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Mineralogical ClassificationStavelotite-(La) (IMA 2004-014)

26th Oct 2005 18:27 UTCJim Ferraiolo

reference:

Stavelotite-(La), a new lanthanum-manganese-sorosilicate mineral from the Stavelot Massif, Belgium. E.-J. Bernhardt, T. Armbruster, A.-M. Fransolet & W. Schreyer, European Journal of Mineralogy, 17(5), 2005, pp 703–714.



abstract:

Stavelotite-(La) occurs as an accessory opaque mineral in a late quartz vein cross-cutting the deep purple manganiferous phyllites of Ordovician age at Le Coreux, near Salmchâteau, Belgium; it is associated with hematite, braunite, hollandite-strontiomelane, kanonaite, Mn-oxides, albite and muscovite. It appears as roundish to rectangular, generally equidimensional masses 10–160 µm in diameter; reflectance ~ 14 %; in reflected light it is grey; Dcalc 4.489 g/cm3. EPMA gave SiO2 20.17, TiO2 0.44, MnO2 4.83, Mn2O3 31.67, MnO 5.99, Al2O3 3.30, Fe2O3 13.08, Sc2O3 1.47, La2O3 8.39, Nd2O3 3.39, Ce2O3 0.44, CaO 0.33, MgO 1.06, CuO 2.11, CoO 0.18, = 96.86, leading to the simplified formula La3Mn2+3Cu(Mn3+,Fe3+,Mn4+)266O30. Indexed XRD powder data are tabulated; strongest lines 2.7232(100), 2.7789(40),1.6635(40), 5.446(31), 1.6687(28) Å; a 11.525, c 33.347 Å, V 3836 Å3, Z = 3, space group P31. The crystal structure has been determined and refined to R 13.32%; it is very complex and densely packed, with four different polyhedral layers stacked parallel to the c axis: layer 1 consists of eight-coordinated distorted cube sites for Mn2+ sharing edges with strongly distorted Mn3+ polyhedra (tetragonal bipyramids), and also contains one planar square site for Cu2+. Layer 2 has more regular octahedra for Mn3+ as well as for the Mn4+ required for charge balance, and it contains SiO4 tetrahedra oriented in one direction. Layer 3 contains additional SiO4 tetrahedra oriented in two different opposing direction in order to link with those of layers 2 and 4, thus forming groups, and it carries La in eightfold coordination. Layer 4 is of similar architecture to layer 2, but in an inverted position. Layers 1, 2 and 4 are geometrically similar to respective layers in the structure of långbanite. Stavelotite-(La) is the first REE silicate mineral to contain only large amounts of Mn (with some Fe) as additional cations

27th Oct 2005 15:30 UTCPeter Haas

Updated.
 
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