Cation distribution in some natural spinels from X-ray diffraction and Mössbauer spectroscopy

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.

The influence of the iron content on the reductive decomposition of A3−xFexAl2Si3O12 garnets (A = Mg, Mn; 0.47 ≤ x ≤ 2.85)

“…This result is close to the theoretical occupancy for iron in olivine structure [31]. The hyperfine parameters of two hercynite doublets (T1 and T2 tetrahedral sites) agree with those published for natural Fe-bearing spinel [32]. In the XRD patterns [20], a slight shift of hercynite diffractions of Prp 4 Alm 96 heated up to 1200 °C with respect to Prp 4 Alm 96 heated up to 1070 °C could be explained in terms of incorporation of extra iron into the hercynite or some grade of inversion of its structure, as the studied dependence of interplanar distance on iron content according to the Vegard's law in the hercynite-spinel series [33].…”

Following the Amphibolite to Greenschist Metamorphic Path through the Structural Parameters of Spinels from Amsaga (Mauritania)

“…Several different procedures may be adopted to determine cation distribution [47,48], and satisfying results can generally be obtained by combining data from single-crystal X-ray structural refinements and electron microprobe analyses. This approach simultaneously takes into account both the structural and chemical data and reproduces the observed parameters by optimizing cation distributions.…”

Si-Disordering in MgAl2O4-Spinel under High P-T Conditions, with Implications for Si-Mg Disorder in Mg2SiO4-Ringwoodite

“…By the same token, Si in the MgAl2O4-Sp should occupy the M-site. However, existing single-crystal XRD studies on natural 2-3 Sp locate Si on the T-site [73,[75][76][77]. The coupled substitution of Si and Mg for 2Al as observed in our high-P synthetic MgAl2O4-Sp and the site-occupation knowledge to be revealed by our Raman spectroscopic data should shed light on the Si distribution.…”

“…From the single-crystal XRD data we directly obtained an x value of 0.129, probably with relatively large uncertainty due to the similar scattering factors of Mg and Al. Following the method of Carbonin et al [73], with the bond distances from Lavina et al [74] and with x = 0.129 as one of the input variables; further, we calculated a new x value, which was in turn used as an input in the next round of crystal structural analysis. The final cycles of the least-squares refinement, including atomic coordinates and anisotropic thermal parameters for the atoms [I > 2sigma(I)], converged at R 1 = 0.0164, wR 2 = 0.0730 and S = 1.065, and yielded x = 0.162 (see Supplementary Materials for the details).…”

“…Other than the weakest T2g(3) peak, all of the other four peaks were routinely observed on natural MgAl2O4-Sp with very low and insignificant amounts of impurities like SiO2, TiO2, Cr2O3, FeO and/or ZnO [49,57,78,86,87]. [73,[75][76][77]. The coupled substitution of Si and Mg for 2Al as observed in our high-P synthetic MgAl 2 O 4 -Sp and the site-occupation knowledge to be revealed by our Raman spectroscopic data should shed light on the Si distribution.…”