Lithium-bearing fluor-arfvedsonite from Hurricane Mountain, New Hampshire: A crystal-chemical study

The structures of two crystals of Li-bearing fluor-arfvedsonite (1) (K0.32Na0.68)Na-2(Li0.48Fe2.832+Mn0.102+Zn0.06Fe1.463+Ti0.07) (Si7.88Al0.12)O-22[F-1.15(OH)(0.85)] and (2) (K0.25Na0.75)Na-2(Li0.48Fe2.842+Mn0.112+Zn0.05Fe1.453+Ti0.07)(Si7.89Al0.11)O-22[F-1.35(OH)(0.65)] from a granitic pegmatite, Hurricane Mountain, New Hampshire, have been refined to R indices of 1.5(1.6)% based on 1380(1387) reflections measured with MoK alpha X-radiation. The unit cell parameters are (1) a 9.838(4), b 17.991(6), c 5.315(2) Angstrom, beta 103.78(3)degrees, V 913.7 Angstrom(3) and (2) a 9.832(3), b 17.990(7), c 5.316(3) Angstrom, beta 103.79(3)degrees, V 913.2 Angstrom(3). Site-scattering refinement shows Li to be completely ordered at the M(3) site in these crystals. The amphibole composition is intermediate between fluor-arfvedsonite and fluor-ferro-leakeite with a small component (similar to 10%) of fluor-ferro-ferri-nyboite. These amphibole crystals project into miarolitic cavities in a pegmatitic phase of a riebeckite granite. The early-crystallizing amphibole is close to fluor-ferro-leakeite in composition, but becomes progressively depleted in Li and F as crystals project out into miarolitic cavities; the final amphibole to crystallize is a fibrous Li-poor riebeckite. Li plays a significant role in late-stage fractionation involving the crystallization of alkali amphibole in peralkaline granitic environments.