F. Martignago

Kinetics of cation ordering in natural Mg(Al,Cr3+)2O4 spinels

Abstract The kinetics of cation ordering (quench method) in two natural Mg(Al2-yCry)O4 spinels (y~ 0.03-0.06 and 0.24), highly ordered in terms of Mg-Al, were studied by means of X-ray single-crystal diffraction. The equilibrium distribution of Mg and Al between tetrahedral and octahedral sites was investigated at 650 °C (in disordering and ordering) and at 850 °C (in ordering), through several time-steps to monitor the rate of cation distribution before equilibrium was achieved. The cation distributions for both disordering and ordering experiments were obtained by measuring the oxygen positional parameter u, which is correlated to the inversion parameter x (Al in T site), and then to the composition of the samples. The Mueller kinetic model, satisfactorily applied to the experimental data, allowed the calculation of the kinetic ordering constants K, linearly related to temperature by means of Arrhenius equations. The kinetics of ordering processes are influenced by Cr content. The equilibrium for both the isotherms at 650 and 850 °C was reached at different elapsed times by the low- and high-Cr spinels: the time for the low-Cr sample was, in both the ordering experiments, about double that of the high- Cr sample. Consequently, the activation energy (186 and 175 kJ/mol for low- and high-Cr samples, respectively) for the intracrystalline Mg-Al ordering decreases with Cr increase.

Crystal chemistry of Cr3+-V3+-rich clinopyroxenes

Abstract Eleven clinopyroxenes from the Sludyanka Crystalline Complex in Russia belonging to the ternary join NaVSi2O6-NaCrSi2O6-CaMgSi2O6 (natalyite-kosmochlor-diopside) were studied by means of X-ray single crystal diffractometry and electron probe microanalysis. The crystal chemical data show that the T site is almost completely occupied by Si, so that the Na (V3+,Cr3+) → Ca Mg substitution mechanism ensures charge balance. Changes in M1 site geometry are explained by the aggregate ionic radius, and are influenced by Mg occupancy and V3+/(V3+ + Cr3+) ratio. The M2 site geometry depends both on Na content and on the (V3+,Cr3+) → Mg substitution in M1 site. Changes in M2-O3c1 bond length are mainly related to Na content, whereas the longest M2-O3c2 bond lengths are significantly affected by the V3+/(V3+ + Cr3+) ratio of the M1 site. The T site geometry is affected by chemical and geometrical variations at the M1 and M2 sites, principally the M1 site occupancy.

A new radiative microfurnace for X-ray single-crystal diffractometry

Microfurnaces available today for X-ray single-crystal investigation (radiative, gas-flame, gas-flow) are generally temperature-monitored by a thermocouple placed in a different site with respect to the crystal. The new F1 microfurnace aims at the following goals: fast crystal mounting; access to a large portion of reciprocal lattice; low absorption of direct beam; efficient in situ temperature measurement; fast power supply regulation; high temperature stability; controlled atmosphere. In the F1, the crystal is directly glued to a thermocouple with refractory cement. The heating body, fixed on the χ and radially shiftable along the ϕ axis circle, hosts the Pt winding and connections of both gas flow and power supply, and is enclosed in a thin pyrolitic boron nitride (PBN) shield. Temperature stability is within ± 1°C. Calibration of the F1 was carried out by collecting X-ray data from a single-crystal synthetic periclase at tem- peratures of 28, 150, 300, 450, 600, 700, 900 and 1000°C, and yielded the equation: a = 0.0000625 (7) (T(K) - 273) + 4.2083 (4), based on the ratio between cell edge and temperature. Structural refinements gave structural parame- ters and thermal expansion values at the investigated temperature. The mean coefficient of linear expansion up to 1000°C was 14.3 × 10-6 °C.

Kinetics of cation ordering in synthetic Mg(Al, Fe3+)2O4 spinels

The kinetics of cation ordering (quench method) for two synthetic Mg(Al 2- y Fe y 3+ )O 4 spinels ( y ~ 0.39 and 0.54, samples F39 and F54, respectively) were studied by means of X-ray single-crystal diffraction. The equilibrium distribution of Mg–Al–Fe 3+ between tetrahedral and octahedral sites was investigated at 750, 650 and 550 °C (in ordering run) starting from an equilibrium ordering state corresponding to 1000 °C, through several time-steps in order to monitor the rate of change of cation distribution. The cation distribution between T and M sites was calculated from the bond lengths using the Minuit program. The Mueller kinetic model, applied to the Mg–Al cation exchange, allowed the calculation of the kinetic ordering constants K , linearly related to temperature by the Arrhenius equations. The Fe 3+ content influences the kinetics of the ordering process. The equilibrium for the three isotherms at 750, 650 and 550 °C was reached at different elapsed times for the F39 and F54 spinels. In particular, the time for the F39 sample was, in all the ordering runs, higher than that of the F54 sample. Consequently, the activation energy (274 and 265 kJ/mol for low- and high-Fe 3+ samples, respectively) for the intra-crystalline Mg–Al ordering slightly decreases with increasing Fe 3+ content. The intracrystalline closure temperature ( T c ) seems to be only marginally affected by the different Fe 3+ content. As magnesiochromite spinels are among the most abundant phases in diamonds such results could have implications for diamond thermometry because magnesiochromite inclusions can contain as much as 45 % total iron as Fe 3+ .

The effect of non-stoichiometry on the high-temperature behaviour of MgAl2O4 spinel

Abstract The effect of cation vacancies upon the thermal expansion and crystal structure of a synthetic defect spinel with composition Mg0.4Al2.4⃞0.2O4 was investigated by X-ray diffraction, in situ, at temperatures up to 1273 K. No evidence of symmetry violations from the Fd3̄m even at the highest temperature were noted. The volume thermal expansion is markedly less than that of stoichiometric MgAl2O4 spinel, regardless of the degree of inversion. The u oxygen atomic coordinate remains constant throughout the temperature range investigated, with the M-O and T-O bond lengths showing identical rates of expansivities. An analysis of the evolution of polyhedral volumes with temperature indicates that at 1273 K the octahedron inflates by 0.099 Å3 and the tetrahedron by 0.056 Å3. The expansion of the octahedron is significantly greater than in stoichiometric MgAl2O4 spinel, whereas the expansion of the tetrahedron is similar. The results demonstrate that an excess of Al in the spinel structure accompanied by the formation of cation vacancies strongly affect an important thermodynamic property, in this case, thermal expansion. Such an effect must be considered for those phases stable in the Earth’s mantle where 4-5 wt.% Al2O3 is thought to be present.

Microchemical and crystallographic analysis of human bones from Nasca, Peru. A possible method of direct dating of archaeological skeletal material

This paper provides two archaeometric equations obtained by crystal–chemical parameters for dating human bone samples from the archaeological site of Nasca, Peru. Based on radiocarbon dating, the burials span from 500 BC to AD 1000. Crystal–chemical modifications in bone hydroxylapatite over time were compared with a twentieth century skeleton from Italy. Chemical analysis was carried out by means of electron microprobe along a profile from the periosteal to the endosteal border of the midshaft of four human femurs. The results indicate that the Ca/P ratio is linearly correlated with time according to the equation t=7880.68Ca/P−12805.90 (r=0.97; r2=0.95). Bone apatite X-ray powder diffraction analysis showed a systematic increase in the (002) reflection intensity with time, according to the equation t=250.49h/a−1961.86 (r=0.98; r2=0.97). The two independent archaeometric equations permit good accuracy in dating osteoarchaeological remains from the south coast area of Peru.