Sergio Lucchesi

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 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.

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.

Intersite distribution of Fe2+ and Mg in the spinel (sensu stricto)–hercynite series by single-crystal X-ray diffraction

Abstract The influence of composition on Fe2+-Mg intracrystalline distribution was studied in eleven synthetic crystals belonging to the spinel (sensu stricto)-hercynite series (Mg1-yFe2+y)Al2O4, with 0 ≤ y ≤ 1, produced by flux-growth at 800 °C. Samples were analyzed by single-crystal X-ray diffraction and electron microprobe methods, and found to be chemically homogeneous with only minor Fe3+, which substitutes for Al and increases up to 0.09 atoms per formula unit with total Fe. Structural parameters a, u, T-O, and M-O increase with hercynite content and, among bond distances, T-O shows the maximum change, from 1.920 to 1.968 Å. The a variation from 8.0855 to 8.1646 Å is essentially caused by the T-O increase that, in turn, is due to the cooperative effects of (1) Fe2+ → Mg substitution and (2) decrease of inversion from 0.23 to 0.15 along the series. Intracrystalline cation distribution was obtained by a minimization procedure that takes into account structural and chemical data. The T site is mainly populated by Mg and Fe2+ but, at a given temperature, Fe2+ shows a marked preference for tetrahedral coordination with respect to Mg. The influence of composition and temperature on Fe-Mg intracrystalline distribution was modeled within the framework of the general thermodynamic model of O’Neill and Navrotsky for spinel binary solid solutions. The inversion values observed in our samples agree very well with those calculated by the model. Both measured and calculated amounts of octahedral Fe2+ (VIFe2+) show a non-linear increase from spinel s.s. to hercynite. Consequently, the VIFe2+/Fe2+ tot. ratio is not constant along the join, but increases from zero to 15% toward the hercynite end-member. This behavior explains the very limited Fe2+ inversion observed in natural spinels, which usually belong to the hercynite-poor part of the join.

Mn-tourmaline from island of Elba (Italy): Crystal chemistry

Abstract The crystal chemistry of seven crystal fragments taken from differing regions of the same colorless to yellow-greenish tourmaline macro-crystal from pegmatite pockets in aplite veins (island of Elba, Italy) was studied with a multi-disciplinary (SREF, XRDT) and multi-analytical approach (EMPA, SIMS). EMPA and XRDT studies showed relationships between color and chemical zoning and crystal- growth evolution, indicating which fragments could be considered representative of the chemical evolution of the genetic micro-environment in which the crystal developed. Results showed that the colorless fragment is an elbaite while the yellow-greenish crystal fragments are Mn2+-rich (up to 1.34 apfu) and belong to the alkali group and fluor subgroup. They are characterized by dehydroxylation and alkali-defect type substitutions that cooperate in reducing Li and increasing Mn contents. The Y site is populated by Al, Li, and Mn2+, and the Z site by Al and Mn2+ (up to 0.10 apfu). In contrast with data in the literature, Mn2+ populates both octahedral sites according to the order-disorder reaction: YMn + ZAl ↔ YAl + ZMn. As Mn2+ content increases, progressive disorder takes place. This disorder is quantitatively lower than that of the ZMg in dravite, due to the low structural tolerance of the small Z cavity in the incorporation of larger cations by the ZR2+ → ZAl substitution. Relationships of direct proportionality between lattice parameters and both and are observed. The expansion of both octahedra, as well as of lattice parameters, increases linearly as a function of YMn2+ and ZMn2+. The latter has greater weight in dictating unit-cell variations, due to the degree of size mismatch between ZMn2+ → ZAl and YMn2+ → YLi substitutions, and the way in which the Z octahedra are articulated in the structure.

Compositional dependence of cation distribution in some synthetic (Mg,Zn)(Al,Fe3+)2O4 spinels

The compositional dependence of the intracrystalline cation distribution was investigated in four synthetic spinels belonging to binary solid solutions. Spinel single crystals were flux-grown in the (Mg,Zn)(Al,Fe 3+ ) 2 O 4 system, and cation distribution was determined by means of single-crystal X-ray structural refinement, electron-microprobe analysis and Mossbauer spectroscopy. Experimental data were processed and a detailed topochemical model was obtained for each sample: IV (Mg 0.76 Al 0.24 ) VI (Al 1.76 Mg 0.24 )O 4 ; IV (Zn 0.65 Mg 0.22 Al 0.13 ) VI (Al 1.87 Mg 0.13 )O 4 ; IV (Mg 0.73 Al 0.18 Fe 3+ 0.09 ) VI (Al 1.62 Mg 0.27 Fe 3+ 0.11 )O 4 and IV (Mg 0.62 Zn 0.15 Al 0.15 Fe 3+ 0.08 ) VI (Al 1.61 Mg 0.23 Fe 3+ 0.16 )O 4 . Zinc was found to occupy only the tetrahedrally coordinated site and its presence strongly influenced intersite cation partitioning. In Zn-bearing crystals both intracrystalline exchanges Mg ⟷ Al and Mg ⟷ Fe 3+ turned out to be limited. Zinc in octahedral coordination, which is rare in natural spinels, was shown to be favoured by high Fe 3+ contents and high equilibration temperatures. The cation distributions determined experimentally were found to be in close agreement with those calculated using the general thermodynamic model for spinel solid solutions by O9Neill & Navrotsky (1984).

Structural refinement and crystal chemistry of Mn-doped spinel: A case for tetrahedrally coordinated Mn3+ in an oxygen-based structure

Abstract Spinel single crystals of four compositions along the MgAl2O4-MgMn2O4 join, with Mn3+ up to 0.25 apfu, were synthesized by use of a flux-growth method. The crystals were analyzed by electron microprobe, X-ray single-crystal diffraction, and optical absorption spectroscopy. Results revealed that Mg contents vary from 0.90 to 0.99 apfu, Mn2+ ≤ 0.11 apfu, Mn3+ varies from zero to 0.25 apfu, and Al-contents from 1.75 to 1.99 apfu. The unit-cell parameter increases linearly from 8.0883(3) to 8.1413(4) Å with increasing Mn3+ content. The crystals show moderately disordered cation distributions, with i = 0.23(1), and different distribution trends have been observed: the Mg content is constant at the T site and is replaced by Mn2+ at the M-site; Al decreases while Mn3+ increases in T- and M-sites. Mn3+ shows a preference for the T site, and a specific bond distance was refined, IVMn3+-O = 1.88(1) Å. Unpolarized room-temperature single-crystal spectra reveal two relatively broad absorption features at ca. 23 000 and 10 800 cm-1, which are assigned to spin-allowed d-d transitions in Mn3+ located at octahedral and tetrahedral sites, respectively. The bond valence approach shows that the bonds are strained in the tetrahedron indicating underbonding in T, whereas the bonds are unstrained in the octahedron. To reduce the M-M repulsion, the steric effect is driven by the movement of the oxygen atoms, which improves the shielding effect around the M-site, thus increasing the distortion of the structure relative to the CCP. As a result the tetrahedron undergoes an isotropic expansion, which constrains the structure to incorporate larger cations such as Mn3+ rather than smaller cations like Al at the T-site. This behavior, which is in disagreement with predictions based on crystal field energy considerations, illustrates the greater importance of steric factors on the cation distribution in spinels.

Mn-tourmaline crystals from island of Elba (Italy): Growth history and growth marks

Abstract The growth history reconstruction of tourmaline macro-crystals from pegmatite pockets in the aplite veins of Grotta d.Oggi (island of Elba, Italy) was carried out using a multi-disciplinary and multi-analytical approach [X-ray diffraction topography (XRDT) and electron microprobe analysis (EMPA)]. The work covered in this paper is closely related to the previous crystal-chemical characterization of the same macro-crystals (Bosi et al. 2005). The tourmalines examined here are bi-colored crystals exhibiting greenish-yellow to colorless zoning perpendicular to and parallel to the c axis: the transition from the inner yellow regions to the colorless ones occurs suddenly. XRDT and EMPA studies show relationships between color and chemical zoning and crystal growth evolution, thus identifying two main growth stages, which developed under different conditions. The first stage corresponds to greenish-yellow Mn-tourmaline grown as a result of a 2D-growth mechanism in a F-rich pegmatitic environment; the second one corresponds to colorless elbaitic tourmaline, which developed chiefly by a spiral growth mechanism in an OH-rich hydrothermal environment. The structural defects found in greenish-yellow Mn-tourmaline (growth bands, absence of dislocations, near-uniform chemical element concentrations) are the same as those observed in beryl crystals from pegmatite pockets of the island of Elba. Therefore these features are growth marks that characterize the pegmatitic crystallization stage and differentiate it from the hydrothermal stage, in which the dislocations normal to the growing faces occurred. Hence, the role of tourmaline as a petrogenetic indicator can be extended from chemical composition to crystal growth, reflecting both chemical environment and growth mechanism, and contributing to a better understanding of mineral genesis in pegmatitic crystallization.