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Tourmaline Gemstones and Minerals

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Black Tourmaline Crystal

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Black Tourmaline Necklace

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Black Tourmaline Video




Black Tourmaline Bracelet

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BlackTourmaline Meaning
Mineralogy and Crystal Chemistry

The general formula of the tourmaline primitive unit cell is XY3Z6T6O18(BO3)3V3W, following Hawthorne & Henry (1999), where the site coordinations and main elements at the different sites are as follows: IXX: Na1+, Ca2+, □, K1+, Pb2+

VIY: Mg2+, Fe2+, Al3+, Li1+, Ti4+, Mn2+, Fe3+, Zn2+

VIZ: Al3+, Cr3+, Fe3+, Mg2+, Fe2+

IVT: Si4+, B3+, Al3+

IIIB: B3+

IVV: OH1–, O2–

IIIW: OH1–, F1–, O2–
Tourmaline is a ring-silicate mineral, with its unit cell consisting of a six-fold ring of tetrahedra (T sites) on top of a concentric arrangement of three Y-site and six Z-site octahedra (Fig. 3). The Z-site octahedra are somewhat smaller than the Y-site octahedra, whereas the latter are more distorted (Bosi & Lucchesi 2007). Within the six-fold ring, all tetrahedra point toward the layer of octahedra, and this arrangement results in the polar characteristics of tourmaline, because it prohibits the presence of symmetry elements perpendicular to the long axis. The X site sits in a nine-coordinated polyhe- dron situated on top of the six-fold ring (Fig. 3). Three trigonal boron polyhedra are further present within the layer of octahedra in the unit cell, roughly perpendicular to the c axis. The V and W sites are anion sites that are occupied by OH1– or O2– (or both) at the V site, and OH1–, F1–, or O2– at the W site. The Cl contents are generally negligible. Extensive effort has been, and is, directed to identi- fying and understanding the cation and anion occupancy among the different sites of the tourmaline structure (e.g., Grice & Ercit 1993, Hawthorne 2002, Bosi & Lucchesi 2004, Pieczka & Kraczka 2004, Bosi & Lucchesi 2007, Bosi 2008, van Hinsberg & Schumacher 2009b, Bosi 2011, Ertl et al. 2011, Henry & Dutrow 2011, Hughes et al. 2011, Clark et al. 2011). The stable site-occupancy will be the state that minimizes the overall structural strain in the crystal lattice, as well as approach local charge-balance, and site occupancies for specic elements may therefore change as the bulk composition of a tourmaline changes (e.g., along the dravite to schorl series). Although there is a signicant degree of consensus about site occupancies, there are still uncertainties. The assignment of cations to the Y and Z sites, in particular, can be ambiguous owing to potential disorder of cations over these sites, especially in tourmalines that contain appreciable amounts of O2– at the W site. The Y- and Z-site disorder in oxy- tourmaline is generally manifested by disorder of diva- lent cations, particularly Mg2+, at the Z site and trivalent
cations, particularly Al3+, at the Y site (Hawthorne et al. 1993). The incorporation of Fe2+ at the Z site related to this type of disorder is more uncertain, but has prompted a number of investigations dealing with this problem (e.g., Bosi & Lucchesi 2004, Pieczka & Kraczka 2004, Ertl et al. 2006, 2010b, Bosi 2008, 2011). Bosi et al. (2010) reported a suite of samples that show both total order of Fe2+ at the Y site, and disorder over the two octahedral sites (albeit with strong preference for Fe2+ at the Y site), suggesting that the presence and extent of order–disorder of Fe depend on bulk composition of a tourmaline. The presence and extent of B and Al substitution at the T site have been debated (e.g., Henry & Dutrow 1996, Hughes et al. 2000), but is now widely accepted as more precise structural renements and light-element analyses are presented (e.g., Marschall et al. 2004, Ertl et al. 2008, Lussier et al. 2011, Clark et
al. 2011). In fact, tetrahedrally coordinated B can relieve both the structural strain and excess charge of incorpo- ration of Al3+ at the Y site (Bosi et al. 2010). Fluorine is known to be restricted to the W site in tourmaline, together with OH1– and O2– (Hawthorne & Henry 1999). The amount of F1– that can be incorporated 6 tHe canaDian MineRaloGiSt into tourmaline is a function of both crystallographic constraints and the local petrological environment (Henry & Dutrow 2011). Potassium and Cl contents of tourmaline are most commonly at the detection limit of the electron microprobe, owing to their large ionic radii compared to Na and OH, respectively. However, rare K-rich tourmaline has been found (e.g., Grice et al. 1993, Ota et al. 2008b). Given the range of elements that can be accom- modated by the tourmaline structure, a diversity of tourmaline compositions ensues, and hence a large number of tourmaline species. Based on the chemical and structural framework of the tourmaline-supergroup minerals, a new nomenclature has recently been adopted by the International Mineralogical Association’s Commission on New Minerals, Nomenclature and Classification (IMA–CNMNC) (Novák et al. 2009, Henry et al. in press). The tourmaline species that are currently accepted by the IMA–CNMNC include those shown in Table 1. In addition to the major-element occupation of the sites, tourmaline accommodates a range of trace elements. This capacity is due to the marked differ- ences in morphology and coordination of the various structural sites in the tourmaline structure, which allows elements of widely varying charge and ionic radius to be incorporated. Henry & Dutrow (1996) listed a compila- tion of the maximum concentrations in tourmaline for a suite of minor and trace elements, and an updated list can be found on D. Henry’s webpage (www.geol.lsu. edu/ henry/Research/tourmaline/TourmalineWorldRe- cords.htm). Where present at trace levels, the site distri- bution of these trace elements can be inferred from their partitioning behavior with melts (van Hinsberg 2011).


Black Tourmaline Ring

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