The Structural Behavior of Natural Cr-Bearing Spinels Compared with Synthetic Ones

Spinels are not only a very important mineral phase present in a great variety of environment from the mantle to the crust but also they are strategic material for the industry. For these reasons their crystal chemistry has been widely studied using both synthetic and natural spinels and different experimental techniques. Due to the composition complexity of natural spinels and in order to better understand the cation distribution between the non-equivalent T and M sites the researchers focused their attention to synthetic binary join. In the past decade several synthetic series have been created by flux-grown technique, among the others MgAl2O4-FeAl2O4, MgAl2O4-MgFe2O4, MgCr2O4-MgFe2O4 and MgCr2O4-FeCr2O4. All these studies demonstrate not only how cell edge, oxygen positional parameter and thermal displacement parameters increase or decrease according to chemistry, but also their relations with the increase of pressure and temperature. These studies are of great relevance to explain the behavior of natural spinels and to better characterize the conditions in which they formed or equilibrated. Among natural spinels, Cr-bearing spinel is one of the most studied. Usually, it formula (Mg, Fe2+)(Cr, Al, Fe3+)2O4. Cr-spinels are ubiquitous accessory phase in ultramafic and mafic intrusive, cumulate and volcanic suites as well as in metamorphic rocks being extremely sensitive to bulk composition, mineralogy and petrogenesis of the host rocks. Spinel main constituents behave very differently during fractional crystallization or partial melting, with Cr and Mg strongly partitioned in the solid and Al in the melt. In addition, partitioning of Mg and Fe2+ between spinel and silicate melts and minerals is strongly temperature dependent and the ratio of Fe2+ and Fe3+ is sensitive to variations in fO2. In this review we discuss a spinel database, comprehending structural parameters and chemistry of all the Cr-bearing spinels studied by means of single crystal X-ray diffraction present in literature as well as other spinels which study is currently in progress. These spinels, coming from different occurrences are discussed in term of MgAl2O4-FeAl2O4-FeCr2O4-MgCr2O4 synthetic end-members considering structural parameters instead of their only chemistry.