Julie B. Selway

Schorl-oxy-schorl to dravite-oxy-dravite tourmaline from granitic pegmatites; examples from the Moldanubicum, Czech Republic

Wet-chemical analyses (41) of tourmaline from granitic pegmatites (barren, barren-pocket, beryl, lepidolite types) in the Moldanubicum, Czech Republic revealed that members of the oxy-subgroup ĄV common oxy-schorl, minor oxy-dravite and rare oxy-foitite are more abundant relative to the relevant members of the hydroxy-subgroup. The primary substitution mechanisms in tourmaline show combination of heterovalent substitutions: YAlWO YR2+-1W(OH)-1, XďYAl2WO XNa-1YR2+-2W(OH)-1, XďYAl XNa-1YR2+-1 and Xď W(OH) XNa-1WO-1, and homovalent substitutions: Fe2+Mg-1 and (OH)F-1. Tourmalines with the chemistry expressed by the general formula X(Na0.5ď0.5)Y(R2+2Al)ZAl6(BO3)3Si6O18V(OH)3W(O0.5OH0.5) crystallized in very similar PT conditions in granitic systems saturated on Na, Al, Si and H2O, it indicates the importance of short-range order requirements on tourmaline chemical composition. Abundance of heterovalent substitutions involving the W-site requires determination of light elements (H, B, F, Li) and Fe2+/Fe3+ in tourmalines to specify substitution mechanisms with certainty. Normalization of electron-microprobe data of (Fe,Mg)-rich, (Ca,Li,F)-poor tourmalines from granitic pegmatites on (OH,F)3.5O0.5, which is more probable than (OH,F)4, seems to be suitable.

Rossmanite, ⃞(LiAl2)Al6(Si6O18)(BO3)3(OH)4, a new alkali-deficient tourmaline: Description and crystal structure

Abstract Rossmanite is a new tourmaline species from near Rožná, western Moravia, Czech Republic. It forms pale pink columnar crystals about 25 mm long and 5 mm thick, elongaten along c with striations parallel to c on the prism faces. It is brittle, H= 7, Dmeas = 3.00 g/cm3, Dcalc = 3.06 g/cm3. In plane-polarized light, it is colorless. Rossmanite is uniaxial negative, to = 1.645(1), e = 1.624(1), trigonal, space group R3m, in the hexagonal setting a = 15.770(2), c = 7.085(1) Å, V = 1525.8(4) Å3, Z= 3. The strongest six X-ray diffraction lines in the powder pattern are at d = 3.950 Å with I = 100% for (hkl) = (220); 2.552 Å, 93%, (051); 1.898 Å, 72%, (342); 4.181 Å, 58%, (211); 2.924 Å, 56%, (122); and 3.434 Å, 53%, (012). Analysis by a combination of electron microprobe, SIMS, H-line extraction, and crystal-structure refinement gave SiO2 38.10 wt%, Al2O3 44.60, Na2O 1.43, Li2O = 1.13, B2O3 = 10.88, H2O = 3.70, F = 0.20, O ≡ F 0.08, sum = 99.96 wt%, Fe, Mg, Ca, Mn, Ti, F, K not detected. The formula unit (31 anions) is x(⃞57Na0.43)Y(Li0.71Al2.17)zAl6(Si5.92O18) (B2.92O9)(OH)3.83F0.10O0.07, with the ideal end-member formula ⃞(LiAl2)Al6(Si6O18)(BO3)3(OH)4; thus rossmanite can be derived from elbaite [Na(Al1.5Li1.5)(Si6O18)(BO3)3(OH)4] by the substitution x⃞2 + YAl → xNa2 + YLi, where ⃞ = vacancy. The crystal structure of rossmanite was refined to an R index of 1.7% using 1094 observed (5σ) reflections collected with MoKα X-radiation from a single crystal. The structure refinement confirmed the low occupancy of the X site and the presence of Li at the Y site. There is considerable positional disorder at the O1 and O2 sites induced by the local variations in bond-valence distribution associated with ⃞-Na disorder at X and Li-Al disorder at Y.

Compositional evolution of tourmaline in the petalite-subtype Nyköpingsgruvan pegmatites, Utö, Stockholm Archipelago, Sweden

Abstract The classic petalite-subtype Nyköpingsgruvan pegmatites are located on the northern part of Utö Island, Stockholm archipelago, Sweden. They consist of two genetically related pegmatite bodies, the southern (minor) and northern (major), transecting the Nyköpingsgruvan iron formation. The pegmatite zones are named according to their most characteristic mineral: (1) spodumene, (2) pink K-feldspar + albite intergrowth, (3) lepidolite, (4) coarse sacchardoidal albite (>1 mm), (5) petalite, and (6) fine saccharoidal albite (<1–2 mm). The internal tourmaline composition evolves through the following crystallization sequence: Al-rich schorl → phases intermediate between schorl and elbaite → “fluor-elbaite” with variable Fe contents → elbaite → phases intermediate between elbaite and rossmanite → Ca-bearing elbaite-rossmanite → Ca-bearing elbaite. The dominant substitutions are Na ⇄ □ (□ = vacancy) with minor Ca variation at the X site, and 2Fe2+ ⇄ Al + Li at the Y site. The negative correlation between Fe and (Al + Li) and between Fe and Mn in tourmaline is due to fractionation of the pegmatite melt. The negative correlation between □ at the X site and F at the O(1) site is caused by crystal-chemical constraints and controlled by f(F2). The presence of Ca-bearing elbaite-rossmanite, Ca-bearing elbaite, apatite and microlite in the fractionated pegmatite zones indicates that late-stage Ca-enrichment is probably due to conservation of Ca during consolidation of the pegmatite by Ca-F complexes in the melt. Most tourmaline throughout the pegmatites is veined by cookeite, indicating decreasing salinity and low F-activity in the late low-temperature hydrothermal fluids. In the BIF, the exocontact tourmaline is dominantly schorl-dravite with variable Ca contents and in the aplitic veinlets within the iron formation, the exocontact tourmaline is dominantly (Ca, Mg)-bearing schorl, whereas “fluor-elbaite”-schorl dominates mica schist along the contacts with the pegmatites. In the contaminated, dolomitic-marble-hosted Grundberg outcrop, the endomorphic tourmaline is Li-rich (mainly liddicoatite and elbaite) with (Mg, Fe)-rich fracture-infillings (mainly dravite and schorl).