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

Non-Destructive In Situ Study of Plastic Deformations in Diamonds: X-ray Diffraction Topography and µFTIR Mapping of Two Super Deep Diamond Crystals from São Luiz (Juina, Brazil)

“…According to the XRDT data, diamond BZ270 consists of an aggregate of slightly misoriented grains (G) that are not visible simultaneously at the individual angular settings due to the different diffraction vector (Figure 2 In the topographs of Figure 2c,d, the diffraction contrast corresponding to two slightly different angular settings of the same diffraction vector (g = [2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20] shows that the diamond region containing the ferro-periclase inclusions is really a grain (G) that is differently oriented with respect to the other diamond regions. In the Figure 2c G is out of contrast, whereas in Figure 2d G is in diffraction contrast and the surrounding grains are out of contrast.…”

Oxy-dravite, Na(Al2Mg)(Al5Mg)(Si6O18)(BO3)3(OH)3O, a new mineral species of the tourmaline supergroup

“…They are known as valuable indicator minerals that can provide information on the compositional evolution of their host rocks, chiefly due to their ability to incorporate many elements (e.g., Novák et al 2004Novák et al , 2011Agrosì et al 2006;Lussier et al 2011a;van Hinsberg et al 2011). However, the chemical composition of tourmalines is also strongly controlled by various crystal-structural constraints (e.g., Hawthorne 1996Hawthorne , 2002aBosi 2010Bosi , 2011Skogby et al 2012) as well as by temperature (e.g., van Hinsberg and Schumacher 2011).…”

Iron redox reactions in the tourmaline structure: High-temperature treatment of Fe3+-rich schorl

“…Tourmaline, the most widespread borosilicate mineral, is commonly used for understanding crustal evolution (e.g., London, 1999;Kasseman et al, 2000;Agrosì et al, 2006;Lussier et al, 2011;van Hinsberg et al, 2011 and references therein). It is a typical accessory mineral, and occasionally even major mineral, in rocks of highly variable origin and chemical composition.…”