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

Abstract

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 (Mg262Fe2+0.27)(Al186Cr016)(Si2 g2Al018)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 IMA Commission on New Minerals and Mineral Names with the code IMA 2014-097. Its name is after Jeffrey W. Harris and Ben Harte, two world-leading scientists in diamond research. The petrological importance of jeffbenite is related to its very deep origin, which may 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.

Slab Transport of Fluids to Deep Focus Earthquake Depths—Thermal Modeling Constraints and Evidence From Diamonds

“…For example, a specimen of pristine Mg‐rich ringwoodite places its host diamond unquestionably in the lower half of the mantle transition zone when the inclusion was incorporated by its diamond host (e.g., Pearson et al., 2014). Conversely, most inclusions have experienced retrograde transitions (Table 1) either to lower pressure polymorphs of the same composition and stoichiometry such as MgSiO 3 , CaSiO 3 , (Mg x Fe 1−x )O (Burnham et al., 2016; Davies et al., 2004; Harte et al., 1999; Hutchison et al., 2001; Stachel, et al., 2000b) or to unmixed‐assemblages (e.g., garnet‐pyroxene, perovskite‐breyite, enstatite‐jeffbenite) depending on composition and phase relations (Brenker et al., 2002; Bulanova et al., 2010; Harte & Cayzer, 2007; Hayman et al., 2005; Kaminsky et al., 2001; Nestola et al., 2016; Walter et al., 2008, 2011; Zedgenizov et al., 2014). Phase associations in single diamonds such as the co‐occurrence of MgSiO 3 as low‐Ni enstatite (former bridgmanite) and CaSiO 3 as breyite (former Ca‐perovskite), jeffbenite, and ferropericlase have been interpreted as indicative of a derivation at deep transition zone and upper lower mantle depths (Burnham et al., 2016; Davies et al., 2004; Harte et al., 1999; Hutchison et al., 2001; Stachel, et al., 2000b).…”

“…j Pressure estimates made using residual pressures, equation of state of Ice VII and H 2 O-based fluid intersection with mantle adiabat (Tschauner et al, 2018). or to unmixed-assemblages (e.g., garnet-pyroxene, perovskite-breyite, enstatite-jeffbenite) depending on composition and phase relations (Brenker et al, 2002;Bulanova et al, 2010;Harte & Cayzer, 2007;Hayman et al, 2005;Kaminsky et al, 2001;Nestola et al, 2016;Walter et al, 2008Walter et al, , 2011Zedgenizov et al, 2014). Phase associations in single diamonds such as the co-occurrence of MgSiO 3 as low-Ni enstatite (former bridgmanite) and CaSiO 3 as breyite (former Ca-perovskite), jeffbenite, and ferropericlase have been interpreted as indicative of a derivation at deep transition zone and upper lower mantle depths (Burnham et al, 2016;Davies et al, 2004;Harte et al, 1999;Hutchison et al, 2001;Stachel, et al, 2000b).…”

Diamonds from the Machado River alluvial deposit, Rondônia, Brazil, derived from both lithospheric and sublithospheric mantle

“…Meta-serpentinite is a conveniently trace-element-poor and magnesium-rich source of water (e.g. Li et al, 2004), with the water primarily held in wadsleyite, ringwoodite and superhydrous phase B at Transition Zone pressures (Ohtani et al, 2004;Komabayashi and Omori, 2006;Harte, 2010). At low pressures (< 5 GPa) two slab-derived fluids are possible: aqueous fluids, which have only a limited capacity for transporting major and trace elements, and hydrous silicate melts, which can mobilise orders of magnitude higher concentrations of most elements (Hermann et al, 2013).…”

Toward a Robust Elastic Geobarometry of Kyanite Inclusions in Eclogitic Diamonds

“…Attempts to analyze Kyanite_1 in situ did not provide any reliable results. The garnet was measured by single‐crystal X‐ray diffraction before and after its release using a Rigaku Oxford Diffraction SuperNova diffractometer, equipped with a Dectris Pilatus 200K area detector and with a Mova X‐ray microsource (beam spot ~0.12 mm; Nestola et al, ). For the measurements, Mo K α‐radiation, operated at 50 kV and 0.8 mA, was used.…”