Ferdinando Bosi

Crystal chemical relationships in the tourmaline group: Structural constraints on chemical variability

Abstract This paper explores some aspects of the crystal chemistry and structural constraints on tourmaline by examining 127 samples from the literature. According to the bond-valence model, the tourmaline structure shows lattice-induced strain at each polyhedron. The overall effect is an expansion of the triangular (BO3) group and compression of the tetrahedron. The X polyhedron can be either compressed or expanded: compression increases with vacancy content, whereas expansion is typical of Ca-rich tourmaline. The Y octahedron changes extensively from compressed through an unstrained to expanded state as a function of increasing Li content. The Z octahedron is almost unstrained in crystals with ΣZR2+ < 0.40 apfu, whereas it is compressed in crystals with ΣZR2+ > 0.40 apfu. The configuration of the six-membered tetrahedral ring is strongly affected by , which is the most important parameter linked to the deviation of the tetrahedral ring from hexagonal symmetry. The whole structure is stable when the channels through the Z octahedron framework are able to accommodate the Y cations. As becomes larger, the less puckered the tetrahedral ring and the more the O7 atom is displaced away from Z. Consequently, the difference between and cannot be too large, otherwise will be too small to be commensurate with shifting of the O7 atom. One possible mechanism to reduce the difference between and , is the disordering reaction YAl + ZR → YR + ZAl, which increases and decreases . In ideal dravite, schorl, and .tsilaisite,. and are incommensurate.

Crystal chemistry of the schorl-dravite series

Nineteen tourmaline samples of various provenances and geological settings were studied by EMPA, SREF and MS to represent the schorl-dravite compositional field. All samples belong to the Alkali group (except one with an X -site vacancy content of 0.53 apfu) and to the Oxy- and Hydroxy-subgroups. Among divalent cations, the main substitution involves Y Mg for Y Fe 2+ , to produce the two end-members dravite and schorl. Site populations were determined by a new minimization procedure that simultaneously accounts for both structural and chemical data. Results show that the crystals are characterized by disordered cation distribution between Y and Z sites: Al populates both sites, with a marked preference for the smaller Z octahedron; Mg is often equally distributed between Y and Z . Both Fe 2+ and Fe 3+ populate both Y and Z sites, but show a strong preference for Y . Specific mean bond distances (A) optimised for major elements are: Y Al-O = 1.908, Y Mg-O = 2.084, Y Fe 2+ -O = 2.139, z Al-O = 1.900, z Mg-O = 2.077 and z Fe 2+ -O = 2.131. In the schorl-dravite solid solution, structural variations appear to be primarily due to Y and Z interactions. These effects are conspicuous over the entire structure, as Y dimensions directly affect the a cell parameter, while Z is similarly correlated with c. The dimensions of Y and Z octahedra are determined by Al contents. Dimensional variations of Z are well described by its bond-distance variations, except for Z -07D. Both octahedra reciprocally interact, influencing their distortions: inverse correlations exist between Y dimension vs. Z quadratic elongation and Z dimension vs. Y quadratic elongation. As a common feature, the effects of the octahedral second coordination sphere are only confined to polyhedral distortions instead of dimensional variations, which only depend on site populations.

Crystal chemistry of the magnetite-ulvöspinel series

Abstract Spinel single crystals of 19 compositions along the magnetite-ulvöspinel join were synthesized by use of a flux-growth method. To obtain quantitative site populations, the crystals were analyzed by single-crystal X-ray diffraction, electron-microprobe techniques, and Mössbauer spectroscopy. All results were processed by using an optimization model. The unit-cell parameter, oxygen fractional coordinate, and tetrahedral bond length increase with increasing ulvöspinel component, whereas the octahedral bond length decreases marginally. These changes result in sigmoidal crystal-chemical relationships consistent with cation substitutions in fully occupied sites. As a first approximation, the Akimoto model T(Fe3+1-XFe2+x)M(Fe2+Fe3+1-XTiX)O4 describes the cation substitutions. Deviations from this model can be explained by an electron exchange reaction TFe2+ + MFe3+ = TFe3+ + MFe2+, which causes MFe2+ ≠ 1 and TFe2+/Ti ≠ 1. The resultant S-shaped trends may be related to a directional change in the electron exchange reaction at Ti ≈ 0.7 apfu. In general, variations in structural parameters over the whole compositional range can be split into two contributions: (1) a linear variation due to the TFe3+ + MFe3+ = TFe2+ + MTi4+ chemical substitution and (2) non-linear variations caused by the internal electron exchange reaction. In accordance with bond-valence theory, strained bonds ascribable to steric effects characterize the structure of magnetite-ulvöspinel crystals. To relax the bonds and thereby minimize the internal strain under retained spinel space group symmetry, the electron exchange reaction occurs.

Crystal chemistry of the elbaite-schorl series

Abstract 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 O2.-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 O2. at the W site. The main substitutional mechanism is the dehydroxylation type: YFe2+ + YFe3+ + WO → YLi + YAl + W(OH+F). The T site is characterized by TSi → TAl substitution. is linearly correlated with vacancy content in crystals with (OH + F) ≤ 4, whereas it is almost constant in crystals with OH at the O2 position. Along the series, is inversely correlated with YAl. The Z site is almost fully occupied by R3+ (with ZAl largely dominant) and the ZFetot ↔ ZAl substitution explains the inverse correlation of with ZAl. 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 [ZO6] 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.

Crystal chemistry of the dravite-chromdravite series

Five Cr-dravite-chromdravite samples, representative of the high-chromium part of the compositional field observed in tourmalines occurring in rocks surrounding Lake Baikal, were selected and studied by electron microprobe analysis and structural refinement. All examined tourmalines belong to the Alkali group, Oxy-subgroup. Their most striking feature is the exceptionally high Cr content (from 3.2 to 4.41 apfu), which substitutes for Al. The main divalent cation is Mg. Site populations were determined by a minimization procedure which simultaneously takes into account both structural and chemical data. Empirical bond distances for Y Cr 3+ -O (1.978 A) and z Cr 3+ -O (1.970 A) were optimised within the minimization procedure. Results indicate that the crystals are characterized by a quite disordered cation distribution between Y and Z octahedral sites, the various cations showing different degrees of preference. Cr 3+ and Mg populate both sites but show opposite behaviour: Mg has a marked preference for the Z octahedron and Cr 3+ for Y. Al almost exclusively populates Z. Most structural features are related to variations in the Z octahedron, whereas the dimensions of other polyhedra remain almost constant. Z dimensions depend on site populations, and are in particular determined by the z Cr 3+ ⟷ Z Al substitution. In the dravite-chromdravite series, both current and literature data show that the unit cell parameter c is strongly and positively correlated with Z dimensions, while correlation with a is evident only within the Cr-dravite-chromdravite subseries. Y does not actively participate in structural variations and, in particular, no correlation was ever observed between Y dimensions and unit cell parameters. Most structural variations are thus due to Z , while the effects of Y are negligible. This behaviour stands out when examining the a vs. c plot, for the whole series: a strong correlation between the two unit cell parameters was observed in the Cr-dravite-chromdravite subseries, but none within the dravite-Cr-dravite subseries or schorl-dravite series.

Linking Mössbauer and structural parameters in elbaite-schorl-dravite tourmalines

Abstract Accurate crystal-chemical analysis of complex minerals such as tourmalines belonging to the elbaite-schorl-dravite series was obtained by combining Mössbauer spectroscopy (MS) and structural information. Well-defined relationships were established between the two approaches, leading to a close match of results obtained and a strong link between observed parameters. Although MS information is a powerful tool for quantifying the amount of Fe2+ and Fe3+, it is not always useful in determining their site distribution. In particular, both for Fe3+ and for (Fe2+-Fe3+) interactions structural information is still essential. Fe3+ MS doublets were identified and assigned to iron in Y and Z sites on the basis of structural information. In a few spectra, Fe3+ doublets with very low parameters (δ ~ 0.2 mm/s) were observed and, in contrast with the T-site assignment of previous works, were assigned to Fe3+ in octahedral coordination. Electron delocalization between Fe2+ and Fe3+ was observed and related to three different interactions (Y-Y, Y-Z, and Z-Z). Notably, MS hyperfine parameters of Fe2+ were self-consistent and particularly reliable in determining Fe2+ site partitioning. Fe2+ at Y was modeled by three doublets (ΔEQ = 2.45, 2.19, and 1.72 mm/s). The sum of their absorption areas perfectly matches the YFe2+ populations derived from structural data (r2 = 0.97). The fourth doublet observed (ΔEQ = 1.38 mm/s) is consistent with Fe2+ at Z, which is an octahedron smaller and less distorted than Y (λZ = 1.014, λY = 1.024). The absorption area of this doublet is highly correlated with the amount of ZFe2+ obtained from site-occupancy refinement (r2 = 0.95). For YFe3+ a link between the quadrupole splitting parameter ΔEQ and variations in the chemical/ structural environment surrounding Fe nucleus was observed. The ΔEQ of YFe3+ increases with ZO6 volume (r2 = 0.84) and is linked to the variation of electrical field gradient generated by the ZR2+ → ZAl substitution. Since the Z skeleton completely surrounds Y islands, ΔEQ of YFe3+ shows much more susceptibility to inductive effects from the second rather than the first coordination sphere.

Behavior of cation vacancy in kenotetrahedral Cr-spinels from Albanian eastern belt ophiolites

Abstract The crystal chemistry of 17 Cr-spinels from the Albanian eastern belt ophiolites was studied by a multi-analytical approach (EMPA, MS, SREF), processing the data with a tested optimization model to obtain site populations. The samples come from the three most important ultramafic massifs of Albania (Tropoja, Bulqiza, and Shebenik), and occur in ultramafic cumulates as well as in ultramafic mantle tectonites, associated with serpentinized olivine. All samples are characterized by Cr ↔ Al and minor Mg ↔ Fe2+ substitutions, and may be classified as magnesiochromite, except one, which is spinel s.s. Cation distributions showed that Cr and Al are ordered in M, and Fe2+ and Mg in the T site. Contents of Fe3+ measured by MS were always higher than those calculated from EMPA, and this non-stoichiometry reveals that the Albanian crystals underwent an increase in fO₂ conditions after mineral formation. Cation vacancies produced by Fe2+ oxidation occur in the T site, and the oxidation mechanism, is described by: 2 TFe2+ + ½ O2 → 2 TFe3+ + O2- + T⃞. T-O variations show a non-linear regression with TFe2+, and this trend is due to both the cooperative effects of TMg ↔ TFe2+ substitution and TFe2+ oxidation. Cation vacancy in the T site does not impart rigidity to the polyhedron, because it cannot have chemical bonds with ligands: this feature, together with the spinel topology, makes the tetrahedron adopt .soft. behavior. In effect, the T⃞-O distance does not have a single value, but changes according to the population of the M site, as confirmed by comparison with literature data and also by application of the Bond Valence Model.

Crystal structure analyses of four tourmaline specimens from the Cleopatra’s Mines (Egypt) and Jabal Zalm (Saudi Arabia), and the role of Al in the tourmaline group

Abstract Fe-rich “oxydravite” and dravite from the Late Proterozoic ophiolitic mélange of the Arabo-Nubian Shield, located in Egypt and Saudi Arabia, were structurally and chemically characterized by using crystal structure refinement based on single-crystal X-ray diffraction data, electron microprobe analysis, and Mössbauer spectroscopy. Structural formulae obtained by optimization procedures indicate disordering of Al, Mg, and Fe2+ over the Y and Z sites, and an ordering of Fe3+ at Y. The disordering can be explained by the substitution mechanisms 2YMg+ZAl+WOH = 2YAl+ZMg+WO2- and 2YFe2++ZFe3++WOH = 2YFe3++ZFe2++WO2-, which are consistent with reducing the mismatch in dimensions between YO6 and ZO6 octahedra. To explain the Mg-Al disordering process, as well as the occurrence of B at the T site in tourmaline, analogies have been drawn between the crystal structure of tourmaline and that of lizardite. A critical constraint in both structures is the geometrical fit of the six-membered tetrahedral ring with the attached group of three YO6 octahedra. In tourmaline, the disordering of Mg and Al over Y and Z relieves the strain due to the misfit in dimensions of the larger triads of edge-sharing MgO6 octahedra and the smaller Si6O18 tetrahedral rings. In Al-rich tourmaline, where the octahedral cluster is smaller, the strain can be relieved by incorporating B in the tetrahedra. An opposite effect is observed by substitution of Al for Si at the tetrahedral site in Mg-rich tourmaline. Because the Al radius is intermediate between those of Mg and Si, Al plays an important structural role in accommodating the potential misfit between YO6, ZO6, and TO4 polyhedra. The amount of Al and its distribution in the structure strongly affects the values of the unit-cell parameters of tourmaline and yields volume variations according to a quadratic model. This results from the effect of ZAl combined with the occurrence of B at T in Al-rich tourmaline. ZAl has a greater effect than YAl as long as Al does not fully occupy the Z site.

Disordering of Fe2+ over octahedrally coordinated sites of tourmaline

Abstract The partitioning of iron among octahedrally coordinated sites in tourmaline, and its stereochemical consequences, were investigated in a Fe-rich dravite in a skarn rock from Utö, Sweden. A multianalytical approach using structure refinement (SREF), electron microprobe analysis (EMPA), and Mössbauer spectroscopy (MS) established the chemical and structural nature of the tourmaline. A structural formula obtained by optimization procedures indicates disordering of Al, Mg, and Fe2+ over the Y and Z sites, and ordering of Fe3+ at the Y site. Two Fe-rich tourmalines from the literature, reexamined with the optimizing site assignment procedure, appear to have iron partitioning comparable to that of the Utö tourmaline with Fe2+ disordered over the octahedral sites. This is best explained by disordered Fe2+ distributions that minimize the strain state of the Y-O bonds and provide a shielding effect reducing Y-Z repulsion. This is consistent with predictions from bond-valence theory and Pauling’s rules. An indication of Z-site occupancy by Fe2+ in tourmaline may be signaled by a significant correlation between and the c lattice parameter (r2 = 0.96). The c value for a very Fe2+-rich tourmaline and an ideal end-member schorl, with Fe2+ and Al ordered at Y and Z (respectively), yielded values larger than 1.907 Å (the likely bond length for ). These large lengths indicate that Fe2+ occurs at the Z site. The hypothesis of a dragging effect from to explain lengthening of is not supported by experimental evidence.

Structure of the Molten Salt Methyl Ammonium Nitrate Explored by Experiments and Theory

We present an analysis of the structure of the monomethylammonium nitrate (MMAN) compound. Vibrational Raman spectroscopy and X-ray powder diffraction have been used to characterize the bulk phases of MMAN, and assignment of the resonant frequencies has been performed by ab initio (DFT) computations on small clusters of the compound. The theoretical spectra are in excellent agreement with the experimental ones and provide a means by which an interpretation of the hydrogen-bonding network that exists in such compound can be analyzed. In particular, we found that the spectrum of one of the solid phases is structurally very similar to that of the liquid. We present experimental evidence for the existence of such phase both from X-ray data and Raman spectra which, in turn, is easily interpreted with a one-to-one correspondence with the ab initio simulation of the small clusters. A geometric structure of the short-range local arrangement in these two bulk phases is therefore proposed.