The Local Structural State of Aluminosilicate Garnet Solid Solutions: An Investigation of Grospydite Garnet from the Roberts Victor Kimberlite Using Paramagnetically Shifted 27Al and 29Si MAS NMR Resonances

Abstract:

Most rock-forming silicates are substitutional solid solutions. Over the years extensive research has been done to determine their structural and crystal chemical properties. Here, the distribution of cations, or order-disorder behavior, is of central importance. In the case of aluminosilicate garnet solid solutions (X₃Al₂Si₃O₁₂ with X = Mg, Fe²⁺, Mn²⁺ and Ca) it has been shown that both synthetic and natural crystals have random long-range X-cation disorder in space group Ia-3d, as given by X-ray single-crystal diffraction measurements. However, the structural state of natural garnets at the local scale is not known.

Garnet from a grospydite xenolith from the Roberts Victor kimberlite, South Africa, was studied by ²⁷Al and ²⁹Si MAS NMR spectroscopy. The research thrust was placed on measuring and analyzing paramagnetically shifted resonances to determine the local (short range) structural state of the X-cations in a grossular-rich ternary aluminosilicate garnet solid solution. The garnet crystals are compositionally homogeneous based on microprobe analysis, showing no measurable zoning, and have the formula Grs_46.7Prp_30.0Alm_23.3. The garnet is cubic with the standard garnet space group Ia-3d.

The ²⁷Al MAS NMR spectrum shows a very broad asymmetric resonance located between about 100 and -50 ppm. It consists of a number of individual overlapping paramagnetically shifted resonances, which are difficult to analyze quantitatively. The ²⁹Si MAS NMR spectrum, showing better resolution, has two 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 in nature containing many overlapping individual peaks. S0 contains information on local cation configurations whereby an isolated SiO₄ group in the garnet structure does not have an edge-shared Fe²⁺-containing dodecahedron. S4 involves 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 cation mixing. These first results do not indicate any overt short-range cation order in grossular-rich grospydite garnet.