Thermodynamics and kinetics of cation ordering in natural and synthetic Mg(Al,Fe3+)2O4 spinels from in situ high-temperature X-ray diffraction

Abstract

Spinel-group minerals are characterized by a non-convergent, temperature-dependent cation order/disorder, i.e., no change of symmetry is associated with cation disordering. Cation distribution can be described by the IV (A 1-i B i ) VI (B 2-i A i )O 4 structural formula, where IV and VI represent the tetrahedrally and octahedrally coordinated T and M sites, respectively, A and B are cations with different valences, and i represents the inversion parameter. At low temperature, i tends to 0 in normal spinels [e.g., spinel s.s. IV (Mg) VI (Al) 2 O 4 ] and to 1 in inverse spinels [e.g., magnesioferrite IV (Fe 3+ ) VI (MgFe 3+ )O 4 ], which are both ordered. At high temperature, cation disorder increases and i values have been shown to converge asymptotically toward 2/3 for all compositions (Harrison et al. 1998;O'Neill et al. 1992;O'Neill and Dollase 1994;Redfern et al. 1999;Andreozzi et al. 2000;Carbonin et al. 2002;Martignago et al. 2003).The infl uence of non-Al cations on the Mg-Al order-disorder reaction has been recently studied by Martignago et al. (2003) using a natural, highly ordered, Fe 3+ -bearing spinel. Their results indicate that the Fe 3+ cation triggers trivalent-divalent cation exchange at T ≥ 550 °C. This agrees with the results obtained by Andreozzi et al. (2001) for the synthetic spinel-magnesioferrite series, which showed that Fe 3+ partitions between the M and T sites as a function of both magnesioferrite component and temperature. Moreover, Martignago et al. (2003) observed that at the lowest temperatures order-disorder is controlled by a two-stage kinetic process, with a rapid intersite exchange of Fe 3+ with Mg followed by a slow exchange of Mg with Al.Andreozzi and Princivalle (2002) quantitatively described the kinetics of Mg-Al exchange in MgAl 2 O 4 spinel using the Mueller (1967, 1969) model. Following O'Neill (1994, these authors described the order-disorder process with the chemical exchange reaction IV Al + VI Mg = IV Mg + VI Al, the forward reaction accounting for the exchange of Al at the T site with Mg at the M site (ordering), and the backward reaction for the exchange of Mg at the T site with Al at the M site (disordering). These homogeneous reactions were assumed to be second-order chemical reactions, and fi rst-order with respect to the concentration at one site (Ganguly 1982). The time rate of change of concentration of Al at the T site (= i), was expressed as -di/dt = K[i 2 -K D (1-i)(2-i)], where K D , the equilibrium constant of the exchange reaction, is a function of temperature and pressure alone. The variation of i with time was revealed to only depend on the initial and fi nal inversion degrees, and on the rate constant K. The kinetics of Mg-Fe 3+ exchange in MgFe 2 O 4 spinel was * ABSTRACT One natural and two synthetic spinels with compositions Mg(Al 1-z Fe z 3+ ) 2 O 4 (with 2z = 0.078, 0.138, and 0.200, respectively) were studied by in situ, high-temperature, single-crystal X-ray diffraction. All samples were heated from room temperature to 1050 °C. Thermal expansi...

Geothermometric study of Mg-rich spinels from the Somma-Vesuvius volcanic complex (Naples, Italy)

“…The plot of the unit-cell parameter vs. the Al content for both suites (Fig. 2) lies within the field defined by the regression lines belonging to the synthetic spinel sensu stricto-hercinite (Andreozzi and Lucchesi 2002) and spinel sensu stricto-magnesioferrite (Martignago et al 2006) joins. Figure 2 shows that the data from the two vol- Lucchesi et al 1998) spinels.…”

“…Figure 2 shows that the data from the two vol- Lucchesi et al 1998) spinels. Dotted line represents the data regression (R 2 = 0.945) of synthetic spinel sensu stricto-hercynite join (Andreozzi and Lucchesi 2002); dashed line represents the data regression (R 2 = 0.999) of synthetic spinel sensu stricto-magnesioferrite join (Martignago et al 2006 canic suites partially overlap, with the Somma-Vesuvius spinels higher in Al content, and brings out the crystal-chemical linking between the two sets of samples. With respect to the Alban Hills, the Somma-Vesuvius spinels are lower in Fe 3+ content, whereas the amounts of Fe 2+ are comparable.…”

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

“…Throughout the temperature range investigated, no violations of space group symmetry were detected. Refinements were performed isotropically and ionized form factors were used for T and M cations, whereas neutral form factors were used for oxygen following refinement strategies already used in previous studies (Della Giusta et al, 1996;Salviulo et al, 2000;Martignago et al, 2006). Crystal-structure details are reported in Table 1.…”

Geothermometric study of Cr-spinels of peridotite mantle xenoliths from northern Victoria Land (Antarctica)

“…Experimental studies on spinels have demonstrated that the kinetics of intracrystalline cation order-disorder is influenced by Fe 3+ and Cr contents as well as by non-stoichiometry (O'Neill 1994(O'Neill , 1997Harrison and Putnis 1999;Martignago et al 2006;Princivalle et al 2006Princivalle et al , 2012. In particular, the presence of Fe 3+ fosters the intracrystalline cation ordering process, because it was established that, in Fe 3+ -rich aluminate spinels, the Mg-Fe 3+ site exchange proceeds at higher speed and to lower temperature than the Mg-Al exchange ).…”