Darrellhenryite, Na(LiAl_2)Al_6(BO_3)_3Si_6O_(18)(OH)_3O, a new mineral from the tourmaline supergroup

Abstract

Darrellhenryite, Na(LiAl_2)Al_6(BO_3)_3Si_6O_(18)(OH)_3O, a new member of the tourmaline supergroup (related to the alkali-subgroup 4), is a new Li-bearing tourmaline species, which is closely related to elbaite through the substitution ^Y(Al)^W_(0.5)O_1^YLi^W_(−0.5)(OH)_(−1). It occurs in a complex (Li-bearing) petalite-subtype pegmatite with common lepidolite, Li-bearing tourmalines, and amblygonite at Nová Ves near Český Krumlov, southern Bohemia, Moldanubian Zone, Czech Republic. This zoned pegmatite dike cross-cuts a serpentinite body enclosed in leucocratic granulites. Pink darrellhenryite forms columnar crystals (sometimes in parallel arrangement) up to 3 cm long and up 2 cm thick, associated with albite (var. cleavelandite), minor quartz, K-feldspar, petalite, rare polylithionite, and locally rare pollucite. The optical properties and the single-crystal structure study (R1 = 0.019) of darrellhenryite are consistent with trigonal symmetry, ω = 1.636(2), ɛ = 1.619(2), birefringence: 0.017, space group R3m, ɑ = 15.809(2), c = 7.089(1) Å, V = 1534.4(4) Å^3, and Z = 3. The chemical analysis, in combination with the results from the single-crystal structure refinement, gives the formula ^X(Na_(0.58)Ca_(0.01)□_(0.41))_(1.00)^Y(Li_(1.05)Al_(1.95))_(3.00)^ZAl_6 (BO_3)_3^T(Si_6O_(18)) ^V(OH)_3^W(O_(0.66)F_(0.34))_(1.00), which can be simplified to an ideal formula of Na(LiAl_2)Al_6(BO_3)_3Si_6O_(18)(OH)_3O. The strongest lines of the powder pattern [d in Å (I, hkl)] are 4.180 (39, 211), 3.952 (54, 220), 3.431 (73, 012), 2.925 (100, 122), 2.555 (90, 051), 2.326 (42, 511), 2.029 (42, 223), 2.021 (42, 152), 1.901 (50, 342), 1.643 (49, 603). The density is D_(meas) = 3.03(3) g/cm^3, D_(calc) = 3.038 g/cm^3. Darrellhenryite is considered to have crystallized in Li- and B-rich but F-moderate environments in complex pegmatites; no influence of higher activity of O on the darrellhenryite formation is implied from its mineral assemblage. The name is for Darrell J. Henry, Professor of Geology at the Louisiana State University, Baton Rouge, U.S.A., an expert on the mineralogy, petrology, crystal chemistry, and nomenclature of tourmaline-supergroup minerals.

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