Eugenio Scandale

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.

Tsilaisite, NaMn3Al6(Si6O18)(BO3)3(OH)3OH, a new mineral species of the tourmaline supergroup from Grotta d’Oggi, San Pietro in Campo, island of Elba, Italy

Abstract Tsilaisite, NaMn3Al6(Si6O18)(BO3)3(OH)3OH, is a long-expected new mineral of the tourmaline supergroup. It occurs in an aplitic dike of a LCT-type pegmatite body from Grotta d’Oggi, San Pietro in Campo, island of Elba, Italy, in association with quartz, K-feldspar, plagioclase, elbaite, and schorl. Crystals are greenish yellow with a vitreous luster, a white streak, and show no fluorescence. Tsilaisite has a Mohs hardness of approximately 7; it is brittle with a sub-conchoidal fracture, and has a calculated density of 3.133 g/cm3. In plane-polarized light, tsilaisite is pleochroic, O = pale greenish yellow, E =very ale greenish yellow; it is uniaxial negative, ω = 1.645(5), Ɛ = 1.625(5). Tsilaisite is rhombohedral, space group R3m, a = 15.9461(5), c = 7.1380(3) Å, V = 1571.9(1) Å3, Z = 3. The strongest eight ‑raydiffraction lines in the powder pattern [d in Å(I)(hkl)] are: 3.974(100)(220), 2.942(94)(122), 2.570(79) (051), 2.034(49)(152), 4.205(41)(211), 6.329(22)(101), 2.377(21)(003), and 1.592(21)(550). Analysis by a combination of electron microprobe, secondary ion mass spectrometry, and optical absorption spectroscopy gives SiO2 = 36.10(3), TiO2 = 0.32(4), Al2O3 = 37.10(5), MnO = 9.60(10), CaO = 0.09(4), Na2O = 2.11(7), K2O = 0.03(1), F = 0.79(3), B2O3 = 10.2(6), Li2O = 0.8(1), H2O = 3.1(2), sum 99.95 wt%. The unit formula is X(Na0.67⃞0.30Ca0.02K0.01)Y(Mn2+1.34Al1.14Li0.54Ti0.04)ZAl6T(Si5.94Al0.06)B2.91O27V(OH)3W(OH0.39F0.41O0.20). The structure, refined also taking into account the positional disorder of the O1 and O2 anions, converged to statistical indices R1 for all reflections of about 2%. The resulting site populations indicate that the Z site is occupied by Al and that the Y site is dominated by Mn2+. Aluminum is incorporated at Y through two types of substitutions: YAl+WO2- → YMn2++WOH, which has the result of replacing OH at the W site by O2-, and Y(Al+Li)+WF → 2YMn2++WOH, which relates fluor-elbaite to the tsilaisite component. Infrared absorption spectra measured in the principal OH-stretching region were interpreted on the basis of local arrangements consistent with the short-range bond-valence requirements. A compositional trend from fluor-elbaite to tsilaisite is observed in samples from Elba Island. The occurrence of tsilaisite is very rare in nature, as a consequence of both the requirement of extraordinary petrogenetic conditions and limited structural stability.

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.

Transmission Electron Microscope Study of Experimentally Deformed K-Feldspar Single Crystals

AbstractA set of sanidine single crystals were previously deformed at 700° C in a Griggs triaxial press with different crystallographic orientations of the core so as to induce dislocation glide of different slip systems respectively. Deformed crystals have been studied by transmission electron microscopy (TEM) and the activated slip systems have been characterized for two orientations.(010)[001] and (001)1/2[

General and Specific Growth Marks in Pegmatite Beryls

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Fluor-tsilaisite, NaMn3Al6(Si6O18)(BO3)3(OH)3F, a new tourmaline from San Piero in Campo (Elba, Italy) and new data on tsilaisitic tourmaline from the holotype specimen locality

10] systems expected for one orientation (main stress nearly parallel to [012]) are observed, whereas the (001)[100] system expected for the other orientation (main stress nearly parallel to [101]) is never observed. In the latter specimen the deformation is rather difficult and occurs through unexpected systems characterized as (110)1/2[1

A 94-layer long-period mica polytype: A TEM study

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