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Crystal chemistry of the axinite-group minerals: A multi-analytical approach

Giovanni B. Andreozzi^1,*,, Luisa Ottolini², Sergio Lucchesi¹, Giorgio Graziani¹ and Umberto Russo³

¹ Dipartimento di Scienze della Terra, Università di Roma "La Sapienza," Piazzale Aldo Moro 5, I-00185 Roma, Italy
² C.N.R. Centro di Studio per la Cristallochimica e la Cristallografia, Via Ferrata 1, I-27100 Pavia, Italy
³ Dipartimento di Chimica Inorganica, Metallorganica e Analitica, Università di Padova, Via Loredan 4, I-35131 Padova, Italy

Correspondence: ^* E-mail: graziani{at}uniroma1.it

Sixty axinite samples from 24 localities worldwide were characterized by electron microprobe analysis (EMPA) to define the limits of compositional variation. Three samples are very close to the Mn, Fe, and Mg end-members. Ternary (Mn,Fe²⁺,Mg)-compositions occur mostly in the ferroaxinite and manganaxinite fields, and are constrained by the relation Mg Fe. Core-rim chemical zoning was observed in 20 samples, with systematic enrichment of Fe in the core and Mn in the rim, independent of sample provenance.

The chemical composition (including B, H, and Fe²⁺/Fe³⁺) of 17 homogeneous samples was investigated using electron-microprobe analysis, thermo-gravimetry (TG), ion microprobe (SIMS), crystal-structure refinement (SREF), and Mössbauer spectroscopy (MS). For all samples except pure manganaxinite, most of the iron is Fe²⁺. The content of Fe³⁺ and the Fe³⁺/Fe ratio increase with Mn content up to 0.31 atoms per formula unit (apfu) and 0.80, respectively. Fe³⁺ may substitute for Al or also for divalent cations balanced by the OH deficiency:

Boron content ranges from 1.88 to 2.07 apfu (±2.5% relative) and shows an inverse relation with Si content. Direct measurement of the B-tetrahedron size provides structural confirmation of the Si B exchange. Hydroxyl deficiency accompanies this substitution and the following coupled mechanism is proposed:

Hydrogen content ranges from 1.7 to 2.1 apfu (±5% relative). The deficiency of OH from the stoichiometric value of 2.0 per formula unit is related directly to the number of trivalent and tetravalent cations, as OH content plays a crucial role in charge-balance relations.

A revised chemical formula for the axinite-group minerals is proposed:

where x 1, u < 1, v < 1, y < 1, z << 1, and w = (u + v + z).

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