Distribution of isomorphous cations within octahedral sheets in montmorillonite from Camp-Bertaux

 Ni-saturated montmorillonite from Camp-Bertaux heated at different temperatures has been studied by X-ray powder diffraction, X-ray absorption (EXAFS) and vibration IR spectroscopy. Analysis of the experimental data has shown that heating of samples at temperatures higher than 150° C was accompanied by migration of Ni cations into vacant cis-octahedra of 2:1 layers. In the octahedral sheet the Ni cation has two “heavy” (Fe) and four “light” (Al and Mg) nearest octahedral cations. A model for the octahedral cation distribution in Camp-Bertaux montmorillonite was proposed in which Fe and Mg octahedral cations are segregated in small clusters. Received July 7, 1996 / Revised, accepted August 23, 1996     

Computational study of tetrahedral Al–Si and octahedral Al–Mg ordering in phengite

 As part of a wider study of the nature and origins of cation order–disorder in micas, a variety of computational techniques have been used to investigate the nature of tetrahedral and octahedral ordering in phengite, K₂ ^[6](Al₃Mg)^[4](Si₇Al)O₂₀(OH)₄. Values of the atomic exchange interaction parameters J _n used to model the energies of order–disorder were calculated. Both tetrahedral Al–Si and octahedral Al–Mg ordering were studied and hence three types of interaction parameter were necessary: for T–T, O–O and T–O interactions (where T denotes tetrahedral sites and O denotes octahedral sites). Values for the T–T and O–O interactions were taken from results on other systems, whilst we calculated new values for the T–O interactions. We have demonstrated that modelling the octahedral and tetrahedral sheets alone and independently produces different results from modelling a whole T–O–T layer, hence justifying the inclusion of the T–O interactions. Simulations of a whole T–O–T layer of phengite indicated the presence of short-range order, but no long-range order was observed. Received: 8 August 2002 / Accepted: 14 February 2003 Acknowledgements The authors are grateful to EPSRC (EJP) and the Royal Society (CIS) for financial support. Monte Carlo simulations were performed on the Mineral Physics Group's Beowulf cluster and the University of Cambridge's High Performance Computing Facility.     

In situ high-temperature X-ray and neutron powder diffraction study of cation partitioning in synthetic Mg(Fe<Subscript>0.5</Subscript>Al<Subscript>0.5</Subscript>)<Subscript>2</Subscript>O<Subscript>4</Subscript> spinel

The intra-crystalline cation partitioning over T- and M-sites in a synthetic Mg(Fe,Al)₂O₄ spinel sample has been determined as a function of temperature by Rietveld structure refinements from powder diffraction data, combining in situ high-temperature neutron powder diffraction (NPD; POLARIS diffractometer, at ISIS, Rutherford Appleton Laboratory, UK), to determine the Mg and Al occupancy factors, with in situ high-temperature X-ray powder diffraction, to fix the Fe³⁺ distribution. The results obtained agree with a two-stage reaction, in which an initial exchange between Fe³⁺ and Mg, the former leaving and the latter entering tetrahedral sites, is successively followed by a rearrangement involving also Al. The measured cation distribution has then been compared and discussed with that calculated by the Maximum Configuration Entropy principle, for which only NPD patterns have been used. The cation partitioning has finally been interpreted in the light of the configuration model of O’Neill and Navrotsky.     

How stacking disorder can conceal the actual structure of micas: the case of phengites

The present study deals with how stochastic stackings of tetrahedral/octahedral phengitic sheets bearing diverse cation distributions affect diffraction signals and the structural inferences therefrom derived. The interest for such minerals is dictated by that the stability of phengite polytypes, their cation distributions and P/T conditions of crystallization are related to each other. We focus our attention on layers’ probabilistic sequences that preserve the topology of the polytypes 2M ₁(SG: C2/c) and 3T(SG: P3₁12). Neutron diffraction intensities are modelled by a Monte Carlo approach and then used as artificial experimental data for conventional structure refinements that yield the occupancy factors in the fourfold (Si, Al) and sixfold (Al, Mg) coordination sites of 2M ₁ and 3T. The cation ordering from structure refinement tallies with the one of the “average structure” of a stochastic stacking, but it can significantly differ from those of the individual tetrahedral/octahedral sheets. For instance, sheets having ordered cation arrangements can lead to a stochastic structure which is supposed to bear a fully disordered cation partitioning according to structure refinement. This affects the configuration entropy contributions: the values obtained by conventional refinements can deviate from the correct ones up to 30 %. The analysis of the equivalent reflection intensities brings to light the anomalies hinting at the occurrence of such stacking disorder (using modelled reflections, the mean ratio between standard deviation and average intensity of symmetry equivalent reflections is ideally 0 for perfect crystal structures, but it can amount up to 6 in stochastically disordered phengites). However, taking into account the instrumental uncertainties and the deviations from ideality of actual crystals, such phenomena are very difficult to be detected experimentally.     

Calcium self-diffusion in diopside at high temperature: implications for transport properties

We have investigated ⁴⁴Ca self-diffusion in natural diopside single crystals (containing ∼2 atomic % Fe) at temperatures up to 1320 °C (i.e. 30 °C below the nominal melting point). Oxygen fugacity was controlled by gaseous mixtures. Diffusion profiles ranging from ∼50 to 500 nm were analysed by Rutherford Back-Scattering Spectrometry (RBS). The present results are complementary to previous studies, and show that in both synthetic (Fe-poor) and natural (Fe-rich) diopside, there are two different diffusion regimes for Ca with a transition at ∼1230±15 °C. Below this temperature diffusion is characterised by an activation enthalpy of ∼284±10 kJ/mol, while at higher temperatures it increases up to ∼1006±75 kJ/mol. These regimes are proposed to be respectively extrinsic and intrinsic. For the intrinsic regime Ca self-diffusion may involve Ca-Frenkel point defects. These are pairs of a vacancy on a M₂ site and a calcium cation on an interstitial (normally unoccupied) site. The concentration of such point defects depends only on temperature, and it is especially important at very high temperatures. The activation enthalpy for intrinsic diffusion may represent the half defect formation enthalpy plus the migration enthalpy for movement through interstitial sites. For the extrinsic regime we propose Ca self-diffusion to involve extrinsic interstitial point defects with concentration proportional to ()^{–0.19±0.03}. We suggest that for both regimes, Ca diffusion involves the well known M₃ sites in the octahedral layers, as well as sites in the tetrahedral layers, that we call M_4. These sites are especially convenient to explain the observed isotropic diffusion. Increasing concentration of Ca-Frenkel point defects may be related to the onset of premelting, which affects the thermodynamic properties of Fe-“free” diopside above 1250 °C. In the light of the present results, premelting is also expected to occur in natural Fe-bearing diopside and it could strongly influence its thermodynamic and transport properties. Subsequently, in deep upper mantle conditions (T≈1250 °C–1300 °C) where premelting could occur, diffusional cation exchanges with surrounding phases and diffusion controlled creep might be facilitated. Finally, our diffusion data support a previous suggestion that electrical conductivity may be electronic rather than ionic. Received: 17 December 1997 / Revised, accepted: 17 April 1998     

Octahedral cation ordering of illite and smectite. Theoretical exchange potential determination and Monte Carlo simulations

The distributions of Al ³⁺/Mg ²⁺ and Al ³⁺/ Fe ³⁺ were studied in the octahedral sheet of illites and smectites. Cation exchange interaction parameters J _i, as first, second, third and fourth neighbours were calculated by means of empirical interatomic potentials. Several compositions with different interlayer cations and tetrahedral charge were studied in both Al/Mg and Al/Fe systems. The values of J _i parameters were similar in all Al/Mg samples. From these J _i values, a strong trend to form AlMg pairs was observed in the Al/Mg system. In the Al/Fe system, the values of J _i are very small, indicating no preference for Al/Fe mixing. From these J _i parameters, Monte Carlo simulations of octahedral cation ordering were performed. In the Al/Mg system, an order/disorder phase transition was observed obtaining a fully ordered distribution without presence of an MgMg pair, according to experimental data. Similar phase transitions were observed for the octahedral compositions Al/Mg 1/1 and 3/1. In the Al/Fe system an order/disorder phase transition was also detected but at very low temperature for illite and smectite. Complete Al/Fe mixing is observed in the most stable ordered distribution. This is consistent with experimental results for synthetic Fe/Al smectites.     

Cation ordering in BaAl2Ge2O8-feldspar: implications for the phase transition in anorthite

BaAl₂Ge₂O₈-Feldspar undergoes an order-disorder phase transition I2/c↔C2/m at T _tr ≈1690 K. The thermodynamics of the Al,Ge cation ordering process is described in terms of the compressible Ising model in mean field approximation. The mean field potential predicts a first order character of the phase transition. This is compared to antiferromagnetic ordering in a two-dimensional square Ising model with NN-pair interactions and four-spin interactions on alternating squares. Calculated order parameters and short range ordering are in good agreement with the corresponding properties observed in BaAl₂Ge₂O₈-feldspar by means of X-ray diffraction, hard mode infrared spectroscopy and TEM. Using known calorimetric data a similar model is postulated for Al,Si ordering in anorthite, CaAl₂Si₂O₈, for which the derived potential describes a transition with slightly stronger first order character at T _tr ≈1928 K. Received: 30 January 1998 / Revised, accepted: 29 August 1998     

Computational study of tetrahedral Al–Si ordering in muscovite

 The nature of Al–Si ordering across the tetrahedral sites in muscovite, K₂Al₄(Si₆Al₂O₂₀)(OH)₄, was investigated using various computational techniques. Values of the atomic exchange interaction parameters J _l were obtained. From these parameters, a two-dimensional Al–Si ordering scheme was deduced. The transition temperature T _c for this two-dimensional ordering is 1900 K. There are several possible ordering schemes in three dimensions, based on different stacking sequences of ordered sheets of tetrahedral sites. Monte Carlo simulations of both two-dimensional and three-dimensional ordering were performed, but in the three-dimensional simulation only the two-dimensional ordering is seen, implying that three-dimensional ordering is too slow to be attained during the timescale of the simulation. The effect of the three-dimensional interactions is to raise the two-dimensional ordering temperature to 2140 K. From the three-dimensional Monte Carlo simulation, the frequency of occurrence of 4Si0Al, 3Si1Al, 2Si2Al and 1Si3Al clusters was determined, which match those inferred by ²⁹Si MAS–NMR measurements reasonably well. In fact, the match suggests that the cation ordering seen in experiments corresponds to a configuration with considerable short-range order but no long-range order, similar to a state that is at a temperature just above an ordering phase transition. Received: 28 August 2000 / Accepted: 12 March 2001     

Phase transition and mixing behaviour of the cummingtonite–grunerite solid solution

 Natural amphiboles with composition close to the binary join cummingtonite–grunerite and crystals of the same samples annealed at 700 °C for 55.5 h, in order to obtain different degrees of non-convergent cation order, have been characterised by means of X-ray single-crystal diffraction and IR spectroscopy. Long-range order parameters describing the non-convergent order of Mg/Fe among the different octahedral sites have been calculated from the site occupancies of the investigated samples. Values of the O6-O5-O6 angles and of the 〈M4-O〉 mean bond distances depend on the C2/m → P2₁/m phase transition for a given degree of order. In the IR spectra, only two phonon lines dominated by the bending of the tetrahedral chains are sensitive to the displacive phase transition and to the different degree of cation order; all the other wavenumber shifts are correlated with compositional changes only. The local strains arising from the cation substitution, ordering and phase transition have been quantified by means of the autocorrelation function. Very small local heterogeneities are associated with the Mg/Fe substitution and disordering in samples at intermediate composition. The displacive phase transition seems to occur in order to reduce local distortions and the P2₁/m samples are as homogeneous as orthorhombic anthophyllites. The orthorhombic structure, however, appears less flexible than the monoclinic in accommodating cations larger than Mg at the octahedral sites. Received: 9 February 2000 / Accepted: 30 September 2000     

Fe,Ti ordering and octahedral distortions in acentric neptunite: Temperature dependent X-ray and neutron structure refinements and Mössbauer spectroscopy

Single-crystal X-ray and neutron structure refinements carried out on neptunite (KNa₂Li(Fe, Mg, Mn)₂Ti₂Si₈O₂₄) from San Benito, California at various temperatures (neutrons: 15 K and 293 K; X-rays: 110 K, 293 K and 493 K) indicate that this mineral crystallizes in the acentric space group Cc (T=293K: a=16.427 Å, b=12.478 Å, c=9.975 Å, = 115.56°, Z=4, V=1844.53 ų) due to ordering of octahedrally coordinated metals (Ti, Fe, Mn, Mg). In the neptunite structure, Ti and (Fe, Mn, Mg) octahedra share edges to form chains that run along [110] and [110]. These chains are, in turn, linked through shared corners along [001]. The resulting octahedral framework is interwoven by a similar [Si₈O₂₂] tetrahedral framework. Li, Na and K occupy 6-, 8- and 10- coordinated sites within the framework. The metal-containing polyhedra show strong distortions at all temperatures. In particular, Ti exhibits a strong off-center displacement (0.25 Å) within its octahedron, leading to four Ti-O distances of 2.0 Å, one of 2.2 Å and one of 1.7 Å. The displaced Ti position is in good agreement with a position that minimizes differences between ionic bond strengths and is interpreted as an energy minimum in an ionic potential model. Mössbauer spectra collected at 77 K, 293 K and 400 K indicate all Fe to be present as octahedral Fe²⁺. Although two distinct Fe positions were found in the structure, 77 K and 293 K spectra display only one quadrupole doublet. Two Fe sites can only be resolved in the 400 K spectrum. It is suggested that the temperature dependence of octahedral edge distortions is responsible for the separation of the Mössbauer doublets.     

Thermobarometry of mafic igneous rocks based on clinopyroxene-liquid equilibria, 0–30 kbar

 Models for estimating the pressure and temperature of igneous rocks from co-existing clino-pyroxene and liquid compositions are calibrated from existing data and from new data obtained from experiments performed on several mafic bulk compositions (from 8–30 kbar and 1100–1475° C). The resulting geothermobarometers involve thermodynamic expressions that relate temperature and pressure to equilibrium constants. Specifically, the jadeite (Jd; NaAlSi₂O₆)–diopside/hedenbergite (DiHd; Ca(Mg, Fe) Si₂O₆) exchange equilibrium between clinopyroxene and liquid is temperature sensitive. When compositional corrections are made to the calibrated equilibrium constant the resulting geothermometer is (i) 10⁴ T=6.73−0.26^* ln [Jd^px*Ca^liq*Fm^liqDiHd^px*Na^liq*Al^liq] −0.86^* ln [Mg^liqMg^liq+Fe^liq]+0.52^*ln [Ca^liq] an expression which estimates temperature to ±27 K. Compared to (i), the equilibrium constant for jadeite formation is more sensitive to pressure resulting in a thermobarometer (ii) P=−54.3+299^* T10⁴+36.4^* T10⁴ ln [Jd^px[Si^liq]^2*Na^liq*Al^liq] +367^*[Na^liq*Al^liq] which estimates pressure to ± 1.4 kbar. Pressure is in kbar, T is in Kelvin. Quantities such as Na^liq represent the cation fraction of the given oxide (NaO_0.5) in the liquid and Fm=MgO+FeO. The mole fractions of Jd and diopside+hedenbergite (DiHd) components are calculated from a normative scheme which assigns the lesser of Na or octahedral Al to form Jd; any excess Al^VI forms Calcium Tschermak’s component (CaTs; CaAlAlSiO₆); Ca remaining after forming CaTs and CaTiAl₂O₆ is taken as DiHd. Experimental data not included in the regressions were used to test models (i) and (ii). Error on predictions of T using model (i) is ±40 K. A pressure-dependent form of (i) reduces this error to ±30 K. Using model (ii) to predict pressures, the error on mean values of 10 isobaric data sets (0–25 kbar, 118 data) is ±0.3 kbar. Calculating thermodynamic properties from regression coefficients in (ii) gives V^Jd _f of 23.4 ±1.3 cm³/mol, close to the value anticipated from bar molar volume data (23.5 cm³/mol). Applied to clinopyroxene phenocrysts from Mauna Kea, Hawaii lavas, the expressions estimate equilibration depths as great as 40 km. This result indicates that transport was sufficiently rapid that at least some phenocrysts had insufficient time to re-equilibrate at lower pressures. Received: 16 May 1994/Accepted: 15 June 1995     

Temperature dependence of the cation distribution in nickel aluminate (NiAl2O4) spinel: a powder XRD study

NiAl₂O₄ is a largely inverse spinel, which in detail shows increasing randomisation with temperature of Ni and Al between the octahedral and tetrahedral cation sites of the spinel structure. We have used powder XRD to determine this cation distribution in various samples of NiAl₂O₄ quenched after annealing between 700 and 1400° C. The inversion parameter (x) can be measured with a precision of ± 0.004 (one standard deviation), and a comparison of different methods of synthesis, X-ray diffraction and refinement techniques, suggests a probable accuracy of better than 0.01. The results are supported by some preliminary single crystal refinements on flux-grown samples.Below 800° C the rate of cation ordering becomes very slow, and, despite reaching an apparently steady state, it is doubtful if our samples attained complete internal equilibrium. Above 1250° C the cation redistribution becomes so fast that the quenching method becomes unreliable. Between 800 and 1250° C inclusive, the degree of inversion changes smoothly from 0.87 at 800° C to 0.79 at 1250° C, and is accompanied by linear changes in u, the oxygen parameter, from 0.2555 to 0.2563 (±0.0002), and a₀, the lattice parameter, from 8.0462 to 8.0522 Å (±0.0002 Å).     

An empirical model for the calculation of spinel-melt equilibria in mafic igneous systems at atmospheric pressure: 1. Chromian spinels

 In order to develop a model for simulating naturally occurring chromian spinel compositions, we have processed published experimental data on chromian spinel-melt equilibrium. Out of 259 co-existing spinel-melt experiments reported in the literature, we have selected 118 compositions on the basis of run time, melt composition and experimental technique. These data cover a range of temperatures 1150–1500° C, oxygen fugacities of −13<log f _O2< −0.7, and bulk compositions ranging from basalt and norite, to komatiite. Six major spinel components with Cr³⁺, Al³⁺, Ti⁴⁺, Mg²⁺, Fe³⁺ and Fe²⁺-bearing end-members were considered for the purpose of describing chromite saturation as a function of melt composition, temperature and oxygen fugacity at 1 atmosphere pressure (0.101 MPa). The empirically calibrated mineral-melt expression based on multiple linear regressions is: K ^Sp _i =A/T(K)+B log f _O2+C ln (Fe³⁺/Fe²⁺)_L+D ln R _L +E, where K ^Sp _i is an equilibrium constant and R _L is a melt structure-chemical parameter (MSCP). Twenty-eight forms of equilibrium constants were considered, including single distribution coefficients, exchange equilibrium constants, formation constants for AB₂O₄ components, as well as simple “spinel cation ratios”. For each form of the equilibrium constants, a set of 16 combinations of the MSCPs have been investigated. The MSCP is present in the form of composite ratios [e.g., Si/O, NBO/T,(Al+Si)/Si, or (Na+K)/Al] or as simple cation ratios (e.g., Mg/Fe²⁺). For the calculation of Fe³⁺ and Fe²⁺ species in silicate melts, we used existing equations, whereas the Fe³⁺/Fe²⁺ ratio of spinels was calculated from the spinel stoichiometry. The regression parameters that best repoduce the experimental data were for the following constants: (Fe³⁺/Fe²⁺) _Sp , (Mg/Fe²⁺) _Sp /(Mg/Fe²⁺) _L , (Cr/Al) _Sp / (Cr/Al) _L , K _FeCr2O4, and Ti _Sp /Ti _L . These expressions have been combined into a single program called SPINMELT, which calculates chromite crystallization temperature and composition at a given f _O2 with an average accuracy of ∼10° C and 1–2 mol%. An example of the use of SPINMELT is presented for a magma parental to the Bushveld Complex. Received: 30 May 1995/Accepted: 1 November 1995     

Evolution of parental magmas of Miocene shield basalts of Gran Canaria (Canary Islands): constraints from crystal,melt and fluid inclusions in minerals

 Picritic units of the Miocene shield volcanics on Gran Canaria, Canary Islands, contain olivine and clinopyroxene phenocrysts with abundant primary melt, crystal and fluid inclusions. Composition and crystallization conditions of primary magmas in equilibrium with olivine Fo_90-92 were inferred from high-temperature microthermometric quench experiments, low-temperature microthermometry of fluid inclusions and simulation of the reverse path of olivine fractional crystallization based on major element composition of melt inclusions. Primary magmas parental for the Miocene shield basalts range from transitional to alkaline picrites (14.7–19.3 wt% MgO, 43.2–45.7 wt% SiO₂). Crystallization of these primary magmas is believed to have occurred over the temperature range 1490–1150° C at pressures ≈5 kbar producing olivine of Fo_80.6-90.2, high-Ti chrome spinel [Mg/ (Mg+Fe²⁺)=0.32–0.56, Cr/(Cr+Al)=0.50–0.78, 2.52–8.58 wt% TiO₂], and clinopyroxene [Mg/(Mg+Fe)=0.79–0.88, Wo_44.1-45.3, En_43.9-48.0, Fs_6.8-11.0] which appeared on the liquidus together with olivine≈Fo₈₆. Redox conditions evolved from intermediate between the QFM and WM buffers to late-stage conditions of NNO+1 to NNO+2. The primary magmas crystallized in the presence of an essentially pure CO₂ fluid. The primary magmas originated at pressures >30 kbar and temperatures of 1500–1600° C, assuming equilibrium with mantle peridotite. This implies melting of the mantle source at a depth of ≈100 km within the garnet stability field followed by migration of melts into magma reservoirs located at the boundary between the upper mantle and lower crust. The temperatures and pressures of primary magma generation suggest that the Canarian plume originated in the lower mantle at depth ≈900 km that supports the plume concept of origin of the Canary Islands. Received: 23 October 1995/Accepted: 21 February 1996     

Low-temperature single-crystal Raman spectrum of pyrope

 The single-crystal polarized Raman spectra of synthetic pyrope, Mg₃Al₂Si₃O₁₂, were measured at room temperature and 5 K, as were the room-temperature unpolarized spectra of two natural pyrope-rich crystals. No major differences in the spectra between room temperature and 5 K are observed or are present between the synthetic and the natural crystals. The spectra are consistent with the proposal that the Mg cation is dynamically disordered and not statically distributed over subsites in the large triangular-dodecahedral E-site in pyrope. A low-energy band at about 135 cm⁻¹ softens and shows a large decrease in its line width with decreasing temperature. The presence of a weak, broad band at about 280 cm⁻¹ may be due to anharmonic effects, as could the one at 135 cm⁻¹. The latter is assigned to the rattling motion of Mg in pyrope in the plane of the longer Mg-O(4) bonds (Kolesov and Geiger 1998). The successful modeling of the anisotropic motion of the Mg cation in pyrope, which has an anharmonic character, provides a valuable test of the validity of empirical or semi-empirical lattice-dynamic calculations for silicates. Received: 10 May 1999 / Accepted: 10 April 2000     

Manganese olivine II: point defect relaxation

To investigate the point defect chemistry and the kinetic properties of manganese olivine Mn₂SiO₄, the point defect relaxation time (τ) characterizing the rate of re-equilibration of electrical conductivity following a change in oxygen fugacity was measured for single crystals oriented for electrical conduction along the [010] direction. The experiments were carried out at temperatures T = 1173–1473 K and oxygen fugacities with the MnSiO₃ activity controlled at unity. The value of τ, which ranges from 130 to 1463 s, increases with decreasing temperature. At 1273 K, the value of τ in the regime is a factor of ∼1.8 smaller than that in the regime. The point defect relaxation time was used to calculate the chemical diffusivity (). Values of lie in the range 2.2× 10^{− 10} − 2.5×10^{− 9} m²/s. For the regime, a semi-log plot of vs 1/T yields a concave downward curve. Based on these results combined with those from Part I of this work for the point defect structure and electrical conductivity in Mn-olivine, it is proposed that the relaxation rate of electrical conduction is determined by the coupled diffusion of manganese vacancies and electron holes which rate-control the relaxation process at low and high temperatures, respectively. Deconvolution of the curve into two straight-line segments yields values for the mobilities and the diffusivities for Mn vacancies and electron holes. These results, combined with the measured electrical conductivity data, were used to calculate the concentrations of Mn vacancies and electron holes. These results in conjunction with those published for other transition-metal silicate olivines reveal the following: The mobility of electron holes in Mn-olivine is about two orders of magnitude smaller than that in Fe-olivine and is somewhat larger than that in (Mg_0.9Fe_0.1)-olivine. The mobilities of metal vacancies in these three olivines are similar both in magnitude and in temperature dependence. The concentrations of the majority point defects are the highest in Fe-olivine crystals and the lowest in Co-olivine crystals, while those in Mn-olivine and (Mg_0.9Fe_0.1)-olivine crystals lie in between. The deviation from stoichiometry in the cation sublattice for a transition-metal silicate olivine is about two orders of magnitude lower than that for the corresponding transition-metal oxide. Received: 10 January 1996 / Revised, accepted: 29 May 1997     

H T -XRD study of synthetic ferrian magnesian spodumene: the effect of site dimension on the P 21/ c → C 2/ c phase transition

 Ferrian magnesian spodumene was synthesized in the MLFSH system at P=0.4 GPa, T=700 °C, fO₂=NNO+2.3. The space group at room T is P2₁/c [a=9.638(3) ?, b=8.709(2) ?, c=5.258(2) ?, β=109.83(3)^∘, V=415.2 ?³]. The structure is topologically equivalent to that of ferrian spodumene, LiFeSi₂O₆, and has two symmetrically independent tetrahedral chains, A and B, and two independent octahedral sites, M1 and M2. The crystal-chemical composition was determined combining EMP, SIMS and single-crystal XRD analysis, yielding ^M2(Li_0.85Mg_0.09Fe²⁺ _0.06) ^M1(Fe³⁺ _0.85Mg_0.15)Si₂O₆. Li is ordered at the M2 site and Fe³⁺ is ordered at the M1 site, whereas Mg (and Fe²⁺) distribute over both octahedral sites. Structure refinements done at different temperatures (25, 70, 95, 125, 150 and 200 °C) allowed characterization of a reversible displacive P2₁/c→C2/c transition at 106 °C. Previous HT-XRD studies of Li-clinopyroxenes had shown that the transition temperature is inversely related to the size of the M1 cation. For the crystal of this work, the aggregate ionic radius at M1 is longer than that of ferrian spodumene, for which the transition temperature is −44 °C. The higher transition temperature observed can only be explained on the basis of the shorter aggregate radius at the M2 site (due to the presence of Mg substituting after Li), in keeping with the results obtained for ferromagnesian P2₁/c pyroxenes. The effects of all the chemical substitutions must be considered when modelling transition temperatures and thermodynamic behaviour in clinopyroxenes. Received: 7 May 2002 / Accepted: 23 October 2002     

Pressure dependence on partition coefficients for trace elements between olivine and the coexisting melts

Partition coefficients between olivine and melt at upper mantle conditions, 3 to 14 GPa, have been determined for 27 trace elements (Li, Be, B, Na, Mg, Al, Si, P, K, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Rb, Sr, Y, Zr, Cs, Ba, La and Ce) using secondary-ion mass-spectrometry (SIMS) and electron-probe microanalysis (EPMA). The general pattern of olivine/melt partitioning on Onuma diagrams resembles those reported previously for natural systems. This agreement strongly supports the argument that partitioning is under structural control of olivine even at high pressure. The partition coefficients for mono- and tri-valent cations show significant pressure dependence, both becoming larger with pressure, and are strongly correlated with coupled substitution into cation sites in the olivine structure. The dominant type of trace element substitution for mono- and tri-valent cations into olivine changes gradually from (Si, Mg)↔(Al, Cr) at low pressure to (Si, Mg)↔(Al, Al) and (Mg, Mg)↔(Na, Al) at high pressure. The change in substitution type results in an increase in partition coefficients of Al and Na with pressure. An inverse correlation between the partition coefficients for divalent cations and pressure has been observed, especially for Ni, Co and Fe. The order of decreasing rate of partition coefficient with pressure correlates to strength of crystal field effect of the cation. The pressure dependence of olivine/melt partitioning can be attributed to the compression of cation polyhedra induced by pressure and the compensation of electrostatic valence by cation substitution. Received: March 6, 1997 / Revised, accepted: March 12, 1998     

Structure of magnetite (Fe<Subscript>3</Subscript>O<Subscript>4</Subscript>) above the Curie temperature: a cation ordering study

A pristine magnetite (Fe₃O₄) specimen was studied by means of Neutron Powder Diffraction in the 273–1,073 K temperature range, in order to characterize its structural and magnetic behavior at high temperatures. An accurate analysis of the collected data allowed the understanding of the behavior of the main structural and magnetic features of magnetite as a function of temperature. The magnetic moments of both tetrahedral and octahedral sites were extracted by means of magnetic diffraction up to the Curie temperature (between 773 and 873 K). A change in the thermal expansion coefficient around the Curie temperature together with an increase in the oxygen coordinate value above 700 K can be observed, both features being the result of a change in the thermal expansion of the tetrahedral site. This anomaly is not related to the magnetic transition but can be explained with an intervened cation reordering, as magnetite gradually transforms from a disordered configuration into a partially ordered one. Based on a simple model which takes into account the cation-oxygen bond length, the degree of order as a function of temperature and consequently the enthalpy and entropy of the reordering process were determined. The refined values are ΔH⁰ = −23.2(1.7) kJ mol⁻¹ and ΔS⁰ = −16(2) J K⁻¹ mol⁻¹. These results are in perfect agreement with values reported in literature (Mack et al. in Solid State Ion 135(1–4):625–630, 2000; Wu and Mason in J Am Ceramic Soc 64(9):520–522, 1981).     

The low-pressure stability of phase A,Mg7Si2O8(OH)6

 Phase A, Mg₇Si₂O₈(OH)₆, is a dense hydrous magnesium silicate whose importance as a host of H₂O in the Earth’s mantle is a subject of debate. We have investigated the low-pressure stability of phase A in experiments on the reaction phase A=brucite+forsterite. Experiments were conducted in piston-cylinder and multi-anvil apparatus, using mixtures of synthetic phase A, brucite and forsterite. The reaction was bracketed between 2.60 and 2.75 GPa at 500° C, between 3.25 and 3.48 GPa at 600° C and between 3.75 and 3.95 GPa at 650^° C. These pressures are much lower than observed in the synthesis experiments of Yamamoto and Akimoto (1977). At 750^° C the stability field of brucite + chondrodite was entered. The enthalpy of formation and entropy of phase A at 1 bar (10⁵ Pa), 298 K, were derived from the experimental brackets on the reaction phase A=brucite+forsterite using a modified version of the thermodynamic dataset THERMOCALC of Holland and Powell (1990), which includes a new equation of state of H₂O derived from the molecular dynamics simulations of Brodholt and Wood (1993). The data for phase A are: ΔH ^o _f =−7126±8 kJ mol^-1, S ^o=351 J K^-1 mol^-1. Incorporating these data into THERMOCALC allows the positions of other reactions involving phase A to be calculated, for example the reaction phase A + enstatite=forsterite+vapour, which limits the stability of phase A in equilibrium with enstatite. The calculated position of this reaction (753° C at 7 GPa to 937° C at 10 GPa) is in excellent agreement with the experimental brackets of Luth (1995) between 7 and 10 GPa, supporting the choice of equation of state of H₂O used in THERMOCALC. Comparison of our results with calculated P-T paths of subducting slabs (Peacock et al. 1994) suggests that, in the system MgO–SiO₂–H₂O, phase A could crystallise in compositions with Mg/Si>2 at pressures as low as 3 GPa. In less Mg rich compositions phase A could crystallise at pressures above approximately 6 GPa. Received: 3 July 1995/Accepted: 14 December 1995