Tetragonal Almandine-Pyrope Phase, TAPP: Finally a name for it, the new mineral jeffbenite

Jeffbenite, ideally Mg₃Al₂Si₃O₈, previously known as tetragonal-almandine-pyrope-phase ('TAPP'), has been characterized as a new mineral from an inclusion in an alluvial diamond from São Luiz river, Juina district of Mato Grosso, Brazil. Its density is 3.576 g/cm³ and its microhardness is ∼7. Jeffbenite is uniaxial (-) with refractive indexes ω = 1.733(5) and ϵ = 1.721(5). The crystals are in general transparent emerald green. Its approximate chemical formula is (Mg_2.62Fe²⁺ _0.27)(Al_1.86Cr_0.16)(Si_2.82Al_0.18)O₁₂ with very minor amounts of Mn, Na and Ca. Laser ablation ICP-MS showed that jeffbenite has a very low concentration of trace elements. Jeffbenite is tetragonal with space group I42d, cell edges being a = 6.5231(1) and c = 18.1756(3) Å. The main diffraction lines of the powder diagram are [d (in Å), intensity, hkl]: 2.647, 100, 2 0 4; 1.625, 44, 3 2 5; 2.881, 24, 2 1 1; 2.220, 19, 2 0 6; 1.390, 13, 4 2 4; 3.069, 11, 2 0 2; 2.056, 11, 2 2 4; 1.372, 11, 2 0 12. The structural formula of jeffbenite can be written as (M1)(M2)₂(M3)₂(T1)(T2)₂O₁₂ with M1 dominated by Mg, M2 dominated by Al, M3 dominated again by Mg and both T1 and T2 almost fully occupied by Si. The two tetrahedra do not share any oxygen with each other (i.e. jeffbenite is classified as an orthosilicate). Jeffbenite was approved as a new mineral by the IMACommission on New Minerals and Mineral Names with the code IMA2014-097. Its name is after JeffreyW. Harris and Ben Harte, twoworld-leading scientists in diamond research. The petrological importance of jeffbenite is related to its very deep origin, whichmay allow its use as a pressure marker for detecting super-deep diamonds. Previous experimental work carried out on a Ti-rich jeffbenite establishes that it can be formed at 13 GPa and 1700 K as maximum P-T conditions.