The spinel structure is based on a nearly ideal cubic close-packed array of oxygen atoms with tetrahedral (T) and octahedral (M) cavities. In common 2-3 spinels, one eighth of the T sites and one half of the M sites are occupied by heterovalent cations A and B in the ratio AB2O4, where A = (Mg, Fe2+, Zn, Mn2+) and B = (Al, Fe3+, Cr3+). Commonly there exists octa-hedral-tetrahedral disorder of A and B cations depending on thermal history. Consequently, the crystal-chemistry of spinels is described by the general formula IV(A1-iBi)VI(AiB2-i)O4, where “i” refers to the inversion parameter. There are two ordered configurations stable at low tem-perature, one with i = 0 (normal spinel; e.g., MgAl2O4, FeAl2O4, MgCr2O4, FeCr2O4) and one with i = 1 (inverse spinel; e.g. MgFe2O4, FeFe2O4). With increasing temperature disorder takes place, i.e. A and B cations undergo increasing intersite exchange over the three cation sites per formula unit. Modifications of T-O and M-O bond distances to accommodate various chemical compositions and/or cation distribution determine variations in the oxygen positional parameter u and the cell edge a0.
Cr-bearing spinels can be found in different geological environments, usually related to mafic and ultramafic rocks from great depth in the mantle as inclusions in diamonds (Lenaz et al., 2009; Nestola et al., 2014) to extra-terrestrial material (Lenaz et al., 2015). In nature, several cations can enter into the structure originating a solid solution with a general formula (Mg, Fe2+)(Al, Cr)2O4. Other cations such as Fe3+ or Ti can be present in low amounts - although in some cases the Fe3+ content can be very high. It’s important to notice that almost all the major cations can move among the two sites apart from Cr that prefers the octahedral site. Given that, spinels with higher Cr content will show a limited possibility of inversion while those with high Mg and Al and low Cr could undergo different degrees of inversion.
In order to see the effects of order/disorder phenomena via Raman spectroscopy we ana-lysed spinels already characterised by X-ray single crystal diffraction and electron microprobe, and many other spinels from the same suites characterised by electron microbe for a total of about 300 different natural Cr-bearing spinels with 0.03 < Cr < 1.68 atoms per formula unit.
Preliminary data show that the relative intensities and shifts of the A1g peak can be related to the Cr/(Cr+Al) ratio, being at about 690 cm-1 and 745 cm-1for high and low Cr content, re-spectively. As far as concern the low Cr spinels, the Raman spectra of disordered spinels is characterised by a peak at about 162-166 cm-1 and a succession of smooth curves in the range 300-650 cm-1. Ordered low Cr-spinels do not show the peak at 162-166 cm-1 but a peak at about 404 cm-1 and two others not well developed peaks/curves in the range 600-650 cm-1.
