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

Abstract

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,Fe 2+ ,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 Fe 2+ /Fe 3+ ) 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 Fe 2+ . The content of Fe 3+ and the Fe 3+ /ΣFe ratio increase with Mn content up to 0.31 atoms per formula unit (apfu) and 0.80, respectively. Fe 3+ 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: [6]

Bond-valence constraints around the O1 site of tourmaline

“…This is much smaller than the typical experimental error (0.1À0.2 atoms per formula unit) associated with (OH)-measurements in borosilicates (e.g. Andreozzi et al, 2000;Bosi et al, 2005). Hence, equation 3 appears to be a good tool for estimation of (OH) contents in tourmaline.…”

Crystal chemistry of the elbaite-schorl series

“…In a few cases (60fc, L4e, L3h, L1v, and L1z), there was insuffi cient material to perform Mössbauer analysis, so Fe 2+ /Fe 3+ ratios were calculated on the basis of the best match between total scattering values obtained by structural refi nement and chemical analysis. Based on previous work with borosilicates (Andreozzi et al 2000), Fe 2+ /Fe 3+ ratios calculated with this procedure are considered to be reliable within ±20% relative. (7) 0.61145(6) U eq 0.00524 (7) 0.00498 (6) 0.00460 (7) 0.00496 (7) 0.00537 (8) (5) 0.00358 (6) 0.00389 (5) 0.00428 (6) 0.00379 (9) 0.00378 (8) O1 (27) (7) 0.0386 (7) 0.0450 (9) 0.0460(9) 0.055 (1) (11) …”

High-temperature behavior of axinite-(Mn), kornerupine and leucosphenite

“…Кристаллохимия и химическое разнообразие аксинитов изучены комплексом методов рентгеноструктурного анализа (РСА), термогравиметрии (ТГ), электронной микроскопии (ЭМ), мессбауэровской спектроскопии и др. для шести десятков образцов из 24 месторождений, в том числе из Дальнегорского месторождения, в работе Г. Б. Андреоцци с соавторами (Andreozzi et al, 2000). Подобное кристаллохимическое исследование минералов корнерупин-призматиновой серии представлено в серии работ М. Купера и Ф. К. Хоторна, в частности в работе (Cooper, Hawthorne, 2009), где приводятся данные по химическому составу и кристаллическим структурам для 47 кристаллов из различных месторождений, в том числе из месторождения Дараи-Стаж.…”

“…Целью настоящей работы являлось исследование методами высокотемпературной терморентгенографии, ДСК и ТГ термического поведения аксинита-(Mn), корнерупина и лейкосфенита, в частности, определение для них температур плавления/разложения и фазовых переходов, а также расчет коэффициентов термического расширения (КТР). (Andreozzi et al, 2000). Корнерупин из месторождения Дараи-Стаж практически не содержит железа, что подтверждается его почти бесцветной окраской.…”