Abstract

Aspedamite, ideally □12(Fe3+,Fe2+)3Nb4[Th(Nb,Fe3+)12O42]{(H2O),(OH)}12, is a new heteropolyniobate mineral species from the Herrebøkasa quarry, Aspedammen, Østfold, southern Norway. It occurs as small euhedral crystals of dodecahedra and cubes to a maximum of 50 μm across, perched on a white mat of an Al–Nb–Fe–Ti–Ca–K-bearing silicate on a partly altered 12 × 12 × 6 mm crystal of monazite penetrated by plates of columbite-(Fe) and muscovite. Aspedamite is brownish orange with a very pale orange streak and an adamantine luster; it does not fluoresce under ultraviolet light. The Mohs hardness is 3–4, and it is brittle with a hackly fracture. The calculated density is 4.070 g/cm3, and the calculated index of refraction is 2.084. Aspedamite is cubic, space group Im3̅, a 12.9078(6) Å, V 2150.6(3) Å3, Z = 2. The strongest seven lines in the X-ray powder-diffraction pattern [d in Å(I)hkl] are: 9.107(100)011, 2.635(36)224, 2.889(33)024, 1.726(29)246, 3.233(28)004, 3.454(18)123, 4.567(15)022. A chemical analysis with an electron microprobe gave Nb2O5 65.64, Ta2O5 1.78, SiO2 0.78, ThO2 5.64, TiO2 2.15, Fe2O3 10.56, FeO 2.73, MnO 0.82, CaO 0.28, K2O 0.16, La2O3 0.52, Ce2O3 1.62, Nd2O3 0.44, H2O (calc) 7.20, sum 100.32 wt.%; the H2O content was determined by crystal-structure analysis. The empirical formula of aspedamite on the basis of 54 anions with Fe3+/(Fe3+ + Fe2+) = 0.67 (estimated from crystal-chemical arguments) and O(4) = (H2O)9 + (OH)3 is K0.09 Ca0.13Ce0.26La0.08Nd0.07Fe2+1.00Mn0.30Fe3+3.48Th0.56Ti4+0.71Si0.34Nb12.98Ta0.21O42(H2O)9(OH)3. The crystal structure of aspedamite was solved by direct methods and refined to an R1 index of 1.6% based on 596 observed reflections collected on a three-circle rotating-anode (MoKα X-radiation) diffractometer equipped with multilayer optics and an APEX-II detector. The structure is based on the heteropolyanion [DA12O42] (D = Th, sum A = Nb9Fe3+2Ti), which consists of twelve face- and corner-sharing AO6 octahedra that surround the [12]-coordinated D cation. There are eight heteropolyanions at the corners of the unit cell, with an additional heteropolyanion at the center, forming an I-centered arrangement. Each heteropolyhedral cluster is decorated by eight B octahedra, each of which bridges two adjacent clusters along the body diagonals of the cell. Further intercluster linkage is provided by the C octahedra, which link pairs of adjacent clusters in the a direction. Aspedamite is isostructural with menezesite.

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