Multi-anvil High-pressure Apparatus Research Articles

THE FORSTERITE-DIOPSIDE-SILICA SYSTEM AT 7 GPA AND ITS SIGNIFICANCE IN THE MELTING BEHAVIOR OF THE MANTLE

Investigation of the liquidus phase relations in the forsterite–diopside–enstatite system has been made at 7 GPa and variable temperatures under anhydrous condition using a Walker-type multi-anvil high pressure apparatus. A total of 16 compositions were synthesized to facilitate study of this system. Location of the piercing point on the forsterite–diopside bounding join has been established at Fo 39 Di 61 (all compositions are in wt.%) and 1980 ± 20 °C. Electron microprobe analyses of the phases indicate that forsterite ss can incorporate about 1 to 1.5 wt.% monticellite (CaMgSiO 4 ) in solid solution. In the diopside–enstatite join, a temperature minimum occurs at Di 66 En 34 and 1975 ± 30 °C, whereas the peritectic point, where orthopyroxene ss reacts with liquid to form clinopyroxene ss , occurs at En 65 Di 35 and 2010 ± 20 °C. The maximum amount of MgSiO 3 that can be incorporated in diopside ss has been measured to be about 82 wt.%. Because of the increase in the solid solution proportion of MgSiO 3 in diopside ss compared to that at 2 and 5.1 GPa, the peritectic point in the diopside–enstatite bounding join also shifts further toward the enstatite apex. In the ternary system, the invariant point has been located at Fo 54 Di 35 Q 11 and 2010 ± 30 °C, where orthopyroxene ss + forsterite ss + liquid ↔ clinopyroxene ss + forsterite ss + liquid occurs. Our present study confirms that the primary phase field of forsterite shrinks and that of clinopyroxene expands further in comparison to that observed at 2 and 5.1 GPa. Preliminary experimental results from composition Fo 80 Di 12 Qtz 8 with 4 wt.% Al 2 O 3 indicate that garnet (Pyrope 98 Grossularite 2 ) and forsterite (with 0.31 wt.% monticellite) are the liquidus phases, followed by clinopyroxene (Al 2 O 3 contents between 3.4 to 2.7 wt.%). At 1970 °C , the average composition of the liquids obtained through electron microprobe analysis was as follows: SiO 2 49.81, MgO 34.72, CaO 7.29, and Al 2 O 3 6.75 wt.%, which is somewhat similar to a komatiitic melt. When projected onto the forsterite–diopside–enstatite plane this liquid composition plots very close to our cotectic line. Taking model mantle composition of a garnet peridotite, a shift in the compositional path of the residual mantle materials has been determined after extraction of the volume percent of the melt using the Lever rule.

Diamond crystallization in the Fe-Co-S-C and Fe-Ni-S-C systems and the role of sulfide-metal melts in the genesis of diamond

Diamond crystals 0.1–0.8 carats were synthesized in experiments conducted in a BARS split-sphere multianvil high-pressure apparatus in the systems Fe-Co-S-C and Fe-Ni-S-C at a pressure of 5.5 GPa and temperature of 1300°C. The microtextures of the samples and the phases accompanying diamond (carbides, graphite, monoslufide solid solution, pentlandite, and taenite) are examined in much detail, the properties of metal-sulfide-carbon alloys are discussed, and issues related to the genesis of sulfide inclusions in diamonds and graphite crystallization in the diamond stability field are considered. The experiments demonstrate that diamonds can be synthesized and grow in pre-eutectic metal-sulfide melts with up to 14 wt % sulfur at relatively low P-T parameters, which correspond to the probable temperatures and pressures of natural diamond-forming processes at depths of approximately 150 km in the Earth’s upper mantle.

Acoustic velocity measurements on solids and liquids under extreme conditions of pressure and temperature

Elastic properties of materials under extreme conditions of pressure and temperature are of great interests to researchers in many disciplines. In the last decade, acoustic velocity measurements using an improved ultrasonic interferometry method have been developed in both MA-6 and MA-8 types of multi-anvil high pressure apparatus. By placing the piezoelectric transducer (lithium niobate, 10 deg Y-cut) in a stress-free location and using extended delayline, high S/N acoustic signal can be maintained, while sample is under high pressure andtemperature. By combining with synchrotron X-radiation, measurements of sound velocities using ultrasonic interferometry, crystalstructure and unit cell parameters using X-ray diffraction, and sample strain (length) using X-radiographic imaging, can be made simultaneously, all in-situ at a high pressure and temperature, enabling a pressure-standard-free characterization of solid and liquid materials to 25 GPa and 1800 K. Results on ceramic and metallic materials from recent experiments will be presented to show velocities as a function of pressure and temperature, absolute pressure determination, equation of state for glass, and the application to liquids. Other new developments, such as controlling sample stress state at high P and T, the study of composites, and materials undergoing phase transformations will also be reviewed [Work sponsored by NSF and DOE/NNSA.]

In situ observation of a garnet/perovskite transition in CaGeO3

We used an in situ measurement method to investigate the phase transition of CaGeO3 polymorphs under high pressures and temperatures. A multi-anvil high-pressure apparatus combined with intense synchrotron X-ray radiation was used. The transition boundary between a garnet and a perovskite phase at T = 900–1,650 K and P = 3–8 GPa was determined as occurring at P (GPa) = 9.0−0.0023 × T (K). The transition pressure determined in our study is in general agreement with that observed in previous high-pressure experiments. The slope, dP/dT, of the transition determined in our study is consistent with that calculated from calorimetry data.

Electrical conductivity of MgCO 3 at high pressures and high temperatures

The electrical conductivity of polycrystalline magnesite (MgCO 3) was measured at 3–6 GPa at high temperatures using complex impedance spectroscopy in a multi-anvil high-pressure apparatus. The electrical conductivity increased with increasing pressure. The activation enthalpy calculated in the temperature range 650–1000 K also increased with increasing pressure. The effect of pressure was interpreted as being the activation volume in the Arrhenius equation, and the fitted data gave an activation energy and volume of 1.76±0.03 eV and −3.95±0.78 cm 3/mole, respectively. The negative activation volume and relatively large activation energy observed in this study suggests that the hopping of large polarons is the dominant mechanism for the electrical conductivity over the pressure and temperature range investigated.

HIP Production of Diamond-SiC Composites and Their Application to High-Pressure Anvils

Using a hot isostatic pressing (HIP) technique, we synthesized diamond-SiC composites from diamond and Si powders. At an HIP condition of 1450°C and 100 MPa, a pressure much lower than that of the diamond stability field, diamond powders react with molten Si to form well-sintered diamond-SiC composites. Cubes of the composites were thereby fabricated, and an application to the second stage anvils in the Kawai-type multi-anvil high-pressure apparatus was attempted. Since the diamond-SiC composites are transparent to X rays, the present system is applicable not only to conventional energy dispersive X-ray diffraction studies but also to angle dispersive diffraction and radiographic studies that need a larger window for X-ray images. In this article, our recent advances were reviewed.

Effect of hydrogen on the melting temperature of FeS at high pressure: Implications for the core of Ganymede

We have carried out in situ X-ray diffraction experiments on the FeS–H system up to 16.5GPa and 1723K using a Kawai-type multianvil high-pressure apparatus employing synchrotron X-ray radiation. Hydrogen was supplied to FeS from the thermal decomposition of LiAlH4, and FeSHx was formed at high pressures and temperatures. The melting temperature and phase relationships of FeSHx were determined based on in situ powder X-ray diffraction data. The melting temperature of FeSHx was reduced by 150–250K comparing with that of pure FeS. The hydrogen concentration in FeSHx was determined to be x=0.2–0.4 just before melting occurred between 3.0 and 16.5GPa. It is considered that sulfur is the major light element in the core of Ganymede, one of the Galilean satellites of Jupiter. Although the interior of Ganymede is differentiated today, the silicate rock and the iron alloy mixed with H2O, and the iron alloy could react with H2O (as ice or water) or the hydrous silicate before the differentiation occurred in an early period, resulting in a formation of iron hydride. Therefore, Ganymede's core may be composed of an Fe–S–H system. According to our results, hydrogen dissolved in Ganymede's core lowers the melting temperature of the core composition, and so today, the core could have solid FeSHx inner core and liquid FeHx–FeSHx outer core and the present core temperature is considered to be relatively low.

High-pressure high-temperature phase relations in MgAl2O4

High-pressure high-temperature phase relation experiments in MgAl2O4 were performed in the pressure and temperature ranges of 18-27 GPa and 1400-2500 °C using a Kawai-type multi-anvil high-pressure apparatus. It was clarified that MgAl2O4 spinel directly transforms to the Mg2Al2O5 + Al2O3 assemblage at about 20 GPa and temperature higher than 2100 °C. The experimental results indicates that the phase assemblage of Mg2Al2O5 + Al2O3 is stable in the P-T region of 20 < P < 25 GPa and T > 2000 °C. The crystal structure of the Mg2Al2O5 phase was refined by the Rietveld method using a structure model based on that of ludwigite. The calculated density of ρcalc = 3.801(1) g/cm3 for the Mg2Al2O5 phase is very consistent with the phase relations determined by the high-pressure high-temperature experiments.

Reaserch on pressure generation efficiency of 6—8 type multianvil high pressure apparatus

In the present study, we investigated the major factors and mechanisms of the pressure generation efficiency of the two-stage multianvils high pressure apparatus, which was recently designed and assembled in our laboratory. A simplified mechanical model of the pressure cell assembly was also proposed to interprete the experimental data. The results with the 10/4 (with octahedral edge-length of 10 mm, and WC cube truncation edge-length of 4 mm) cell assembly indicate that the pressure efficiency is very sensitive to the gaskets size and can be improved with the increased initial density of MgO pressure transmitting medium when cell pressure is about 12 GPa. Above 15 GPa, the pressure efficiency is mainly limited by the yield strength of the second stage WC cubes, as well as the composition and size of gaskets.

Hydrogen partitioning between iron and ringwoodite: Implications for water transport into the Martian core

We determined the exchange partition coefficients of hydrogen between solid iron and ringwoodite between 16.6 and 20.9 GPa at temperatures up to 1273 K using a Kawai-type multianvil high-pressure apparatus with synchrotron X-ray radiation at the BL04B1 beamline at SPring-8, Japan. The hydrogen concentration in iron hydride was estimated from the volume expansion of iron caused by hydrogenation determined by in situ X-ray diffractions at high pressure and high temperature, and the water content of ringwoodite in the recovered samples was estimated using the Fourier transform infrared spectroscopy (FTIR). According to our results, the exchange partition coefficients of hydrogen between the solid iron and ringwoodite were almost constant, 26, with pressure between 16.6 and 20.9 GPa and 1273 K. These results revealed that hydrogen was strongly partitioned to metallic iron and that iron hydride formed, coexisting with dry ringwoodite under the experimental pressures. Ringwoodite, found in the Martian core–mantle boundary region, is an important hydrogen reservoir. The pattern of quasi-parallel bands of uniformly magnetized crust with alternating positive and negative polarity measured by the Mars Global Surveyor spacecraft strongly shows that a magnetic field did exist in ancient Mars suggesting a possible plate tectonic activity on ancient Mars. Thus, water could have been transported to the deep Martian interior by hydrous minerals during the plate subduction process and stored in ringwoodite in the deep Martian slabs, as is suggested on the Earth today. Our experiments suggested that hydrogen stored in ringwoodite was absorbed by the Martian core at the Martian core–mantle boundary. Thus, water from the ancient Martian ocean may be stored now in the Martian core.

Use of a pressless multianvil high-pressure split-sphere apparatus to measure the silicate melt viscosity

The possibility of using a pressless multianvil high-pressure split-sphere apparatus to measure the viscosity of silicate melts was studied experimentally using as an example diopside melt (CaMgSi2O6) at a pressure of 4.0 GPa and a temperature of 1800°C. The viscosity was calculated by the Stokes formula. The measurement error of the melt viscosity was estimated by the falling ball method. Prospects for using devices of this type to estimate the viscosity of silicate melts at high pressures and temperatures are shown.

Si and O diffusion in (Mg,Fe) 2SiO 4 wadsleyite and ringwoodite and its implications for the rheology of the mantle transition zone

Si and O diffusion rates on polycrystalline (Mg,Fe) 2SiO 4 wadsleyite and ringwoodite have been determined at pressures between 16 and 22 GPa and temperatures between 1400 and 1600 °C using a Kawai-type multi-anvil high-pressure apparatus. Pre-synthesized polycrystalline wadsleyite or ringwoodite was used as starting materials. Diffusing sources of 29Si and 18O enriched (Mg,Fe) 2SiO 4 thin film were deposited on the surface of wadsleyite and ringwoodite by pulsed laser deposition. The diffusion profiles were obtained using secondary ion mass spectrometry in the depth-profiling mode. All measured diffusion profiles were composed of volume and grain-boundary diffusion regimes. Arrhenius relations for volume and grain-boundary diffusion rates of Si and O in wadsleyite and ringwoodite have been determined. The results show that Si is the slowest diffusing element among the major elements in both wadsleyite and ringwoodite under mantle transition zone as well as slab conditions. Therefore, Si should be the rate-controlling species for high-temperature creep in wadsleyite and ringwoodite. Comparisons with Si and O diffusion rates obtained in this study and those in olivine and silicate perovskite suggest that Si and O diffusion rates are enhanced at both 410 and 660 km seismic discontinuities. The deformation mechanism maps constructed from Si diffusion data in wadsleyite and ringwoodite suggest that the dominant deformation mechanism operating in the mantle transition zone is dislocation creep, which may be the origin of the observed global seismic anisotropy. Under cold subduction zone conditions, grain-size sensitive diffusion creep becomes dominant when the grain size is reduced to less than 10–100 µm below the metastable olivine wedge.

SiO2 solubility in rutile at high temperature and high pressure

Silicon-bearing rutile has been found in chromitite from the Luobusa (罗布莎) ophiolite, Tibet. However, the extent of SiO2 solubility in rutile and the nature of its origin are still unclear. At high pressure, SiO2 takes a rutile structure with Si in 6-fold coordination. Thus, high pressures may enhance its solubility in rutile because of possible isovalent exchange in the octahedral site. In this study, we report new experimental results on SiO2 solubility in rutile up to 23 GPa and 2 000 °C. Starting materials were mixtures of powdered pure rutile and pure quartz, with compositions of (Ti0.5Si0.5)O2, (Ti0.93Si0.07)O2, and (Ti0.75Si0.25)O2. The mixtures were loaded into either platinum capsules (for a 10/5 assembly) or rhenium capsules (for an 8/3 assembly). The experiments were carried out using multi-anvil high-pressure apparatus with a rhenium resistance heater. Sample temperatures were measured with a W5%Re-W26%Re thermocouple and were controlled within ±1 °C of the set temperature. TiO2-rich and SiO2-rich phases were produced in all the quenched samples. Microprobe analyses of the phases show that the solubility of SiO2 in rutile increases with increasing pressure, from 1.5 wt.% SiO2 at 10 GPa to 3.8 wt.% SiO2 at 23 GPa at a temperature of 1 800 °C. The solubility also increases with increasing temperature from 0.5 wt.% SiO2 at 1 500° to 4.5 wt.% SiO2 at 2 000° at a pressure of 18 GPa. On the other hand, the solubility of TiO2 in coesite or stishovite is very limited, with an average of 0.6 wt.% TiO2 over the experimental P-T ranges. Temperature has a much larger effect on the solubility of SiO2 in rutile than pressure. At high pressure, the melting point of SiO2 is definitely higher than that of TiO2 and the eutectic point moves towards SiO2 in the TiO2-SiO2 system. Lower oxygen fugacity decreases the solubility of SiO2 in rutile, whereas water has little effect on the solubility. Our experimental data are extremely useful for determining the depth of origin of the SiO2-bearing rutile found in nature.

The temperature-pressure-volume equation of state of platinum

High-temperature and high-pressure equations of state (EOSs) of Pt have been developed using measured shock compression data up to 290 GPa and volume thermal expansion data between 100 and nearly 2000 K and 0 GPa. The lattice thermal pressures at high temperatures have been estimated based on the Mie–Grüneisen relation with the Debye thermal model and the Vinet isothermal EOS. The contribution of electronic thermal pressure at high temperatures has also been included here. The optimized EOS parameters of Pt are K0T=273 GPa, K0T′=5.20, γ0=2.70, and q=1.10 with Θ0=230 K, where the subscript 0 refers to the ambient conditions. The temperature-pressure-volume (T-P-V) data of Pt have also been measured up to 1600 K and 42 GPa, using synchrotron powder x-ray diffraction experiments combined with a Kawai-type multianvil high pressure apparatus and sintered diamond anvils. We find that the newly developed T-P-V EOS of Pt is fully consistent with not only the shock compression data up to 290 GPa and volume thermal expansion data up to near 2000 K, but also the present measured synchrotron T-P-V data and recently measured T-P-V data of Pt up to 1900 K and 80 GPa. Thus we find that there is no need to include a volume dependence of q over a wide pressure range up to more than 300 GPa. The present EOS has been developed without any pressure scale. Such excellent consistency between the EOS and experimental values over wide temperature and pressure ranges shows that the present EOS can be used as a reliable primary pressure standard for static experiments up to 300 GPa and 3000 K.

An exploratory study of the viscoelasticity of phase-transforming material

Attenuation and modulus dispersion are typically associated with shear stress and strain. Time-dependent volume changes accompanying pressure variations can give rise to bulk modulus attenuation and dispersion. Phase transformations in a two-phase region are candidates for such phenomena. Here we report laboratory data that are consistent with bulk modulus softening as pressure is cycled in a region of coexisting olivine and spinel. We use Fay70For30 olivine as our sample. Experiments are performed in a multi-anvil high-pressure apparatus (Deformation DIA) using synchrotron (NSLS) X-ray radiation as the probing tool. Pressure is up to 12 GPa and temperature is up to 1450 °C. Measurements were carried out within the binary loop where the olivine and spinel phases coexist. We apply a uniaxial oscillation stress onto the sample and Young’s modulus and Q −1 are measured at frequencies of 0.1–0.01 Hz. Our results indicate that the sinusoidal force applied to the sample in olivine-ringwoodite region has much lower bulk modulus and higher Q −1 than in the single-phase regions. Our data are consistent with the diffusion controlled model of [Jackson, I., 2007. Physical origins of anelasticity and attenuation in rock, In: Price, G.D. (Ed.) Mineral Physics. Treatise On Geophysics. Elsevier], where the characteristic time decreases with decreasing strain.

Compression behavior of WC and WC-6%Co up to 50 GPa determined by synchrotron x-ray diffraction and ultrasonic techniques

The equations of state (pressure-volume relations) for WC and WC-6%Co have been determined by synchrotron x-ray diffraction measurements on polycrystalline powder samples loaded in a diamond anvil cell as well as by ultrasonic measurements on hot-pressed polycrystalline, cylindrical samples loaded in a multianvil high-pressure apparatus. The third-order Birch–Murnaghan equation of state fitted to the x-ray diffraction pressure-density sets of data, collected up to 50 GPa, yields ambient pressure isothermal bulk moduli of KoT=411.8±12.1 GPa and KoT=402.4±14.1 GPa, with pressure derivatives of KoT′=5.45±0.73 and KoT′=7.50±0.86 for WC and WC-6%Co, respectively. The ultrasonic measurements, conducted up to 14 GPa, enabled the determination of the pressure dependences of both bulk and shear moduli. Using Eulerian finite strain equations to fit the ultrasonic data, we obtain for WC an ambient pressure adiabatic bulk modulus of Kos=383.8±0.8 GPa, and Kos′=2.61±0.07 for its pressure derivative, while values of Gos=304.0±0.3 GPa and Gos′=1.50±0.09 were determined for the shear modulus and its pressure derivative, respectively. Meanwhile, for WC-6%Co, we obtain Kos=357.5±1.0 GPa, Kos′=5.18±0.14, Gos=253.5±0.3 GPa, and Gos′=1.09±0.09. The equations of state derived from the ultrasonic data are in good agreement with extrapolated results reported previously by Day and Ruoff [J. Appl. Phys. 44, 2447 (1973)] and Gerlich and Kennedy [J. Appl. Phys. 50, 3331 (1978)] who carried out measurements up to 0.2 and 1.0 GPa, respectively.

Temperature-pressure-volume equation of state of the B2 phase of sodium chloride

The temperature-pressure-volume (T-P-V) data of the B2 phase of sodium chloride (NaCl) were measured at high temperatures between 1023 and 1973K, and high pressures between 22.9 and 26.3GPa, using synchrotron powder x-ray diffraction experiments with a Kawai-type multianvil high pressure apparatus. The Mie–Grüneisen-type thermal pressure analysis was made to obtain the high temperature and high pressure T-P-V equation of state (EOS) of the B2 phase based on the present measured T-P-V data together with the 300K volume compression data previously reported using diamond-anvil-cell experiments. Some molecular dynamics calculations using a breathing shell model interionic potential, recently developed for the NaCl system, were also carried out to investigate the behavior of thermal pressure of the B2 phase at high temperatures and high pressures. The resulting T-P-V EOS agrees very well with recently measured volume compression data at 1000K. Here we present the T-P-V EOS of the B2 phase up to 3000K and more than 150GPa, as a reliable pressure standard at high temperatures and high pressures.

Dislocation microstructures of MgSiO 3 perovskite at a high pressure and temperature condition

Dislocation microstructures of polycrystalline MgSiO 3 perovskite, synthesized in a multi-anvil type high-pressure apparatus at 26 GPa and 2023 K for 600 min, have been investigated using transmission electron microscopy (TEM). The recovered MgSiO 3 perovskite displayed deformation lamellae under cross-polarized optical microscopy, suggesting that the sample was deformed during the high-pressure experiment. TEM observations showed that curved dislocations with Burgers vectors lying in the {1 1 0} plane, potentially b = 〈1 1 0〉 and [ 0 0 w ] (orthorhombic symmetry) were nucleated. Low-angle tilt boundaries controlled by 〈1 1 0〉 dislocation climb were also activated at high temperature and pressure. The potential Burgers vector of b = 〈1 1 0〉 is consistent with previous results of slip to 〈1 0 0〉 cubic, [1 0 0] pc and [0 1 0] pc (cubic symmetry and pseudo-cubic symmetry) on non-silicate perovskite analogues deformed at high temperatures and ambient pressure. The microstructures of climb-accommodated dislocation creep controlled by diffusion in the MgSiO 3 perovskite can provide useful information on the rheology at high temperatures and slow strain rates corresponding to the Earth's lower mantle.

In situ observation of the decomposition of kyanite at high pressures and high temperatures

In situ observations of the decomposition of kyanite, Al 2 SiO 5 , were carried out in a multi-anvil high-pressure apparatus using synchrotron radiation, where the phase change from kyanite to stishovite + corundum was observed at high pressures and high temperatures. The phase boundary of this decomposition at T = 1200–1900 K and P = 5–15 GPa was determined to be, P (GPa) = 10.2 + 0.0016 × T (K). Previous studies using the quench method showed a discrepancy in the transition pressure of the decomposition. Our results using in-situ observations resolve this dispute.

In-situ control of different oxygen fugacity experimental study on the electrical conductivity of lherzolite at high temperature and high pressure

At pressure 1.0–4.0 GPa and temperature 1073–1423 K and under the control of oxygen fugacity (Mo+MoO 2, Fe+FeO and Ni+NiO), a YJ-3000t multi-anvil solid high-temperature and high-pressure apparatus and Solartron-1260 impedance/Gain-Phase analyzer were employed to analyze the electrical conductivity of lherzolite. The experimental results showed that: (1) within the range of the selected frequencies (10 3–10 6 Hz), either as viewed from the relationship between the real or imaginary part of complex impedance and the frequency, or from the relationship between modulus, phase angle and frequency, it can be seen clearly that the complex impedance has a strong dependence on frequency; (2) with the rise of temperature ( T), the electrical conductivity ( σ) increased, and Lg σ and 1/ T follows the Arrhenius relationship; (3) with the rise of pressure, the electrical conductivity decreased, and activation enthalpy and temperature-independent pre-exponential factor decreased as well. And the activation energy and activation bulk volume of the main charge carrier in the lherzolite have been obtained for the first time, which are 1.68±0.02 eV and 0.04±0.01 cm 3/mol, respectively; (4) under the given pressure and temperature, the electrical conductivity tends to increase with increasing oxygen fugacity, and under the given pressure, the activation enthalpy and pre-exponential factor tend to decrease with the rise of oxygen fugacity; (5) at 2.0 GPa and the control of the three solid buffers, Mo+MoO 2, Fe+FeO and Ni+NiO, the exponential factors of electrical conductivity variation range with oxygen fugacity are 1 3.19 - 1 3.67 , and the theoretical model for the relationship between the electrical conductivity of lherzolite and the oxygen fugacity under high pressure has been established for the first time; (6) the electrical conduction mechanism of small polarons provides a reasonable explanation to the variation of conductivity of lherzolite with oxygen fugacity.