An investigation of local Fe2+ order-disorder in a mantle grospydite garnet using paramagnetically shifted Al-27 and Si-29 MAS NMR resonances

摘要

An attempt is made for the first time to measure the local (short-range) distribution of the X-cations in a ternary almandine-pyrope-grossular garnet solid solution using paramagnetically shifted NMR resonances. Initial Al-27 MAS NMR measurements made on an almandine-rich garnet of composition Alm(45)Prp(42)Grs(13) proved to be uninterpretable because of the high Fe2+ concentration. Thus, a grossular-rich and almandine-poorer garnet occurring in a mantle grospydite xenolith taken from the Roberts Victor ldmberlite, South Africa, was chosen for detailed Al-27 and Si-29 MAS NMR spectroscopic study. The crystals are compositionally homogeneous based on electron microprobe analysis, showing no measurable zoning, and have the formula Alm(23)Prp(30)Grs(47) (Fe0.23Mg0.30Ca0.47)(3)Al2Si3O12. The garnet is cubic with the conventional space group Ia (3) over bard. The composition and, also importantly, the lack of measurable zoning or compositional heterogeneity, as well as the mode of occurrence, make this garnet a key sample for investigating the possible presence of short-range X cation order. The Al-27 MAS NMR spectrum shows a very broad asymmetric resonance located between about 100 and -50 ppm. It consists of a number of individual closely overlapping paramagnetically shifted resonances, which are difficult to analyze quantitatively. The Si-29 MAS NMR spectrum, showing better resolution, has two broad but observable resonances termed S0 and S4. S0 is located between about -60 ppm and -160 ppm and S4 is centered at roughly 95 ppm. Both S0 and S4 are composite resonances containing many overlapping individual peaks. S0 contains information on local X-cation configurations, where an isolated SiO4 group in the garnet structure does not have any edge-shared Fe2+-containing dodecahedra. S4 involves various local configurations where there is one edge-shared dodecahedron containing Fe2+. The measured intensity of the resonances S0 and S4 are roughly similar to calculated intensities assuming random Fe2+-cation mixing. These first results do not indicate any overt short-range X-cation order in grospydite garnet.