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

Published online by Cambridge University Press:  02 January 2018

Fabrizio Nestola*
Affiliation:
Dipartimento di Geoscienze, Università di Padova, Via Gradenigo, 6, I-35131 Padova, Italy
Antony D. Burnham
Affiliation:
School of Earth Sciences, University of Bristol, Queen's Road, Bristol BS8 1RJ, UK Now at Research School of Earth Sciences, Australian National University, Canberra, Australia
Luca Peruzzo
Affiliation:
CNR-IGG, Padova, Via Gradenigo, 6, I-35131 Padova, Italy
Leonardo Tauro
Affiliation:
Dipartimento di Geoscienze, Università di Padova, Via Gradenigo, 6, I-35131 Padova, Italy
Matteo Alvaro
Affiliation:
Dipartimento di Scienze della Terra e dell'Ambiente, Università di Pavia, Via Ferrata 1, 27100, Pavia, Italy
Michael J. Walter
Affiliation:
School of Earth Sciences, University of Bristol, Queen's Road, Bristol BS8 1RJ, UK
Mickey Gunter
Affiliation:
Geological Sciences, University of Idaho, 875 Perimeter MS 3022, Moscow, 83844-3022, USA
Chiara Anzolini
Affiliation:
Dipartimento di Geoscienze, Università di Padova, Via Gradenigo, 6, I-35131 Padova, Italy
Simon C. Kohn
Affiliation:
School of Earth Sciences, University of Bristol, Queen's Road, Bristol BS8 1RJ, UK

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.

Information

Type
Research Article
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 2016

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Structure factors

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