Aspedamite, Ideally 12(Fe3+, Fe2+)3Nb4[Th(Nb,Fe3+)12O42] 12, a New Heteropolyniobate Mineral Species from the Herrebokasa Guarry, Aspedammen, Ostfold, Southern Norway: Description and Crystal Structure

Abstract

Aspedamite, ideally □ 12 (Fe 3+ ,Fe 2+ ) 3 Nb 4 [Th(Nb,Fe 3+ ) 12 O 42 ]{(H 2 O),(OH)} 12 , is a new heteropolyniobate mineral species from the Herrebokasa quarry, Aspedammen, Ostfold, 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/cm 3 , and the calculated index of refraction is 2.084. Aspedamite is cubic, space group Im 3, a 12.9078(6) A, V 2150.6(3) A 3 , Z = 2. The strongest seven lines in the X-ray powder-diffraction pattern [ d in A( 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 Nb 2 O 5 65.64, Ta 2 O 5 1.78, SiO 2 0.78, ThO 2 5.64, TiO 2 2.15, Fe 2 O 3 10.56, FeO 2.73, MnO 0.82, CaO 0.28, K 2 O 0.16, La 2 O 3 0.52, Ce 2 O 3 1.62, Nd 2 O 3 0.44, H 2 O (calc) 7.20, sum 100.32 wt.%; the H 2 O content was determined by crystal-structure analysis. The empirical formula of aspedamite on the basis of 54 anions with Fe 3+ /(Fe 3+ + Fe 2+ ) = 0.67 (estimated from crystal-chemical arguments) and O(4) = (H 2 O) 9 + (OH) 3 is K 0.09 Ca 0.13 Ce 0.26 La 0.08 Nd 0.07 Fe 2+ 1.00 Mn 0.30 Fe 3+ 3.48 Th 0.56 Ti 4+ 0.71 Si 0.34 Nb 12.98 Ta 0.21 O 42 (H 2 O) 9 (OH) 3 . The crystal structure of aspedamite was solved by direct methods and refined to an R 1 index of 1.6% based on 596 observed reflections collected on a three-circle rotating-anode (Mo K α X-radiation) diffractometer equipped with multilayer optics and an APEX-II detector. The structure is based on the heteropolyanion [ DA 12 O 42 ] ( D = Th, sum A = Nb 9 Fe 3+ 2 Ti), which consists of twelve face- and corner-sharing AO 6 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.