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

Abstract

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 27 Al 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 Fe 2+ concentration. Thus, a grossular-rich and almandine-poorer garnet occurring in a mantle grospydite xenolith taken from the Roberts Victor kimberlite, South Africa, was chosen for detailed 27 Al and 29 Si 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 [(Fe 0.23 Mg 0.30 Ca 0.47 ) 3 Al 2 Si 3 O 12 ]. The garnet is cubic with the conventional space group Ia 3 d . 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 27 Al 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 29 Si 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 SiO 4 group in the garnet structure does not have any edge-shared Fe 2+ -containing dodecahedra. S4 involves various local configurations where there is one edge-shared dodecahedron containing Fe 2+ . The measured intensity of the resonances S0 and S4 are roughly similar to calculated intensities assuming random Fe 2+ -cation mixing. These first results do not indicate any overt short-range X-cation order in grospydite garnet.