Alison R. Pawley

The stability of antigorite in the systems MgO-SiO2-H2O (MSH) and MgO-Al2O3-SiO2-H2O (MASH): The effects of al3+ substitution on high-pressure stability

Abstract The high-pressure stability of antigorite in the systems MSH and MASH was investigated using two structurally and chemically well-constrained natural samples. Careful sample selection and characterization ensured that the only significant difference between the samples was Al content, one sample being essentially Al free, and the other containing 3.06(2) wt% Al2O3. In the system MSH, the reaction antigorite = forsterite + clinoenstatite + water was bracketed, under water-saturated conditions, between 630 and 650 °C at 1.6 GPa, between 620 and 660 °C at 2.5 GPa, between 620 and 660 °C at 3.9 GPa, and between 4.5 and 5.0 GPa at 520 °C. In the system MASH, the reaction antigorite = forsterite + clinoenstatite + chlorite + water was bracketed, under water-saturated conditions, between 660 and 700 °C at 2.0 GPa, between 660 and 680 °C at 2.9 GPa, and between 5.0 and 5.5 GPa at 600 °C. At pressures above 5.8 GPa, intersection of this reaction with the reaction chlorite + clinoenstatite = pyrope + forsterite + water leads to an additional reaction whereby the Al component of the antigorite is transferred to pyrope upon antigorite breakdown. The addition of a few weight percent Al2O3 into antigorite is shown to stabilize the antigorite structure to significantly higher temperatures and pressures, possibly by minimizing structural misfit among the component octahedral and tetrahedral sheets. This effect partially explains the considerable discrepancies noted between previous studies on the stability of antigorite at high pressure. In addition, antigorite breakdown in the system MASH transfers a significant volume of water to chlorite-bearing assemblages, thereby greatly increasing the range of temperatures over which water is tied up in hydrous phases relative to the system MSH.

Neutron diffraction at simultaneous high temperatures and pressures, with measurement of temperature by neutron radiography

Abstract The commissioning and operation of apparatus for neutron diffraction at simultaneous high temperatures and pressures is reported. The basic design is based on the Paris-Edinburgh cell using opposed anvils, with internal heating. Temperature is measured using neutron radiography. The apparatus has been shown in both on-line and off-line tests to operate to a pressure of 7 GPa and temperature of 1700°C. The apparatus has been used in a neutron diffraction study of the crystal structure of deuterated brucite, and results for 520°C and 5.15 GPa are presented. The diffraction data that can be obtained from the apparatus are of comparable quality to previous high-pressure studies at ambient temperatures, and are clearly good enough for Rietveld refinement analysis to give structural data of reasonable quality.

An infrared spectroscopic study of the OH stretching frequencies of talc and 10-Å phase to 10 GPa

Abstract The effects of pressure on the OH stretching frequencies of natural talc and two samples of synthetic 10-Å phase have been measured using a diamond-anvil cell and a synchrotron infrared source. The 10-Å phase was synthesized at 6.0-6.5 GPa, 600 °C for 46 hours (sample 10Å-46) and 160 hours (10Å-160). Spectra were collected up to 9.0 GPa (talc), 9.9 GPa (10Å-46), and 9.6 GPa (10Å-160). The OH stretching vibration of Mg3OH groups in talc occurs at 3677 cm-1 at ambient pressure, and increases linearly with pressure at 0.97(2) cm-1 GPa-1. The same vibration occurs in 10-Å phase, but shows negligible pressure shift up to 2 GPa, above which the frequency increases linearly to the maximum pressure studied, at a rate of 0.96(3) cm-1 GPa-1 (10Å-46) and 0.87(3) cm-1 GPa-1 (10Å- 160). Two other prominent bands in the 10-Å phase spectrum are suggested to be due to stretching of interlayer H2O, hydrogen-bonded to the nearest tetrahedral sheet. These bands also show little change over the first 2 GPa of compression, as most of the compression of the structure is taken up by closing non-hydrogen bonded gaps between interlayer H2O and tetrahedral sheets. Between 2 and 4 GPa, changes in band intensities suggest a rearrangement of the interlayer H2O.

A high-pressure structural study of lawsonite using angle-dispersive powder-diffraction methods with synchrotron radiation

Abstract Structural refinements of lawsonite have been obtained at pressures up to 16.5 GPa using angledispersive powder diffraction with synchrotron radiation on a natural sample contained in a diamond anvil cell. Lawsonite compresses smoothly and relatively isotropically up to 10 GPa. Its bulk modulus is 126.1(6) GPa (for K’ = 4), consistent with previous results. A trend of decreasing Si −O−Si angle indicates that compression is accommodated partly through the narrowing of the cavities containing Ca and H2O in the [001]ortho direction. At 10−11 GPa there is a phase transition from Cmcm to P21/m symmetry. The occurrence of a mixed-phase region, spanning >1 GPa, indicates that the transition is first order in character. The phase transition occurs through a shearing of (010)ortho sheets containing AlO6 octahedral chains in the [100]ortho direction, which causes an increase in βmono. Across the transition, the number of oxygens coordinated to Ca increases from 8 to 9, causing an increase in the average Ca O bond length. The compressibility of P21/m lawsonite could not be determined due to solidification of the methanol/ethanol pressure-transmitting medium. On the basis of an experiment in which the P21/m lawsonite structure was heated to 200°C at 12.0 GPa, we predict a shallow positive P-T slope for the phase transition, and therefore no stability field for P21/m lawsonite in the Earth.

The equation of state of lawsonite to 7 GPa and 873 K, and calculation of its high pressure stability

Abstract The volume of lawsonite, CaAl2Si2O7(OH)2 · H2O, has been measured up to 7 GPa and 873 K using in situ energy dispersive powder diffraction and a multi-anvil high pressure-temperature cell at the Synchrotron Radiation Source, Daresbury Laboratory, U.K. Measurements were made on isotherms at 298, 323, 373, 473, 573, 673, 773, and 873 K within the pressure range. Sample pressure was measured from a NaCl standard mixed with the sample; the unit-cell volume of lawsonite was taken from the same diffraction pattern. The data gave an ambient temperature isothermal bulk modulus of K298 112 ± 6 GPa, similar to previous values. This value overestimates the temperature stability of lawsonite in thermodynamic calculations. A fit of the Birch-Murnaghan equation of state to the whole high pressure and temperature data set gave an isothermal bulk modulus of K298 = 125 ±5 GPa and a dK/dT of -0.01 ± 0.01/K, with K1 set to a value of 4 and the expansivity set to 3.16 × 10-5/K. Using these values to calculate the pressure-temperature positions of three of lawsonite’s dehydration reactions improved the agreement between observed and calculated positions of the lawsonite dehydration reactions to within experimental and calculation error. This work shows that the ambient temperature bulk modulus and ambient pressure expansivity do not adequately describe the volume behavior of lawsonite at combined high pressure and temperature.

Si vacancies in the 10-A phase

Abstract 29Si MAS NMR spectroscopy on samples of 10-Å phase synthesized from oxides (6.0 GPa/600 °C/400 h) and from partial transformation of talc (6.5 GPa/650 °C/12.5 h) reveals that this phase contains Q2-type Si sites in a ratio Q3:Q2 of 5.33:1. It is proposed that the Q2 arise from adjacent vacancies in the tetrahedral sheets for which charge balance is most likely achieved by hydroxylation via a hydrogarnet-like substitution involving the formation of Q2 silanol groups. Variable-contacttime 29Si {1H} CP/MAS NMR spectra of the talc/10-Å phase product support the assignment of Q2 Si to the proposed SiO3(OH) groups. Electron microprobe analysis, including oxygen, gives the following empirical formula normalized to three Mg apfu and inferring a hydrogarnet component Si → 4H associated with Si vacancies: Mg3Si3.83(8)O9.32(OH)2.68.1.1(4)H2O. The observed Mg:Si indicates a significant Si deficiency relative to talc. Comparison of the 29Si MAS NMR and microprobe data indicates that Si vacancies likely occur as single isolated entities, rather than as pairs or clusters, and that between 1 in 18 and 1 in 23 Si sites is vacant. The results suggest new and intriguing possibilities for the incorporation of excess H into the 10-Å phase and, potentially, other phyllosilicates under upper-mantle conditions.

Volume measurements of zoisite at simultaneously elevated pressure and temperature

Abstract Unit-cell parameters of zoisite, Ca2Al3Si3O12(OH), have been measured at simultaneously high pressures and temperatures (up to 6.1 GPa and 800 °C) in a Walker-style multi-anvil apparatus at the synchrotron radiation source at Daresbury Laboratory, U.K. Measurements were made in a series of heating cycles at increasing loads. Sample pressure, measured using an internal NaCl standard, increased during heating. Cell parameters vary smoothly with pressure and temperature; individual expansivities and compressibilities vary in the order c > b > a. Isothermal bulk moduli were calculated from the volumes measured at 30, 200, 400, 600, and 800 °C by fitting the Murnaghan equation of state to each isothermal data set. This assumes K′ = 4. Ambient-pressure volumes calculated from previous measurements of thermal expansivity of zoisite were included in the Murnaghan fits. A linear fit of the bulk moduli with temperature gave values for the bulk modulus at 298 K, K298 = 125(3) GPa, and its variation with temperature, ∂KT /∂T = -0.029(6) GPa K-1. K298 is slightly higher than the recent value for a single crystal in a diamond-anvil cell, indicating a lower maximum pressure stability of zoisite than would be calculated using that value. Our data allow zoisite volumes to be calculated at P-T conditions relevant to the Earth and show that, in a typical subduction zone, zoisite becomes more dense as subduction proceeds, helping to stabilize it to high pressures.

Pressure-temperature studies of talc plus water using X-ray diffraction

Abstract X-ray diffraction measurements of natural talc plus water at combined pressures and temperatures of 0-15 GPa and 23-400 °C reveal the presence of a structural change that could be interpreted as a new high-pressure phase at 4.0 (±0.5) GPa, and raise the possibility that the newly inferred phase transition takes place in cold subducting slabs as a precursor to appearance of the 10 Å phase of talc.

Volume behavior of the 10 Å phase at high pressures and temperatures, with implications for H2O content

Abstract The 10 Å phase is a high-pressure hydrous magnesium silicate whose composition appears to depend on synthesis conditions. We have measured the compressibility to 10.5 GPa and thermal expansivity to 400 °C of samples of 10 Å phase synthesized in long experiments (400 and 169 h, respectively) designed to maximize compositional equilibrium. The structure was refined using a metrically trigonal unit cell. Compression is highly anisotropic, especially over the first 2 GPa of compression, indicating weak bonding across the interlayer. There is an inflection in the compression curve of c at 8 GPa, suggesting a change in compression mechanism or the onset of non-hydrostaticity in the pressure medium. Fitting the compression data collected below 8 GPa to a Murnaghan equation-of-state gives V0 = 734.8(7) Å3, K0 = 25(1) GPa, K′ = 18(1). Thermal expansion is also strongly anisotropic: coefficients for data up to 200 °C are αa = 0.15(5) × 10-5 K-1, αc = 3.1(2) × 10-5 K-1, αV = 3.4(2) × 10-5 K-1. Above 200 °C, the expansivity of c decreased, and all parameters showed a contraction after the experiment, suggesting partial dehydration at high temperatures. Comparison of our compressibility data with those of previous studies suggests that 10 Å phase synthesized in short experiments does not retain all of its interlayer H2O during quenching and decompression. In contrast, samples annealed for many hours at high pressure and temperature are stabilized by small amounts of hydrogarnet-type substitution and consequent hydrogen bond strengthening.

Further complexities of the 10 Å phase revealed by infrared spectroscopy and X-ray diffraction

Abstract Infrared spectroscopy and X-ray diffraction are used to evaluate the OH and H2O environments in 10 Å phase (“TAP”), nominally Mg3Si4O10(OH)2·H2O. Two partially deuterated samples of TAP synthesized under different conditions have very similar IR spectra, indicating that the phase has a reproducible structural state. IR spectra were also collected of samples of fully Ni-substituted and partially deuterated TAP, and of samples heated for 1-2 h at 500 °C to remove structural H2O/D2O and leave behind bands due to OH/OD of the 2:1 layer. A high-pressure study of the Ni-TAP sample confirmed that the behavior of its H2O and OH/OD bands was analogous to that observed in previous studies of Mg-TAP. Comparison of the IR spectra of unheated, heated, and compressed samples has allowed three different types of Mg-OH (Mg-OD) stretching bands to be identified, two of which are further split, indicating subtle complexities in the TAP structure. The third band is identical to the band in talc. Two interlayer H2O stretching bands have been identified. The presence of an absorption feature that is broader than these interlayer H2O bands suggests that there is a second type of more weakly bonded H2O. On heating to 500 °C, the main interlayer H2O bands are lost, the talc-like band is unchanged, and shifts in the other Mg-OH band frequencies indicate a change in environment following the loss of the interlayer H2O. At the same time the signature of a silanol group is possibly revealed from the coincidence of band positions in the Mg-TAP and Ni-TAP spectra. The recognition of three distinct Mg-OH (Ni-OH) environments in Mg-TAP (Ni-TAP) is consistent with the structural model of TAP proposed by Welch et al. (2006) and Phillips et al. (2007), in which the transformation from talc to TAP involves a key change from hydrophobic to hydrophilic character that enables hydration of the interlayer. A final level of complexity is indicated by the identification of a 3c trigonal superstructure from single-crystal XRD, implying a structure analogous to that of the 3T phengite polytype, with interlayer H2O fulfilling the role of K. The formation of additional OH groups when talc transforms to 10 Å phase increases the amount of water contained in 10 Å phase and may also occur in closely related phyllosilicates in the Earth’s mantle, such as intergrowths of chlorite with 10 Å phase. Moreover, the reproducibility of the key features of the IR spectra for different samples implies that this water content is fixed.