Fluor-schorl, a new member of the tourmaline supergroup, and new data on schorl from the cotype localities

Abstract

Fluor-schorl, NaFe^(2+) _3Al_6Si_6O_(18)(BO_3)_3(OH)_3F, is a new mineral species of the tourmaline supergroup from alluvial tin deposits near Steinberg, Zschorlau, Erzgebirge (Saxonian Ore Mountains), Saxony, Germany, and from pegmatites near Grasstein (area from Mittewald to Sachsenklemme), Trentino, South Tyrol, Italy. Fluor-schorl was formed as a pneumatolytic phase and in high-temperature hydrothermal veins in granitic pegmatites. Crystals are black (pale brownish to pale greyish-bluish, if <0.3 mm in diameter) with a bluish-white streak. Fluorschorl is brittle and has a Mohs hardness of 7; it is non fluorescent, has no observable parting and a poor/indistinct cleavage parallel to {0001}. It has a calculated density of ~3.23 g/cm^3. In plane-polarized light, it is pleochroic, O = brown to grey-brown (Zschorlau), blue (Grasstein), E = pale grey-brown (Zschorlau), cream (Grasstein). Fluor-schorl is uniaxial negative, ω = 1.660(2)–1.661(2), ɛ = 1.636(2)–1.637(2). The mineral is rhombohedral, space group R3m, ɑ = 16.005(2), c = 7.176(1) Å, V = 1591.9(4) Å^3 (Zschorlau), ɑ = 15.995(1), c = 7.166(1) Å, V = 1587.7(9) Å^3 (Grasstein), Z = 3. The eight strongest observed X-ray diffraction lines in the powder pattern [d in Å (I)hkl] are: 2.584(100)(051), 3.469(99)(012), 2.959(83)(122), 2.044(80)(152), 4.234(40)(211), 4.005(39)(220), 6.382(37)(101), 1.454(36)(514) (Grasstein). Analyses by a combination of electron microprobe, secondary-ion mass spectrometry (SIMS), Mössbauer spectroscopic data and crystal-structure refinement result in the structural formulae ^X(Na_(0.82)K_(0.01)Ca_(0.01)□_(0.16)) ^Y(Fe^(2+) _(2.30)Al_(0.38)Mg_(0.23)Li_(0.03)Mn^(2+) _(0.02)Zn_(0.01)□_(0.03))_(∑3.00) ^Z(Al_(5.80)Fe^(3+) _(0.10)Ti^(4+) 0.10) ^T(Si_(5.81)Al_(0.19)O_(18)) (BO_3)_3 ^V(OH)_3 ^W[F_(0.66)(OH)_(0.34)] (Zschorlau) and ^X(Na_(0.78)K_(0.01)□_(0.21)) ^Y(Fe^(2+)_(1.89)Al_(0.58)Fe^(3+)_(0.13)Mn^(2+)_(0.13)Ti^(4+)_(0.02)Mg_(0.02)Zn_(0.02)□_(0.21))_(∑3.00) ^Z(Al_(5.74)Fe^(3+)_(0.26)) ^T(Si_(5.90)Al_(0.10)O_(18)) (BO_3)_3 ^V(OH)_3 W[F_(0.76)(OH)_(0.24)] (Grasstein). Several additional, newly confirmed occurrences of fluor-schorl are reported. Fluor-schorl, ideally NaFe^(2+) _3Al_6Si_6O_(18)(BO_3)_3(OH)_3F, is related to end-member schorl by the substution F → (OH). The chemical compositions and refined crystal structures of several schorl samples from cotype localities for schorl (alluvial tin deposits and tin mines in the Erzgebirge, including Zschorlau) are also reported. The unit-cell parameters of schorl from these localities are slightly variable, ɑ = 15.98–15.99, c = 7.15–7.16 Å, corresponding to structural formulae ranging from ~^X(Na_(0.5)□_(0.5)) ^Y(Fe^(2+) _(1.8)Al_(0.9)Mg_(0.2)□_(0.1)) ^Z(Al_(5.8)Fe^(3+) _(0.1)Ti^(4+) _(0.1)) ^T(Si_(5.7)Al_(0.3)O_(18)) (BO_3)_3 ^V(OH)_3 ^W[(OH)_(0.9)F_(0.1)] to ~^X(Na_(0.7)□_(0.3)) ^Y(Fe^(2+) _(2.1)Al_(0.7)Mg_(0.1)□_(0.1)) ^Z(Al_(5.9)Fe^(3+) _(0.1)) ^T(Si_(5.8)Al_(0.2)O_(18)) (BO_3)_3 ^V(OH)_3 ^W[(OH)_(0.6)F_(0.4)]. The investigated tourmalines from the Erzgebirge show that there exists a complete fluor-schorl–schorl solid-solution series. For all studied tourmaline samples, a distinct inverse correlation was observed between the X–O2 distance (which reflects the mean ionic radius of the X-site occupants) and the F content (r^2 = 0.92). A strong positive correlation was found to exist between the F content and the distance (r^2 = 0.93). This correlation indicates that Fe^(2+)-rich tourmalines from the investigated localities clearly tend to have a F-rich or F-dominant composition. A further strong positive correlation (r^2 = 0.82) exists between the refined F content and the Y–W (F,OH) distance, and the latter may be used to quickly estimate the F content.

Files

Additional details