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Clayey soils are treated with binding agents to improve their mechanical properties, as these soils are widely used in construction. The production of binding agents is an energy-intensive process and emits significant amounts of CO2. In addition, the interest in recycling industry waste materials has increased, and the management of significant waste from biomass power plants remains an issue. We used three biomass ashes derived from pellet, olive, and grapevine combustion as stabilizing agents of a clayey soil. The mechanical effects of the treatment on clay-ash mixtures were evaluated using confined compressive tests. The mixtures' chemo-mineralogical evolution was evaluated through X-ray powder diffraction and quantitative Rietveld analysis, Fourier transform infrared spectroscopy (FT-IR), and energy-dispersive X-ray spectroscopy coupled with scanning electron microscopy (EDS-SEM). The FT-IR spectra showed an evolution of the Si-O-Si/Al features, with shifting band positions due to polymerization of the tetrahedral units. The EDS-SEM analysis showed an evolution of the Ca/Si distribution and the growth of pozzolanic reaction products, such as C-S-H nanocrystals and gels. This evidence confirms that the pozzolanic reaction occurs by dissolution of clay minerals and/or the amorphous phase of the ash, which affects the macroscopic behavior of clayey soils in terms of stiffening and strengthening, as confirmed by mechanical tests, albeit these effects are non-homogenous and continuous. [ABSTRACT FROM AUTHOR]

The crystal structure of gypsum-II, a polymorph of CaS[O.sub.4]*2[H.sub.2]O stable above 4 GPa, has been solved using single-crystal synchrotron X-ray diffraction data collected at 5.35 and 6.74 GPa. Gypsum-II is monoclinic, space group [P2.sub.1]/n, with lattice parameters a = 5.865(12), b = 15.045(14), c = 5.478(12) [Angstrom], [beta] = 115.3(2)[degrees], and V 437.0(14) [[Angstrom].sup.3] at 5.35 GPa, and a = 5.776(2), b = 15.017(2), c = 5.473(2) [Angstrom], [beta] = 114.98(4)[degrees], and V= 430.3(2) [[Angstrom].sup.3] at 6.74 GPa. The crystal structure has been refined to [R.sub.1] = 3.7 (5.35 GPa) and 3.9% (6.74 GPa). It closely resembles that of gypsum at room pressure with a stacking of Ca[O.sub.8] and S[O.sub.4] polyhedra along the b-axis to form layers. With increasing pressure, a continuous increase in distortion of the S[O.sub.4] tetrahedron and a strong change in the bonding style of the water molecules are observed. The mechanism of phase transformations previously hypothesized in gypsum, on the basis &high-pressure spectroscopic data, is here clarified for the polymorph stable between 4-8 GPa. Keywords: Gypsum, high pressure, crystal structure, phase transition, Ca-sulfate DOI: 10.2138/am.2010.3452

The pressure evolution of bassanite (CaS[O.sub.4]- 1/2[H.sub.2]0) was investigated by synchrotron X-ray powder diffraction along three isotherms: at room temperature up to 33 GPa, at 109[degrees]C up to 22 GPa, and at 200[degrees]C up to 12 GPa. The room-temperature cell-volume data, from 0.001 to 33 GPa, were fitted to a third-order BirchMumaghan equation-of-state, and a bulk modulus [K.sub.0] = 86(7) GPa with K' - 2.5(3) was obtained. The axial compressibility values are [[beta].sub.a], = 3.7(2), [[beta].sub.b] = 3.6(1), and [[beta].sub.c]. 2.8(1) [GPa.sup.-1] (x [l0.sup.3]) showing a slightly anisotropic behavior, with the least compressible direction along c axis. The strain tensor analysis shows that the main deformation occurs in the (010) plane in a direction 18[degrees] from the a axis. The bulk moduli for isotherms 109 and 200[degrees]C, were obtained by fitting cell-volume data with a second-order Birch-Murnaghan equation-of-state, with K' fixed at 4, and were found to be [K.sub.109] = 79(4) GPa and [K.sub.200] - 63(7) GPa, respectively. The axial compressibility values for isotherm 109[degrees]C are [[beta].sub.a], = 2.4(1), [[beta].sub.b] --3.0(1), [[beta].sub.c] 2.5(1) (x [10.sup.3]) [GPa.sup.1], and for isotherm 200 [degrees]C they are [[beta].sub.a] - 3.5(3), [[beta].sub.b] = 3.4(3), [[beta].sub.c] 2.6(4) (x [10.sup.-3]) [GPa.sup.-1]. These two bulk moduli and the 20[degrees]C bulk modulus, [K.sub.0.20] 69(8) recalculated to a second-order Birch-Murnaghan EoS to be consistent, as well as the axial compressibilities, are similar for the three isotherms indicating that the thermal effect on the bulk moduli is not significant up to 200[degrees]C. The size variation of the pseudo-hexagonal channel with pressure and temperature indicates that the sulfate 'host' lattice and the [H.sub.2]O 'guest' molecule in bassanite do not undergo strong change up to 33 GPa and 200[degrees]C. Keywords: Bassanite, Ca-sulfate, compressibility, high temperature, high pressure, X-ray diffraction DOI: 10.2138/am.2009.3245

This paper described a baseline for a multidisciplinary approach to the mineralogical and chemical characterization of eighteen fragments of archaic transport amphoras (7th-6th centuries BC) found at Gravisca archaeological site (Lazio, Italy) attributed to an Eastern Aegean provenance (six from Samos, three from Miletus, three from Chios and three from Klazomenai). The samples have been typed on the evidence concerning archaic manufactory techniques by using X-ray powder diffraction (XRPD) associated with quantitative phase analysis (QPA), optical (OM) and electron microscopy (SEM). Following Inductive Coupled Plasma Spectroscopy (ICP), compositional data analysis has been for the first time applied to selected geochemical proxies combining rare earth elements and selected heavy metals concentration in Hierarchical Agglomerative Cluster (HAC). This innovative approach allows to distinguish different sources of the raw materials used from different geological and geographical areas. Multivariate statistical treatments via Principal Component Analyses (PCA) open the possibility to group the sherds into clusters, comparing the results with the HAC application allowing to validate the methodology. Two distinct centres associated with Samos manufactory appear to be operating during 7th-6th centuries BC, which could be related to different production periods or even different production areas. The new data concur with a more straightforward historical reconstruction of archaic Poleis manufactory and commercial activity toward Etruria and the Mediterranean and with a new methodological approach to archaeometry. [ABSTRACT FROM AUTHOR]

High-pressure X-ray diffraction was carried out on a single crystal of gypsum compressed in a diamond anvil cell. The sample maintained its crystal structure up to 4.0 [+ or -] 0.1 GPa. The fit of pressure dependence of the unit-cell volume to the third-order Birch-Murnaghan equation yielded [K.sub.T0] = 44(3) GPa and [([partial derivative][K.sub.T]/[partial derivative]P).sub.0] = 3.3(3), where [K.sub.T0] and [([partial derivative][K.sub.T]/[partial derivative]P).sub.0] are the isothermal bulk modulus and its pressure derivative in ambient conditions. The axial compressibility values, fitting data collected up to 3.94 GPa, were [[beta].sup.EoS.sub.0a] = 6.1(1) and [[beta].sup.EoS.sub.0c] = 5.6(1) [10.sup.-3] [GPa.sup.-1]. The value of [[beta].sup.EoS.sub.0b] was 6.2(8) [10.sup.-3] [GPa.sup.-1] fitting the data collected up to 2 GPa, due to non-linearity above this pressure; axial compressibility of gypsum is almost isotropic ([[beta].sub.0a]:[[beta].sub.0b:[[beta].sub.0c] = 1:1:0.9). This behavior is partly unexpected for a layered mineral based on alternate layers of Ca-and S-polyhedral chains separated by interlayers occupied by water molecules. Above 4.0 GPa the compression curve of gypsum shows a discontinuity with a 2.5% contraction in volume. Structural refinements indicate that S[O.sub.4] volume and average S-O bond distances remain almost unchanged from room pressure to 3.9 GPa [range 1.637(4)-1.66(9) [[Angstrom].sup.3]; 1.4733-1.48 [Angstrom]]. The S[O.sub.4] tetrahedron undergoes distortion: the smaller distance decreases from 1.4731(9) to 1.45(2) [Angstrom] and the larger increases from 1.4735(9) to 1.51(2) [Angstrom]. In contrast, the calcium polyhedra show expected high-pressure behavior, becoming more regular and decreasing in volume from 25.84(8) [[Angstrom].sup.3] at ambient P to 24.7(1) [[Angstrom].sup.3] at 3.9 GPa. The largest variations were observed in the interlayer region where the water molecules are located. Along the b axis, the two structural layers have very different compressibilities: the polyhedral layer is almost incompressible in the pressure range studied, whereas water layer compressibility is 9.7(3) [10.sup.-3] [GPa.sup.-1], about twice that of the other two lattice parameters. At ambient conditions, water molecules form weak hydrogen bonds with the O atoms of Ca and S polyhedra. With increasing pressure, the weakest hydrogen bond becomes the strongest one: from 0.001 to 4 GPa, the distance changes from 2.806(1) to 2.73(2) [Angstrom] for OW-H1 ... O2, and from 2.883(2) to 2.69(3) [Angstrom] for OW-H2 ... O2. Structure refinements show that water remains in the structure when P increases. The observed distortion of sulfate tetrahedra explains the splitting of the [v.sub.1] sulfate stretching mode, and the various measured compressibilities of the two hydrogen bonds and the coalescence of the Raman stretching mode observed at pressures over 5 GPa. Keywords: Gypsum, high pressure, single-crystal X-ray diffraction, phase transition

A hypersilicic Cl-bearing mica was synthesized at 4 GPa and 1200-1250[degrees]C, close to the solidus of the join diopside-jadeite-KCl, in association with diopside-jadeite pyroxene, K-rich aluminosilicate glass and/or sanidine and (K,Na)Cl. The mica shows a negative correlation between tetrahedral Si and octahedral (Al + Mg), suggesting an Al-celadonitic substitution (Si + [sup.VI]Al + [sup.VI][] = [sup.VI]Al + [sup.VI]Mg) and a chemical formula: [K.sub.1.01][([Mg.sub.2.45][Al.sub.0.19][[].sub.0.35]) [SIGMA]=3[([Si.sub.3.52][Al.sub.0.48])[SIGMA]=4[O.sub.10][[(OH,O) .sub.1.66] [Cl.sub.0.34])][SIGMA]=2. The presence of hydroxyl was confirmed by OH stretching modes at 3734 and 3606 [cm.sup.-1] in the Raman spectra. Single-crystal X-ray diffraction data provide the unit-cell parameters (space group C2/m, 1 M polytype): a = 5.299(4), b = 9.167(3), c = 10.226(3) [Angstrom], [beta] = 100.06(4)[degrees], V= 489.1 (4) [[Angstrom].sup.3]. The structure refinement shows the presence of vacancies on the octahedral sites (15% for M 1 and 6.5% for M2). Chlorine occupies a position about 0.5 [angstrom] from O4 with partial occupancy (0.39 apfu). Crystal-chemical mechanisms seem to govern chlorine incorporation in mica, since a large A site is necessary to locate the anion in the structure. A large A site results when the six-tetrahedra ring is hexagonal and the tetrahedral rotation angle [alpha] is 0[degrees]. Such a geometry is achieved either by increasing the annite component in biotite or by increasing the hypersilicic character of phlogopite through the Al-celadonite substitution. The present Si-rich mica shows a partial dioctahedral character due to the Al-celadonite substitution, which lowers the [alpha] angle and expands its stability field at high pressure. High [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] conditions, like in potassium-rich brine or potassic carbonatitic melts, increase the Al-celadonite component in the phlogopite solid solution, explaining the association of Si-rich micas with inclusions of potassic liquids in kimberlitic diamonds. Keywords: Phlogopite, Al-celadonite, chlorine, high pressure, solid solution, kimberlites, inclusions in diamonds

Elastic behavior and high-pressure (HP) structural evolution of a natural zeolite levyne, [([Ca.sub.0.5], Na,K).sub.6]([Al.sub.6][Si.sub.12][O.sub.36],) 18[H.sub.2]O, has been investigated up to 5 GPa by means of in situ single-crystal X-ray diffraction with a diamond-anvil cell using a non-penetrating pressure-transmitting medium. Peculiar elastic behavior has been observed in the range 0-1 GPa: the c parameter decreases between 0 and 0.2 GPa, then increases up to 0.5-0.6 GPa. Above this P-value, the parameter decreases as expected. Anomalous behavior is also shown by the a-parameter, which first increases up to 0.2 GPa, then decreases as expected. However, these anomalous lattice variations are only slightly reflected in the cell volume behavior. The low-P trend (P < 1 GPa) is also followed by the lattice parameters in decompression. The isothermal Equation-of-State (EoS) at P > 1 GPa, refined with a second-order Birch-Murnaghan EoS, yields the following parameters: [V.sub.0] = 3539(3) [A.sup.3] and [K.sub.T0] = 48(1) GPa. Comparison of structural refinements performed at 0.0001,0.79(5), and 3.00(5) GPa highlights two distinct deformation mechanisms of the Si/Al-framework, one predominant at low pressures (0-1 GPa) and the other at high pressures (1-5 GPa). The extra-framework content does not show any evident modification within the pressure range investigated.

The high-pressure (HP) structural evolution of a natural tetragonal edingtonite from Ice River, Canada, was investigated up to 5.1 GPa using in situ single-crystal X-ray diffraction and a diamond-anvil cell. The isothermal equation of state was determined. The values of [V.sub.0], [K.sub.T0], and K' refined with a third-order Birch-Murnaghan equation of state (BM-EoS) are [V.sub.0] = 601.6(3) [[Angstrom].sup.3],[K.sub.TO] = 59(2) GPa, and K' = 3.4(8). Under high-pressure conditions the main deformation mechanisms can be described by rotation/kinking of 'rigid units,' represented by the 4 = 1 secondary building unit (SBU), due to the tetrahedra tilting. The angle between the SBUs [phi] increased from 17.15(8)[degrees] at 0.0001 GPa to 20.03(9)[degrees] at 4.61 GPa. The bulk structural compression results from the combination of the 'soft' behavior of the not fully occupied channels [[K.sub.TO] = 19(1) GPa for [100]-channels; [K.sub.TO] = 21(1) GPa for [110]-channels] and of the rigid behavior of the tetrahedral framework. The extra-framework cations do not increase in coordination number within the pressure range investigated. The barium occupancy factors for the Ba1 and Ba2 sites change with increasing pressure. For P > 2.3 GPa the Ba2 site is completely empty, and only the position Ba1 is occupied.

The response of staurolite to pressure was studied by single crystal X-ray diffraction (XRD) in a diamond-anvil cell, using crystals with composition: ([Fe.sub.3.365] [Zn.sub.0.025][Li.sub.0.114][Co.sub.0.009] [Mn.sub.0.034])[T2, M4] ([Al.sub.2][Mg.sub.0.307])[M3] ([Al.sub.15.491][Fe.sup.3+.sub.0.104] [Mg.sub.0.394][Cr.sub.0.004][Ti.sub.0.07])[M1, M2] ([Si.sub.7.534] [Al.sub.0.466])[T1] [O.sub.48][H.sub.3]. Lattice parameters, measured at various pressure up to 7.264(6) GPa, were fitted using a third-order Birch-Murnaghan equation of state (EoS). The resulting EoS parameters are: [V.sub.0] = 740.85(7) [[Angstrom].sup.3], [K.sub.0] = 180(2) GPa and K' = 4.7(6), [a.sub.0] = 7.8723 (2) [Angstrom] [K.sub.0] = 189(2) GPa, and K'.sub.a] = 4.1 (6), [b.sub.0] = 16.62453(1) [Angstrom], [K.sub.0] = 179(2) GPa, [K'.sub.b], = 6.1(6) and [c.sub.0] = 5.6604 (4) [Angstrom], [K.sub.0] = 179(5) GPa, [K'.sub.c] = 2(1); whereas the angle [beta] remained almost constant with increasing pressure. These data suggest an almost isotropic compressibility. Structural evolution was studied by comparison of structural refinements carried out with data Collected at 0.0001, 2.48, 4.15, 5.43, 6.84, and 8.74 GPa. All refinements were made in the Ccmm space group. Polyhedral evolution with P is a function of occupancy: whereas the T1 tetrahedron and the M1 and M2 octahedra, occupied by Si and Al, are practically incompressible, the T2 tetrahedron, and the M4 and M3 octahedra, only partially occupied principally by Fe (the first two) and by Al (the last), show larger changes as a function of pressure. As a consequence, the two kyanite and Fe-Al hydroxide layers, which can be used to describe the staurolite structure, have different compressibilities.

Potential containers for fission products must have long-term stability and resistance to radiation damage, and to chemical, mechanical and physical agents. Synroc is a ceramic material with a multi-phase immobilizing barrier using geochemical principles, consisting of rutile, hollandite, zirconolite and perovskite. Hollandite can trap (super135)Cs and (super137)Cs. Cs-tetra-ferri-annite has been proposed as a potential candidate for the storage of radiogenic Cs isotopes. The compressibility of Cs-tetra-ferri-annite has been studied, along with its thermal expansion.

The pressure stability of kyanite was experimentally reversed with the use of a multi-anvil apparatus. Kyanite was found to decompose into its oxides stishovite and corundum between 14 [+ or -] 0.5 GPa (at 1000 [degrees] C) and 17.5 [+ or -] 1.0 GPa (at 2000 [degrees] C). Reliable thermodynamic calculations can be performed to temperatures of approximately 1500 [degrees] C. Up to this temperature, the location of the equilibrium kyanite = corundum + stishovite, determined in this study, constrains the equilibrium coesite = stishovite. A set of thermodynamic data was calculated by linear programming from the kyanite breakdown reaction and the coesite = stishovite equilibrium. Feasible values for the fitted thermodynamic properties are -28.5 to -26.3 MPa/K for the temperature derivative of the bulk modulus [[(dK/dT).sub.P]] of kyanite, -815254 to -813635 J/mol for [Mathematical Expression Omitted] of stishovite, and 24.6 to 26.3 J/mol [center dot] K for [Mathematical Expression Omitted] of stisovite. The experimental results indicate (1) that in peraluminous eclogites of basaltic or sedimentary origin, stishovite may coexist stably with corundum at a depth greater than 420450 km and (2) that in an inhomogeneous Al-enriched mantle, corundum could be a minor constituent in the lower mantle.

The lattice parameters of kyanite were measured at various pressures up to about 60 kbar by single-crystal X-ray diffraction in a diamond anvil cell. Unit-cell dimensions decreased linearly with an almost uniform rate: [[Beta].sub.a] = 2.00(8) x [10.sup.-4], [[Beta].sub.b] = 1.90(4) x [10.sup.-4], [[Beta].sub.c] = 2.00(4) x [10.sup.-4] [kbar.sup.-1]. The principal compressibility coefficients were [[Beta].sub.1] = 2.23 x [10.sup.-4] [[Beta].sub.2] = 2.04 x [10.sup.-4], [[Beta].sub.3] = 1.65 x [10.sup.-4] [kbar.sup.-1], with [[Beta].sub.1] forming an angle of 35 [degrees] with the c axis. [K.sub.0], calculated by fitting pressure-volume data to a third-order Birch-Murnaghan equation of state, was 1560(100) kbar, with K[prime] 5.6(5.5); when K[prime] was set at 4, [K.sub.0] became 1600(30) kbar. Structural refinements were carded out on data collected at 0.001 kbar with the crystal in air and at 0.1, 25.4, 37, and 47 kbar with the crystal in the diamond anvil cell. Whereas the Si tetrahedra and Al4 octahedron were incompressible in this P range, the polyhedral bulk modulus for Al1 and Al2 was 1280(150) kbar and 2380(200) kbar for Al3. These octahedra became more regular with increasing pressure. The almost isotropic compression pattern was due to the many shared edges between the polyhedra, uniformly distributed in the cell. The evolution of Al-Al separation showed that the largest reduction regarded the Al2-Al3 and Al2-Al4 distances, whereas the average Al1-Al2 distance was almost unchanged, resulting from linkage with Si tetrahedra having rigid edges. The result was that the largest reduction did not occur along the c axis but along the Al4-Al1-Al2-Al3 directions. The geometrical structural invariance, expressed by the [[Beta].sub.v]/[[Alpha].sub.v] ratio and obtained from the average compressibility and average thermal expansion of the cell volume (Winter and Ghose 1979), was 23 [degrees] C/kbar. The following equation of state, which applies in crustal P-T conditions, may be defined as: V/[V.sub.0] = 1 + 3.00(7) x [10.sup.-5] T - 5.8(1) x [10.sup.-4] P, where T is in [degrees] C and P in kbar. The present volume-pressure data support multi-anvil experiments by Schmidt et al. (1997) defining the P-T conditions necessary for decomposition of kyanite into stishovite + corundum.

Compressibility data of clinozoisite and zoisite were measured by single-crystal X-ray diffraction in a diamond-anvil cell up to a pressure of about 50 kbar. In both polymorphs, the unit cell parameters varied linearly with pressure but in an anisotropic pattern: [[Beta].sub.a] = 2.1(1) x [10.sup.-4], [[Beta].sub.b] = 2.8(1) x [10.sup.-4], [[Beta].sub.c] = 3.3(1) x [10.sup.-4] [kbar.sup.-1] for clinozoisite, and [[Beta].sub.a] = 2.3(2) x [10.sup.-4], [[Beta].sub.b] = 2.9(1) x [10.sup.-4], [[Beta].sub.c] = 3.7(2) x [10.sup.-4] [kbar.sup.-1] for zoisite. The principal coefficients of the strain ellipsoid of clinozoisite are [[Beta].sub.1] = 2.0 x [10.sup.-4], [[Beta].sub.2] = 2.7 x [10.sup.-4], [[Beta].sub.3] = 3.3 x [10.sup.-4] [kbar.sup.-1]; [[Beta].sub.1] and [[Beta].sub.3] were symmetrically oriented in the (010) plane with an angle of about 12 [degrees] between [[Beta].sub.1] and the a axis, whereas [[Beta].sub.2] coincides with the b axis. Bulk moduli calculated as the reciprocal of cell-volume compressibility were 1300(20) kbar for the monoclinic and 1140(20) for the orthorhombic polymorph. [K.sub.0], determined by fitting the unit-cell parameters with a third-order Birch-Murnaghan equation of state, was 1270(45) kbar, with K[prime] = 0.5(2) for clinozoisite and 1020(65) kbar with K[prime] = 4.8(4) for zoisite. Structural refinements of clinozoisite performed at 0.5, 19.4, and 42 kbar, and also under ambient conditions, showed that the compression mechanism included both shrinking of the polyhedra (i.e., octahedra and Ca polyhedra) and tilting of the [Si.sub.2][O.sub.7] group, with reduction of the Si-O-Si angle. The different effect of these mechanisms explains the anisotropic compressional pattern in clinozoisite and the similar behavior observed in the two polymorphs. Comparison of high-pressure and high-temperature data for clinozoisite showed that a given increase in pressure produced structural effects very similar to those seen after a proportional decrease in temperature. The calculated volume-expansivity-to-compressibility ratio of 38 bar/K indicates that the cell volume of clinozoisite remains unchanged with geothermal gradients of about 10 [degrees] C/km. The crystallographic data support the results of experimental petrology in indicating that epidote is a good candidate for transporting [H.sub.2]O in down-going subduction slabs.

Independent isobaric data on thermal expansion and isothermal compressibility data for lawsonite, Ca[Al.sub.2][Si.sub.2][O.sub.7][(OH).sub.2] [multiplied by] [H.sub.2]O, were determined between 23 and 598 [degrees] C and 0.001 and 37.7 kbar using single-crystal X-ray diffraction in a microfurnace and a diamond-anvil cell, respectively. The crystal structures of lawsonite were also refined from intensity data collected at 23, 444, and 538 [degrees] C and 0.5 and 28.7 kbar. Both expansion and compression patterns are slightly anisotropic, with minor changes along c with respect to a and b. Unit-cell dimensions vary linearly with T and P: [[Alpha].sub.a] = 1.26(4) x [10.sup.-5], [[Alpha].sub.b] = 1.12(4) x [10.sup.-5], [[Alpha].sub.c] = 7.6(3) x [10.sup.-6] [degrees] [C.sup.-1], and [[Beta].sub.a] = 3.4(1) x [10.sup.-4], [[Beta].sub.b] = 3.0(1) x [10.sup.-4], [[Beta].sub.c] = 2.8(2) x [10.sup.-4] k[bar.sup.-1]. Bulk modulus, calculated as the reciprocal of cell-volume compressibility, is 1100(40) kbar. The cavities of the framework accommodating Ca and [H.sub.2]O molecules change by only about [+ or -] 5% in the investigated T and P ranges. Thus, at room pressure [H.sub.2]O molecules can be hosted in the lawsonite structure at least up to above 500 [degrees] C, where the reduction of reflection intensities shows the beginning of dehydration and breakdown of the phase. Like other dense phases, structural changes with T and P essentially affect bond lengths, whereas interpolyhedral variations mainly concern the Si-O-Si[prime] angle between tetrahedral pairs, which increases with temperature and decreases with pressure. The present data define the 'geometric' equation of state for lawsonite on the basis of cell-volume variations: V/[V.sub.0] = 1 + 3.13(9) x [10.sup.-5]T - 9.1(3) x [10.sup.-4], where T is in degrees Celsius, P is in kilobars, and the [Alpha]/[Beta] ratio is 34 bar/[degrees] C. This indicates that the cell volume of lawsonite remains unchanged with geothermal gradients of about 10 [degrees] C/km, a condition actually observed in down-going subduction slabs. Therefore, the results of high-T and high-P structure refinements are in agreement with results from multi-anvil experiments and confirm that lawsonite is a good candidate for carrying water down to mantle depths.