Nestola, FabrizioBurnham, Antony D.Peruzzo, LucaTauro, LeonardoAlvaro, MatteoWalter, Michael J.Gunter, MickeyAnzolini, ChiaraKohn, Simon C.2017-03-310026-461Xhttp://hdl.handle.net/1885/114192Jeffbenite, 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.This research was supported by the ERC Starting Grant 2012 to FN (agreement no. 307322) and NERC grant NE/J008583/1 to MJW and SCK. We are grateful to Chris Smith and Galina Bulanova for access to the Collier-4 diamond RC2-7.application/pdf© 2016 The Mineralogical Society.TAPPjeffbenitephysical propertiespressure/temperature conditionsdiamondSão Luiz riverBrazil2016-1210.1180/minmag.2016.080.059