Comparative crystal chemistry of spinels from some suites of ultramafic rocks

Abstract

Microprobe analyses and X-ray crystal structure refinement data of Mg-Fe-Al-Cr spinels from different environments are compared. The investigated crystals represent both restitic and recrystallized spinels from an Alpine peridotitic massif, and restitic spinels from four suites of ultramafic xenoliths. Within each suite the crystals represent different steps of an increasing melting process, which causes a strong increase in Cr3+ and a moderate increase in Fe2+, with depletion of Al3+ and Mg2+. Within each suite, in spite of relevant bulk chemistry changes, the ratio of the octahedral to tetrahedral coordination distances, and consequently the oxygen positional parameterx, are kept constant. Conversely,x may differ in suites with similar bulk chemistry, mainly due to different Mg-Al ordering between octahedral and tetrahedral sites. This suggests that thex parameter is strongly affected by physical environment, and that consequently, within the range of the investigated compositions, it could be used as a marker

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

“…The present study indicates that at P-T conditions ≤~3-4 GPa and 1500 • C, covering the P-T range of the top upper mantle of the Earth [98], the even smaller Si cations incorporated by the MgAl 2 O 4 -Sp structure appear on the T-site, rather than on the anticipated M-site (y = 1; Figure 8). This result is compatible with existing single-crystal XRD studies on terrestrial Sp, which locate Si on the T-site [73,[75][76][77]99]. The current study further shows that presenting as SiO 4 groups in the Sp, a small amount of SiO 2 like~1 wt % exhibits very intense Raman peaks at~823 and 856 cm −1 , and can completely alter the stereotypical overall appearance of the Raman spectra established with some SiO 2 -poor natural 2-3 Sp.…”

“…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.…”

“…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.…”

Temperature-dependent disorder in a natural Mg-Al-Fe2+-Fe3+-Spinel

“…The oxygen atom is surrounded by three M and one T cations, forming a triangular pyramid with 3m symmetry. Thus the structure is defined by only two cation-to-oxygen distances, T-O and M-O, which are related to the a cell parameter and to u by the following equations: In previous papers (Basso et al, 1984;Della Giusta et al, 1986Princivalle et al, 1989), we investigated several natural samples from various geological environments. One result was that a depends mainly on bulk chemistry, while u, being determined by cation distribution, is strongly affected by physical environment.…”

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

“…In particular, the crystal chemistry of Cr-spinels from different lithologies, including mantle xenoliths [8][9][10][11][12][13], ophiolites [14][15][16][17][18], Alpine peridotites [19,20], komatiites [21], layered complexes [22][23][24], inclusions in diamonds and kimberlites [25] as well as in meteorites [26,27], have previously been studied to better understand genesis and/or oxidation mechanisms.…”