub.c] = 1.00:1.64:1.16. The unit-cell volume decreases non-linearly as well and with a negative variation, by about 6.3% up to 9.56 GPa. The equation of state calculated using high-accuracy volume-pressure data up to 5.5 GPa gave the following coefficients: [V.sub.o] = 846.02(4) [[angstrom].sup.3], [K.sub.T0] = 115.4(6) GPa, K' = 7.7(3). Among the Mgorthopyroxenes investigated at high pressure so far S95 shows the highest bulk modulus. Six complete intensity data were collected at 0, 0.16, 1.72, 3.95, 8.[O.sub.3], and 9.56 GPa. The results confirm previous conclusions regarding the compressional mechanism in orthopyroxenes. At lower pressures, compression is mostly connected to a decrease in volume of the two M-coordination octahedra, accompanied by an increased kink in the B-tetrahedral chain. At higher pressures, compression of the M sites decreases, the kink of the tetrahedral chains stops to change, and reduction in unit-cell volume is accompanied mainly by compression of tetrahedra. This change in compressional trends results in a relatively large K' parameter and a pronounced stiffening of the structure with pressure. The presence oral in the TB tetrahedral site influences the kink of the B tetrahedral chain, which depends on the ratio of sizes of the M2 and TB coordination polyhedra. The increased stiffness of the M polyhedra, caused by the presence of Fe, is the main reason for the high bulk modulus of S95 and its resistance to shortening of the e axis. This explains the limited shortening of the c axis in S95 and the different compressional axial anisotropy with respect to other orthopyroxenes investigated under high pressure. Keywords: Orthopyroxene, high pressure, crystal structure, X-ray diffraction

The equation of state and the crystal structure evolution with pressure were determined for two single crystals of pure natural Mg[Al.sub.2][O.sub.4] spinels with different degrees of order. The two samples studied were cut from a larger single crystal and one of them was experimentally disordered at high temperature. The two crystals, showing an inversion parameter x of 0.27 and 0.15 at ambient conditions, were loaded together in a diamond anvil cell and their unit-cell edge was measured up to about 7.5 GPa at 14 different pressures. The unit-cell volume, [V.sub.0], the bulk modulus, [K.sub.T0], and its first pressure derivative, K', were simultaneously refined using a third-order Birch-Murnaghan equation of state, giving the following coefficients: [V.sub.0] = 529.32(2) [[Angstrom].sup.3], [K.sub.T0] = 193(1) GPa, K' = 5.6(3) for the ordered sample and [V.sub.0] = 528.39(2) [[Angstrom].sup.3], [K.sub.T0] = 192(1) GPa, K' = 5.4(3) for the disordered one. Complete intensity data were collected at 0, 0.44, 2.92, 7.34, and 8.03 GPa in a separate experiment. For the ordered and disordered samples the oxygen atomic coordinate u remains practically unchanged inside the investigated pressure range with an average value of 0.2633(5) and 0.2614(2), respectively. As a consequence, the polyhedral compressibilities are similar and are not influenced by the Mg/Al distribution over the two crystallographic sites. This also suggests that pressure has little or no influence on the degree of order in the Mg[Al.sub.2][O.sub.4] spinel. Keywords: X-ray single-crystal diffraction, spinel, cation ordering, high pressure

The crystal-structures of seven synthetic pyroxenes along the jadeite hedenbergite ([Jd.sub.57][Hd.sub.43], [Jd.sub.26][Hd.sub.74], [Jd.sub.0][Hd.sub.100]) and jadeite-aegirine ([Jd.sub.100][Ae.sub.0][Jd.sub.76][Ae.sub.24], [Jd.sub.35][Ae.sub.65], [Jd.sub.0][Ae.sub.100]) joins were refined using data collected by means of single-crystal X-ray diffraction (space group C2/c, [R.sub.4[sigma]] between 2.2 and 3.4%). The M2 and M1 polyhedral volumes and bond lengths increase with increasing aegirine and hedenbergite content, moreover the Ca for Na substitution along the jadeite-hedenbergite join changes the M2 coordination from 6 + 2 to 4 + 4, with remarkable tilting of the tetrahedral chains. The value of the displacement parameters follows the trend [U.sub.eq]M2 > [U.sub.eq]O2 > [U.sub.eq]O3 > [U.sub.eq]O1 > [U.sub.eq]M1 [approximately equal to] [U.sub.eq]T for all samples belonging to the jadeite-aegirine join and for pure hedenbergite; in contrast, for pyroxenes with intermediate compositions between hedenbergite andjadeite the trend is [U.sub.eq]O1 > [U.sub.eq]O2 > [U.sub.eq]M2 > [U.sub.eq]O3 > [U.sub.eq]M1 [approximately equal to] [U.sub.eq]T, with O1 and O2 having anomalously large displacement parameters, probably due to different local structural configuration around the cations with different size and charge. Cation substitution at the M1 site of Na-pyroxenes gives rise to a different structural deformation with respect of the double substitution at both the M1 and M2 sites in (Na, Ca)([M.sup.3+],[M.sup.2+])[Si.sub.2][O.sub.6] pyroxenes as the rigid tetrahedral chains try to accommodate both the increasing size of the M1 site and the different coordination requirement of the M2 site. Keywords: Single crystal, X-ray diffraction, crystal-structure, clinopyroxenes

The effects of cation substitution and ordering in the otavite (CdC[O.sub.3])-magnesite (MgC[O.sub.3]) solid solution have been investigated on samples synthesized at 1 GPa in the temperature range 500-800 [degrees]C for run durations up to 120 h. A complete, disordered solid solution, with R3c symmetry, was obtained at 800 [degrees]C, whereas Mg and Cd show partial ordering within the cadmium dolomite stability field, with R[bar.3] symmetry, at intermediate compositions in the temperature range 500-650 [degrees]C. Rietveld refinements for X-ray diffraction data show that variation of the a-axis is linear as a function of composition, independent of the degree of order, whereas the c-axis shows a positive deviation from linearity as a function of composition, decreasing with increasing degree of order. Octahedral bond distances of the 800 [degrees]C series vary linearly with composition. Site occupancies were used to determine the long-range order parameter, Q, for samples with R[bar.3] symmetry. [Q.sup.4] vades linearly as a function of temperature, suggesting a tricritical phase transition with a critical transition temperature [T.sub.c] of 702(10) [degrees]C. EDXTEM compositional microanalyses of samples within two-phase regions are in good agreement with Rietveld refinements, and allow better constraint of phase boundaries. Keywords: X-ray powder diffraction, otavite--magnesite solid solution, carbonates, phase transition, cation ordering

The phase transformations occurring in potassium niobiosilicate (KNS) glasses upon annealing at the glass transition temperature and subsequent heating are assessed. High-temperature resolved X-ray diffraction (HTXRD) is used for tracking nanostructuring and high-resolution transmission electron microscopy (HRTEM) is used for imaging the nanostructures of KNS glasses.

The chemical composition of Earth's lower mantle can be constrained by combining seismological observations with mineral physics elasticity measurements. However, the lack of laboratory data for Earth's most abundant mineral, (Mg,Fe,Al)(Al,Fe,Si)O3 bridgmanite (also known as silicate perovskite), has hampered any conclusive result. Here we report single-crystal elasticity data on (Al,Fe)-bearing bridgmanite (Mg0.9Fe0.1Si0.9Al0.1)O3 measured using high-pressure Brillouin spectroscopy and X-ray diffraction. Our measurements show that the elastic behaviour of (Al,Fe)-bearing bridgmanite is markedly different from the behaviour of the MgSiO3 endmember. We use our data to model seismic wave velocities in the top portion of the lower mantle, assuming a pyrolitic mantle composition and accounting for depth-dependent changes in iron partitioning between bridgmanite and ferropericlase. We find excellent agreement between our mineral physics predictions and the seismic Preliminary Reference Earth Model down to at least 1,200 kilometres depth, indicating chemical homogeneity of the upper and shallow lower mantle. A high Fe3+/Fe2+ ratio of about two in shallow-lower-mantle bridgmanite is required to match seismic data, implying the presence of metallic iron in an isochemical mantle. Our calculated velocities are in increasingly poor agreement with those of the lower mantle at depths greater than 1,200 kilometres, indicating either a change in bridgmanite cation ordering or a decrease in the ferric iron content of the lower mantle. [ABSTRACT FROM AUTHOR]

Ambient temperature X-ray diffraction data were collected at different pressures from two crystals of β-As4S4, which were made by heating realgar under vacuum at 295ºC for 24 h. These data were used to calculate the unit-cell parameters at pressures up to 6.86 GPa. Above 2.86 GPa, it was only possible to make an approximate measurement of the unit-cell parameters. As expected for a crystal structure that contains molecular units held together by weak van der Waals interactions, β-As4S4has an exceptionally high compressibility. The compressibility data were fitted to a third-order Birch–Murnaghan equation of state with a resulting volume V0= 808.2(2) Å3, bulk modulus K0= 10.9(2) GPa and K' = 8.9(3). These values are extremely close to those reported for the low-temperature polymorph of As4S4, realgar, which contains the same As4S4cage-molecule. Structural analysis showed that the unit-cell contraction is due mainly to the reduction in intermolecular distances, which causes a substantial reduction in the unit-cell volume (∼21% at 6.86 GPa). The cage-like As4S4molecules are only slightly affected. No phase transitions occur in the pressure range investigated.Micro-Raman spectra, collected across the entire pressure range, show that the peaks associated with As–As stretching have the greatest pressure dependence; the S–As–S bending frequency and the As–S stretching have a much weaker dependence or no variation at all as the pressure increases; this is in excellent agreement with the structural data. [ABSTRACT FROM AUTHOR]

The equation of state and the crystal structure evolution with pressure were determined for two single crystals of pure natural MgAl2O4 spinels with different degrees of order. The two samples studied were cut from a larger single crystal and one of them was experimentally disordered at high temperature. The two crystals, showing an inversion parameter x of 0.27 and 0.15 at ambient conditions, were loaded together in a diamond anvil cell and their unit-cell edge was measured up to about 7.5 GPa at 14 different pressures. The unit-cell volume, V0, the bulk modulus, KT0, and its first pressure derivative, K', were simultaneously refined using a third-order Birch-Murnaghan equation of state, giving the following coefficients: V0 = 529.32(2) ų, KT0 = 193(1) GPa, K' = 5.6(3) for the ordered sample and V0 = 528.39(2) ų, KT0 = 192(1) GPa, K' = 5.4(3) for the disordered one. Complete intensity data were collected at 0, 0.44, 2.92, 7.34, and 8.03 GPa in a separate experiment. For the ordered and disordered samples the oxygen atomic coordinate u remains practically unchanged inside the investigated pressure range with an average value of 0.2633(5) and 0.2614(2), respectively. As a consequence, the polyhedral compressibilities are similar and are not influenced by the Mg/Al distribution over the two crystallographic sites. This also suggests that pressure has little or no influence on the degree of order in the MgAl2O4 spinel. [ABSTRACT FROM AUTHOR]

In Extended Data Table 1 of this Letter, some of the elastic constants were reported incorrectly. This occurred as a result of an error in the script used to generate the numbers. The values of the elastic constants at room pressure cited in the manuscript on page 544 were derived using the same erroneous script, and the correct values and 1σ-uncertainties in the last given digit are C11 = 461.3(17) GPa instead of 462.7(17) GPa; C22 = 509.7(26) GPa instead of 504.9(26) GPa; C33 = 425.7(5) GPa instead of 426.6(5) GPa; C44 = 188.8(6) GPa instead of 188.4(6) GPa; C55 = 166.5(4) GPa instead of 166.6(4) GPa; C66 = 127.2(17) GPa instead of 129.7(17) GPa; C12 = 141.7(14) GPa instead of 140.2(14) GPa; C13 = 130.0(11) GPa instead of 132.2(11) GPa; and C23 = 161.0(12) GPa instead of 159.3(12) GPa. These errors do not affect any of the conclusions and we apologize for any confusion this may have caused. Extended Data Table 1 and the room-pressure values in the text have been corrected online. The Supplementary Information of this Author Correction contains the original, incorrect Extended Data Table 1, for transparency. [ABSTRACT FROM AUTHOR]