• journal_title：The Canadian Mineralogist
• Contributor：Mark A. Cooper ; Yassir A. Abdu ; Neil A. Ball ; Petr Černý ; Frank C. Hawthorne ; Roy Kristiansen
• Publisher：Mineralogical Association of Canada
• Date：2012-08-01
• Format：text/html
• Language：en
• Identifier：10.3749/canmin.50.4.793
• journal_abbrev：Can Mineral
• issn：0008-4476
• volume：50
• issue：4
• firstpage：793
• section：Mineralogy, Crystal Structure and Crystal Chemistry

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) ų, 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 K_0.09 Ca_0.13Ce_0.26La_0.08Nd_0.07Fe²⁺_1.00Mn_0.30Fe³⁺_3.48Th_0.56Ti⁴⁺_0.71Si_0.34Nb_12.98Ta_0.21O₄₂(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.