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Orthopyroxene is an abundant mineral in subducting slabs. Studying its phase transitions at high pressure is important to the understanding of mineralogy of subducting slabs in the deep Earth. Synchrotron-based single-crystal X-ray diffraction experiments were conducted on a synthetic Ni-bearing ferrosilite (Ni-En31Fs65) at pressures up to 33.8 GPa. Three phase transitions were observed at 12.1(6), 15.6(6), and 31.3(25) GPa. The first two phase transitions in Ni-En31Fs65 resemble the previously described phase transitions in Ni-free Fe-rich orthopyroxenes, i.e., the initial α-opx (Pbca) transforms to β-opx (P21/c), then the latter transforms to γ-opx (Pbca). This indicates that the incorporation of a few mol% NiSiO3 does not influence the phase transition path of Fe-rich orthopyroxene. After the third phase transition, the structure (P21ca) of Ni-En31Fs65 resembles the previously reported β-popx observed in En90 at high pressure, although the onset pressure of the phase transition in Ni-En31Fs65 is ~7 GPa lower than that in En90. β-popx has a post-pyroxene structure that contains fivefold- and sixfold-coordinated Si cations. Our results indicate that the post-pyroxene structure is β-popx (P21ca) for either Fe-poor or Fe-rich orthopyroxenes, although the phase transition path before the pyroxene → post-pyroxene is compositionally dependent. Additionally, unlike the second and third transitions, whose onset pressures are monotonously decreased by increasing Fe content, the Fe effect on shifting the first transition is much more significant for orthopyroxenes within En <50 mol% than that within En >50 mol%. [ABSTRACT FROM AUTHOR]

Tephroite (Mn2SiO4), together with some manganese (Mn)-rich mineral inclusions, has been found in ophiolite-hosted diamonds, possibly originating from Mn-nodules and sediments that were once deposited on the oceanic floor and later subducted into the deep mantle, which provides evidence for oceanic crustal recycling. However, the state and behavior of tephroite under high-pressure and high-temperature conditions remain poorly understood. In this study, we conducted in situ synchrotron single-crystal X-ray difraction (XRD) and Raman spectroscopy of synthetic tephroite up to ~30 GPa and ~900 K. The XRD and Raman spectroscopy experiments in this study first show that tephroite undergoes a pressure-induced, irreversible, amorphous transformation above ~20 GPa. Temperature (<900 K) is found to be an insignificant factor governing the process of amorphous transformation. Amorphous tephroite may be a potential phase in a rapidly cooling oceanic lithospheric subduction slab stagnating at the bottom of the mantle transition zone. [ABSTRACT FROM AUTHOR]

Apatite is a mineral of widespread importance in Earth and planetary science. Here we examine the behavior of a natural fluorapatite (FAp) crystal from Durango (Mexico) under compression to 61 GPa. Single-crystal X-ray diffraction experiments were carried out in a diamond-anvil cell using a synchrotron source. The apatite structure persists up to 32.4 GPa. Birch-Murnaghan equation of state parameters were fit to the pressure-volume data for fluorapatite for two cases: fixing V0 at its measured ambient value resulted in a bulk modulus, K0T, of 97.0(8) GPa and a pressure derivative of the bulk modulus, K′0T, of 3.3(1), while fixing V0 and K0T at its ambient value 90.5 GPa (derived from ultrasonically measured elastic constants) resulted in a K′0T value of 4.1(1). At 35.6 GPa, fluorapatite transforms to a triclinic phase (P1, Z = 4), designated here as fluorapatite II (FAp-II). This phase persists up to at least 61 GPa. The major structural differences between FAp and FAp-II involve the buckling of the Ca polyhedra along the c-axis and changes in the number and coordination of the Ca sites. Our study extends the pressure range over which fluorapatite has been examined by more than a factor of three, providing new insights into its structural response to high-pressure conditions. [ABSTRACT FROM AUTHOR]

Epidote is a typical hydrous mineral in subduction zones. Here, we report a synchrotron-based single-crystal X-ray diffraction (XRD) study of natural epidote [Ca 1.97 Al 2.15 Fe 0.84 (SiO 4)(Si 2 O 7)O(OH)] under simultaneously high pressure-temperature (high P - T) conditions to ~17.7 ​GPa and 700 ​K. No phase transition occurs over this P - T range. Using the third-order Birch-Murnaghan equation of state (EoS), we fitted the pressure-volume-temperature (P - V - T) data and obtained the zero-pressure bulk modulus K 0 ​= ​138(2) GPa, its pressure derivative K 0 ' ​= ​3.0(3), the temperature derivative of the bulk modulus ((∂ K/ ∂ T) P ​= ​−0.004(1) GPa/K), and the thermal expansion coefficient at 300 ​K (α 0 ​= ​3.8(5) ​× ​10−5 ​K−1), as the zero-pressure unit-cell volume V 0 was fixed at 465.2(2) Å3 (obtained by a single-crystal XRD experiment at ambient conditions). This study reveals that the bulk moduli of epidote show nonlinear compositional dependence. By discussing the stabilization of epidote and comparing its density with those of other hydrous minerals, we find that epidote, as a significant water transporter in subduction zones, may maintain a metastable state to ~14 ​GPa along the coldest subducting slab geotherm and promote slab subduction into the upper mantle while favoring slab stagnation above the 410 ​km discontinuity. Furthermore, the water released from epidote near 410 ​km may potentially affect the properties of the 410 ​km seismic discontinuity. Image 1 • Epidote may remain metastable in the upper mantle along a relatively cold subducting slab geotherm. • Epidote is denser than other common hydrous minerals and promotes slab subduction into the upper mantle. • The water released from epidote near 410 km may potentially affect the properties of the 410 km seismic discontinuity. [ABSTRACT FROM AUTHOR]

The thermoelastic properties of minerals in mantle xenoliths combined with the thermal states can provide an integrated understanding of the petrophysics of the lithospheric mantle. Here, we conducted high-pressure and high-temperature experiments on the main minerals (e.g. olivine, orthopyroxene, clinopyroxene, spinel, and garnet) in peridotite xenoliths from basalt of the eastern North China Craton (NCC) using in situ synchrotron single-crystal X-ray diffraction combined with diamond anvil cells. The pressure-temperature-volume data were fitted to the third-order Birch-Murnaghan equations of state and yielded the thermoelastic parameters that included bulk modulus, pressure and temperature derivatives, and thermal expansion coefficients. The density profiles of the eastern NCC during the destruction process since the Mesozoic are presented from the temporal and spatial aspects. The lithospheric density dramatically decreased during destruction, and high heat flow may have been a trigger. The spatially distributed density profile also provides firm evidence for lateral heterogeneities in the eastern NCC. This may suggest that the present mantle is characterized by heterogeneous destruction of the NCC. Unlabelled Image • Thermoelastic parameters of the main minerals in peridotite are obtained from a single high P-T XRD experiment. • High heat flow leading to lower density may be a trigger for the destruction of the eastern NCC. • Density results from mineral physics further confirm the heterogeneous destruction in the eastern NCC. [ABSTRACT FROM AUTHOR]

Olivine is the most abundant mineral in the Earth's upper mantle and subducting slabs. Studying the structural evolution and equation of state of olivine at high-pressure is of fundamental importance in constraining the composition and structure of these regions. Hydrogen can be incorporated into olivine and significantly influence its physical and chemical properties. Previous infrared and Raman spectroscopic studies indicated that local structural changes occur in Mg-rich hydrous olivine (Fo ≥ 95; 4883–9000 ppmw water) at high-pressure. Since water contents of natural olivine are commonly <1000 ppmw, it is inevitable to investigate the effects of such water contents on the equation of state (EoS) and structure of olivine at high-pressure. Here we synthesized a low water content hydrous olivine (Fo95; 1538 ppmw water) at low SiO2 activity and identified that the incorporated hydrogens are predominantly associated with the Si sites. We performed high-pressure single-crystal X‑ray diffraction experiments on this olivine to 29.9 GPa. A third-order Birch-Murnaghan equation of state (BM3 EoS) was fit to the pressure-volume data, yielding the following EoS parameters: VT0 = 290.182(1) Å3, KT0 = 130.8(9) GPa, and K T 0 ′ = 4.16 (8). $K_{\mathrm{T} 0}^{\prime}=4.16(8).$ The KT0 is consistent with those of anhydrous Mg-rich olivine, which indicates that such low water content has negligible effects on the bulk modulus of olivine. Furthermore, we carried out the structural refinement of this hydrous olivine as a function of pressure to 29.9 GPa. The results indicate that, similar to the anhydrous olivine, the compression of the M1-O and M2-O bonds are comparable, which are larger than that of the Si-O bonds. The compression of M1-O and M2-O bonds of this hydrous olivine are comparable with those of anhydrous olivine, while the Si-O1 and Si-O2 bonds in the hydrous olivine are more compressible than those in the anhydrous olivine. Therefore, this study suggests that low water content has negligible effects on the EoS of olivine, though the incorporation of water softens the Si-O1 and Si-O2 bond. [ABSTRACT FROM AUTHOR]

High-pressure single-crystal X‑ray diffraction patterns on five synthetic Mg-Al tourmalines with near end-member compositions [dravite NaMg3Al6Si6O18(BO3)3(OH)3OH, K-dravite KMg3Al6Si6O18(BO3)3(OH)3OH, magnesio-foitite ☐(Mg2Al)Al6Si6O18(BO3)3(OH)3OH, oxy-uvite CaMg3Al6Si6O18(BO3)3(OH)3O, and olenite NaAl3Al6Si6O18(BO3)3O3OH, where o represents an X-site vacancy] were collected to 60 GPa at 300 K using a diamond-anvil cell and synchrotron radiation. No phase transitions were observed for any of the investigated compositions. The refined unit-cell parameters were used to constrain third-order Birch-Murnaghan pressure-volume equation of states with the following isothermal bulk moduli (K0 in GPa) and corresponding pressure derivatives (K0ʹ = ∂K0/∂P)T: dravite K0 = 97(6), K0ʹ = 5.0(5); K-dravite K0 = 109(4), K0ʹ = 4.3(2); oxy-uvite K0 = 110(2), K0ʹ = 4.1(1); magnesio-foitite K0 = 116(2), K0ʹ = 3.5(1); olenite K0 = 116(6), K0ʹ = 4.7(4). Each tourmaline exhibits highly anisotropic behavior under compression, with the c axis 2.8–3.6 times more compressible than the a axis at ambient conditions. This anisotropy decreases strongly with increasing pressure and the c axis is only ~14% more compressible than the a axis near 60 GPa. The octahedral Y- and Z-sites' composition exerts a primary control on tourmaline's compressibility, whereby Al content is correlated with a decrease in the c-axis compressibility and a corresponding increase in K0 and Kʹ. Contrary to expectations, the identity of the X-site-occupying ion (Na, 0 K, or Ca) does not have a demonstrable effect on tourmaline's compression curve. The presence of a fully vacant X site in magnesio-foitite results in a decrease of K0ʹ relative to the alkali and Ca tourmalines. The decrease in K0ʹ for magnesio-foitite is accounted for by an increase in compressibility along the a axis at high pressure, reflecting increased compression of tourmaline's ring structure in the presence of a vacant X site. This study highlights the utility of synthetic crystals in untangling the effect of composition on tourmaline's compression behavior. [ABSTRACT FROM AUTHOR]

Abstract: Synchrotron‐based high‐pressure and temperature single‐crystal X‐ray diffraction experiments were conducted on two hydrous orthoenstatite samples (oEn#1: Mg1.004Si0.996O3, ~619 ppm water; oEn#2: Mg0.947Ni0.055Si0.998O3, ~696 ppm water) to ~34 GPa and 700 K, using resistively heated diamond anvil cells. The α‐opx (Pbca space group)→β‐opx (P21/c space group) phase transition of oEn#1 occurs at 12.90(2) GPa, and the β‐opx phase persists to 34.25(1) GPa. The α‐β transition of oEn#2 occurs at 13.50(1) GPa, and a new isosymmetric β‐opx→β‐opxII transition takes place at 29.80(4) GPa. The β‐opxII phase is preserved down to 24.53(3) GPa during decompression. The transition to the monoclinic β‐opxII phase is interpreted as a result of incorporation of Ni2+ into the orthoenstatite structure. Fitting the third‐order Birch‐Murnaghan thermal equation of state to the single‐crystal P‐V‐T data yields the thermoelastic parameters of the α‐ and β‐opx phases for both orthoenstatite samples. This study is the first attempt to determine the thermal equation of state of the β‐opx phase. Our results suggest that several hundred ppm of water has negligible effects on the bulk modulus of orthoenstatite but notably enhances the thermal expansion. The potential effects of metastable orthoenstatite on subduction zone dynamics are discussed, and the possible contributions of displacive phase transitions to enhancement of the transformational faulting mechanism of the deep‐focus earthquakes in subducted slabs are considered. The presence of metastable orthoenstatite within cold slabs could promote slab stagnation above the 660‐km discontinuity. [ABSTRACT FROM AUTHOR]

Orthopyroxene is an abundant mineral in subducting slabs. Studying its phase transitions at high pressure is important to the understanding of mineralogy of subducting slabs in the deep Earth. Synchrotron-based single-crystal X-ray diffraction experiments were conducted on a synthetic Ni-bearing ferrosilite (Ni-En31Fs65) at pressures up to 33.8 GPa. Three phase transitions were observed at 12.1(6), 15.6(6), and 31.3(25) GPa. The first two phase transitions in Ni-En31Fs65 resemble the previously described phase transitions in Ni-free Fe-rich orthopyroxenes, i.e., the initial α-opx (Pbca) transforms to β-opx (P21/c), then the latter transforms to γ-opx (Pbca). This indicates that the incorporation of a few mol% NiSiO3 does not influence the phase transition path of Fe-rich orthopyroxene. After the third phase transition, the structure (P21ca) of Ni-En31Fs65 resembles the previously reported β-popx observed in En90 at high pressure, although the onset pressure of the phase transition in Ni-En31Fs65 is ~7 GPa lower than that in En90. β-popx has a post-pyroxene structure that contains fivefold- and sixfold-coordinated Si cations. Our results indicate that the post-pyroxene structure is β-popx (P21ca) for either Fe-poor or Fe-rich orthopyroxenes, although the phase transition path before the pyroxene → post-pyroxene is compositionally dependent. Additionally, unlike the second and third transitions, whose onset pressures are monotonously decreased by increasing Fe content, the Fe effect on shifting the first transition is much more significant for orthopyroxenes within En <50 mol% than that within En >50 mol%. [ABSTRACT FROM AUTHOR]

The natural occurrence of clinohumite in metabasalts and hydrothermally altered peridotites provides a source of water-rich minerals in subducted slabs, making knowledge of their phase relations and crystal chemistry under high pressure-temperature ( P- T) conditions important for understating volatile recycling and geodynamic process in the Earth's mantle. Here we present a synchrotron-based, single-crystal X-ray diffraction study on two natural Ti-bearing clinohumites up to ~28 GPa and 750 K in order to simulate conditions within subducted slabs. No phase transition occurs in clinohumite over this P- T range. Pressure-volume relationships of both compositions at room temperature were fitted to a third-order Birch-Murnaghan equation of state (EoS) with V0 = 650.4(3) Å3, K T0 = 141(4) GPa, and K T0′ = 4.0(6) for Ti-poor clinohumite (0.07 Ti per formula unit, pfu) and V0 = 650.8(3) Å3, K T0 = 144(4) GPa, and K T0′ = 3.6(7) for Ti-rich clinohumite (0.21 Ti pfu). Both clinohumites exhibit anisotropic compression with β b > β c > β a. We also refined P-V-T equation of state parameters using the high-temperature Birch-Murnaghan EoS, yielding (∂ K T0/∂ T) P = −0.040(10) GPa/K and α T = 5.1(6) × 10−5 K−1 for Ti-poor clinohumite and (∂ K T0/∂ T) P = −0.045(11) GPa/K and α T = 5.7(6) × 10−5 K−1 for Ti-rich clinohumite. Ti-poor and Ti-rich clinohumites display similar equations of state but are ~20% more incompressible than Mg-pure clinohumite and display ~5% higher bulk sound velocity than olivine at upper mantle conditions. Our results provide constraints for modeling geodynamic process related to the subduction and transport of potentially water-rich slabs in the mantle. [ABSTRACT FROM AUTHOR]

(Ca,Mg)-rich clinopyroxenes are abundant in Earth's upper mantle and subduction zones. Experimental studies on the thermoelastic properties of these minerals at simultaneous high pressure and high temperature are important for constraining of the composition and structure of the Earth. Here, we present a synchrotron-based single-crystal X-ray diffraction study of natural diopside-dominated augite [(Cac0.89Nac0.05Mg0.06)(Mg0.74Fe0.11Al0.14Ti0.01)(Si1.88Al0.12)O6.00] at P and T to ~27 GPa and 700 K. The experiment simulates conditions in cold subducting slabs, and results indicate that augite is stable over this pressure and temperature range. A third-order high-temperature Birch-Murnaghan equation was fit with the pressure-volume-temperature data, yielding the following thermoelastic parameters: KT0 = 111(1) GPa, K'T0 = 4.1(1), (∂K0/∂T)P = -0.008(5) GPa/K and αT = 4(1)x10-5 K-1 +2(3)x10-8 K-2 T. A strain analysis shows that the compression along the three principal stress directions is highly anisotropic with ε1:ε2:ε3 = 1.98:2.43:1.00. Additionally, high-pressure structural refinements of room- temperature polyhedral geometry, bond lengths and O3-O3-O3 angle were investigated to ~27 GPa at ambient temperature. Pressure dependences of polyhedral volumes and distortion indicate that the substitution of Al3+ for Si4+ significantly changes the compressional behavior of the TO4-tetrahedron in augite. Density calculations of this augite along a subducting slab geotherm suggest that augite as well as other common clinopyroxenes would promote slab stagnations at transition zone depths if they are metastably preserved in significant quantities. [ABSTRACT FROM AUTHOR]

To better understand the potential presence of light element alloys of Fe and Ni in the Earth's interior, the crystal structure and compressional behavior of the Ni-P binary compound, schreibersite (Ni3P), have been investigated using synchrotron X-ray diffraction experiments. Both powder and two single-crystal samples of synthetic Ni3P (in different orientations with respect to the loading axis of the diamond anvil cell) were compressed up to approximately 50 GPa at ambient temperature. The compressional data obtained for Ni3P were fitted with a 3rd order Birch–Murnaghan equation of state. All data indicated that the c/a ratio of unit cell parameters remained approximately constant up to about 30 GPa but then increased progressively with pressure, exhibiting a second slight discontinuity at approximately 40 GPa. The changes in unit cell parameters at ~30 GPa and ~40 GPa suggested discontinuous changes in magnetic ordering. Moreover, the threshold of these subtle discontinuities is sensitive to the stress state and orientation of the crystal in the diamond anvil cell. This study is the first report on the compressional behavior of both powder and single-crystal schreibersite at high-pressure (up to 50 GPa). It offers insights into the effects of Ni3P components on the compressional behavior of the Earth's core. [ABSTRACT FROM AUTHOR]

High-pressure diamond-anvil cell synchrotron X-ray diffraction experiments were conducted on single-crystal samples of natural orthoamphibole; gedrite; with composition; (K0.002Na0.394)(Mg2)(Mg1.637Fe2.245Mn0.004Ca0.022Cr0.003Na0.037Al1.052)(Si6.517Al1.483)O22(OH)2. The samples were compressed at 298 K up to a maximum pressure of 27(1) GPa. In this pressure regime, we observed a displacive phase transition between 15.1(7) and 21(1) GPa from the orthorhombic Pnma phase to a new structure with space group P21/m; which is different from the familiar P21/m structure of cummingtonite and retains the (+, +, −, −) I-beam stacking sequence of the orthorhombic structure. The unit cell parameters for the new phase at 21(1) GPa are a = 17.514(3), b = 17.077(1), c = 4.9907(2) Å and β = 92.882(6)°. The high-pressure P21/m phase is the first amphibole structure to show the existence of four crystallographically distinct silicate double chains. The orthorhombic to monoclinic phase transition is characterized by an increase in the degree of kinking of the double silicate chains and is analogous to displacive phase changes recently reported in orthopyroxenes, highlighting the parallel structural relations and phase transformation behavior of orthorhombic single- and double-chain silicates. [ABSTRACT FROM AUTHOR]