ASPEDAMITE, IDEALLY □12(Fe~(3+),Fe~(2+))_3Nb_4[Th(Nb,Fe~(3+))_(12)O_(42)]{(H_2O),(OH)}_(12), A NEW HETEROPOLYNIOBATE MINERAL SPECIES FROM THE HERREBOKASA QUARRY, ASPEDAMMEN, 0STFOLD, SOUTHERN NORWAY: DESCRIPTION AND CRYSTAL STRUCTURE

摘要

Aspedamite, ideally □_(12)(Fe~(3+),Fe~(2+))_3Nb_4[Th(Nb,Fe~(3+))_(12)O_(42)]{(H_2O),(OH)}_(12), is a new heteropolyniobate mineral species from the Herrebokasa quarry, Aspedammen, Ostfbld, 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 Im3, 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_2O_5 65.64, Ta_2O_5 1.78, SiO_2 0.78, ThO_2 5.64, TiO_2 2.15, Fe_2O_3 10.56, FeO 2.73, MnO 0.82, CaO 0.28, K_2O 0.16, La_2O_3 0.52, Ce_2O_3 1.62, Nd_2O_3 0.44, H_2O (calc.) 7.20, sum 100.32 wt.%; the H_2O 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_2O)_9 + (OH)_3 is K_(0.09) Ca_(0.13)Ce_(0.26)La_(0.08)Nd_(0.07)Fe_(1.00)~(2+)Mn_(0.30)Fe_(3.48)~(3+)Th_(0.56)Ti_(0.71)~(4+)Si_(0.34)Nb_(12.98)Ta_(0.21)O_(42)(H_2O)_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 (MoKα 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_9Fe_2~(3+)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 /-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.