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Fourteen Cr-spinels from Albanian ophiolites were examined. [Fe.sup.2+]/[Fe.sup.3+] ratios were obtained by [sup.57]Fe Mossbauer spectroscopy (MS) and compared with ratios retrieved by electron microprobe analyses (EMPA). MS spectra were collected at both 298 K (RT) and 77 K (LT), and fitted using various interpretative models. [Fe.sup.3+] contents by EMPA, calculated from spinel stoichiometry, are almost always underestimated with respect to those obtained by MS. Moreover, [Fe.sup.3+] contents by MS-RT are shown to be somewhat overestimated with respect to those by MS-LT, which are proved to be the most reliable. On basis of MS results, Albanian Cr-spinels proved to be non-stoichiometric, with an oxidation degree, z, ranging from 4 to 49%. Our results indicate no dependence of z on sample provenance, but suggest a strong dependence on spinel composition. Chemical data of Albanian spinel and associated olivine were used to estimate [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] via oxygen geobarometry. Notably, [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] values calculated on basis of the EMPA data are always largely underestimated with respect to those obtained on basis of MS data. However, [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] values calculated on basis of MS-RT data are equal to or higher than (up to 0.2 log units) [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] values based on MS-LT data. The increase in [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] responsible for spinel oxidation was tentatively estimated to be less than one log unit for poorly oxidized samples, but up to 6 log units for the most oxidized samples. Keywords: Analysis, chemical (mineral), chromite, olivine, Mossbauer spectroscopy, thermobarometry, olivine-spinel

The crystal-chemistry of 13 elbaite-schorl tourmaline crystals from the Cruzeiro pegmatite (Minas Gerais, Brazil) was studied with a multi-analytical approach (SREF, EMPA, SIMS, MS). Effective cation radii at the Y and Z sites and site populations were refined by a minimization procedure. The results indicate that the crystals belong to the alkali group. Elbaite crystals are [O.sup.2] -free at the W and V sites and show OH content at the O2 site (up to 0.2 apfu). Conversely, schorl crystals always show [O.sup.2-] at the W site. The main substitutional mechanism is the dehydroxylation type: [sup.Y][Fe.sup.2+] + [sup.Y][Fe.sup.3+] + [sup.w]O [right arrow] [sup.Y]Li + [sup.Y]Al + [sup.w][OH+F). The T site is characterized by [sup.T]Si [right arrow] [sup.T]Al substitution. is linearly correlated with vacancy content in crystals with (OH + F) [less than or equal to] or 4, whereas it is almost constant in crystals with OH at the O2 position. Along the series, is inversely correlated with [sup.Y]Al. The Z site is almost fully occupied by [R.sup.3+] (with [sup.Z]Al largely dominant) and the [sup.Z.][Fe.sub.tot] [left and right arrow] [sup.Z]Al substitution explains the inverse correlation of with [sup.Z]Al. In the elbaite compositional range, lattice parameters are functions of , whereas in the schorl range they are essentially functions of . Along the whole elbaite-schorl series, both chemical substitutions and size increase of Y are far larger than those of Z. In spite of this, lattice parameters increase with as much as with . This is due to the role of the [Z[O.sub.6]] polyhedra, which extend along a and c to form the skeleton of the tourmaline structure. Therefore, any change in the size of Z leads to a change in the whole structure.

A set of nine samples of axinite, selected from 60 specimens from worldwide localities, were investigated by single-crystal X-ray diffraction, electron and ion microprobe, and [sup.57]Fe Mossbauer spectroscopy. The selected samples cover the compositional join from almost pure ferroaxinite (80%,) to pure manganaxinite (95%). A new crystal-chemical formula for the axinite mineral group is proposed: [sup.VI][[X1 X2 Y Z1 Z2].sub.2][sup.IV][[T1 T2 T3 T4 T5].sub.2] [O.sub.30]([O.sub.w]O[H.sub.1-w]) where VI and IV are coordination numbers; X1 = Ca and very minor Na; X2 = Ca (in axinites) or Mn (in tinzenite); Y = Mn (in manganaxinite and tinzenite), [Fe.sup.2 (in ferroaxinite) or Mg (in magnesioaxinite), with minor A1 and [Fe.sup.3+]; Z1 = A1 and [Fe.sup.3+]; Z2 = A1; T1, T2, and T3 = Si; T4 = Si (and presumably very minor B): T5 = B and minor Si. Charge unbalance (w), due to heterovalent substitutions, is compensated for by [O.sup.2] [right arrow] O[H.sup.-] substitution. From ferroaxinite to nanganaxinite, cell volume increases linearly from 568.70 to 573.60 [[Angstrom].sup.3]. This is mainly due to an increase in the mean distance from 2.220 to 2.255 [Angstrom], which is directly related to the Mn population (up to 1.89 apfu). [Fe.sup.3+] concentrations, as determined by [sup.57]Fe Mossbauer spectra at 80 K, sub-regularly increase up to 0.27 apfu, and three cases are evidenced: (1) [Fe.sup.3+] << [Fe.sup.2+] (or no [Fe.sup.3+]), in ferroaxinite: (2) [Fe.sup.2+] < [Fe.sup.2+], in intermediate compositions, and (3) [Fe.sup.3+] > [Fe.sup.2+] (or only [Fe.sup.3+]), in manganaxinite. Chemical and structural data were co-processed via a computer minimization program to obtain the cation distribution scheme. Adopting the Hard-Sphere Model, empirical cation-oxygen distances were refined for every cation in the axinite structure. The results revealed that [Fe.sup.2+] is ordered at the octahedral Y site (up to 1.61 apfu), whereas [Fe.sup.3+] is ordered at the octahedral Z1 site (up to 0.26 apfu) and is almost absent in the smallest Z2 site, which is fully populated by A1. The observed [Fe.sup.3+] partitioning is in agreement with the structural results, which show that the Z1 octahedron is always larger than Z2. Moreover, no [Fe.sup.3+] is found at the tetrahedral sites, but Si [right arrow] B substitution occurs at T5. The continuous Y dimensional increase from ferroaxinite to manganaxinite involves progressive enlargement of the edge-sharing Z1 octahedron. As a consequence, the [sup.Z1][Fe.sup.3+] [right arrow] [sup.Z1][A1.sup.3+] substitution is structurally favored toward manganaxinite and points to a new end-member with the suggested name 'ferri-manganaxinite.'

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.sup.2+],Mg)-compositions occur mostly in the ferroaxinite and manganaxinite fields, and are constrained by the relation Mg [is less than or equal to] 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.sup.2+]/[Fe.sup.3+]) of 17 homogeneous samples was investigated using electron-microprobe analysis, thermo-gravimetry (TG), ion microprobe (SIMS), crystal-structure refinement (SREF), and Mossbauer spectroscopy (MS). For all samples except pure manganaxinite, most of the iron is [Fe.sup.2+]. The content of [Fe.sup.3+] and the [Fe.sup.3+]/[Sigma]Fe ratio increase with Mn content up to 0.31 atoms per formula unit (apfu) and 0.80, respectively. [Fe.sup.3+] may substitute for Al or also for divalent cations balanced by the OH deficiency: [Fe.sup.3+] + [O.sup.2-] [equivalence] [Fe.sup.2+] + [OH.sup.-], or [Fe.sup.3+]O[([Fe.sup.2+]OH).sub.-1] Boron content ranges from 1.88 to 2.07 apfu ([+ or -] 2.5% relative) and shows an inverse relation with Si content. Direct measurement of the B-tetrahedron size provides structural confirmation of the Si [equivalence] B exchange. Hydroxyl deficiency accompanies this substitution and the following coupled mechanism is proposed: [Si.sup.4+] + [O.sup.2-] [equivalence] [B.sup.3+] + [OH.sup.-], or [SiO(BOH).sub.-1] Hydrogen content ranges from 1.7 to 2.1 apfu ([+ or -] 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: [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII], where x [is less than or equal to] 1, u [is less than] 1, v [is less than] 1, y [is less than] 1, z [much less than] 1, and w = (u + v + z).