The equilibrium distribution of Mg and Al between the tetrahedral and octahedral sites of a flux grown, stoichiometric MgAl 2 O 4 spinel was investigated between 600 and 1100 °C by single-crystal X-ray diffraction of quenched samples. The cation distribution for both ordering and disordering runs was obtained by minimizing accurate crystallographic parameters and effective ionic radii. Along with the variation of the degree of inversion from 0.18 to 0.29 between 600 and 1100 °C, both unit cell and oxygen positional parameter decreased linearly. Multiple non-linear least-squares fit of our data with the thermodynamic model of O'Neill and Navrotsky (1983) gave α = 23 ± 2 KJ/mol and β = 13 ± 4 KJ/mol. The influence of both cation inversion and thermal expansion on T-O and M-O bond length variation was determined by comparison of our data with previous in situ studies. In the thermal interval investigated, the inversion accounts for change of -0.014 Å for T-O and +0.007 Å for M-O. Mean linear polyhedral thermal expansion coefficients of 6.5 × 10 -6 °C -1 and 8.9 × 10 -6 °C -1 were calculated for T and M sites, respectively. *

No abstract

No abstract

Three natural spinels of different places of occurrence and compositions were investigated by means of microprobe chemical analysis, single crystal X-ray diffraction and MOssbauer spectroscopy. All cation distributions between T and M sites were calculated from microprobe and XRD experimental data, by means of a mathematical model with appropriate assumptions. Fe 2 § and Fe 3 § assignments calculated in this way were compared with those observed in MOssbauer spectra. The satisfactory agreement found proves, at least in the samples studied, the reliability of the model and the assumptions used. In the spinels examined, such results show Fe 2 § and Fe 3 § virtually ordered in T and M sites respectively. M6ssbauer data also revealed Fe 2 § in different tetrahedral sites due to the next-nearest neighbour effect, probably as a consequence of spinel genetic conditions.

A natural Mg-A1-Fe spinel from the Balmuccia peridotite (Italian Western Alps) was annealed at T between 650 and 1150~ under controlled oxygen activity, and quenched in H20. Twenty-three cation distributions were calculated from XRD structural refinements in tandem with microprobe analysis, and verified by Mrssbauer spectroscopy in the case of unheated samples.Unheated crystals showed essentially ordered distribution of Fe 3+ in octahedral and Fe 2+ in tetrahedral sites, the only intracrystalline disorder being represented by ~0.12 atoms per formula unit of [41AI and 161Mg. Thermal runs and quenching maintained substantially ordered distribution of Fe 2 § and Fe 3+ up to ~ 990~ and produced continuous [41Mg-[6IAI exchange. Between 990 and 1150~ the previous order of Fe2 § 3+ appeared to change slightly, t61Fe 2+ reaching ~0.04 afu and [6]Mg ---I41AI ~ 0.24 afu at the highest T. After quenching from this temperature, Fe 2+ still resided mainly in the T site. Some previously heated crystals underwent reordering on lowering of the temperature.Experimental data, integrated with existing literature, enabled cation-oxygen distance in this structure to be improved. Results from annealed samples allowed the formulation of an experimental thermometric function based on Mg-AI intracrystalline disorder.