Abstract
Stavelotite-(La), simplified formula La3Mn2+3Cu(Mn3+,Fe3+,Mn4+)26[SiO2O7]6O30, occurs as an accessory opaque mineral in a late quartz vein cross-cutting the deep-purple, highly oxidised manganiferous phyllites of Ordovician age at Le Coreux near Salmchâteau, Belgium. Accompanying minerals are hematite, braunite, hollandite-strontiomelane, kanonaite, Mn-oxides, albite and muscovite. Stavelotite-(La) is trigonal, space group P31, with a = 11.525(2), c = 33.347(9) Å, V= 3836(1) Å3 and Z = 3. Its very complex, densely packed crystal structure consists of 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 directions in order to make connection with those of layers 2 and 4, thus forming the [Si2O7] groups, and it carries La in eightfold coordination. Layer 4 is of similar architecture as layer 2, but in an inverted position. Layers 1, 2 and 4 are geometrically similar to respective layers in the structure of the mineral långbanite. 65 electron microprobe analyses yielded the following mean composition (Mn valences calculated on the basis of structural data): 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; total 96.86 wt.% giving the empirical formula (45 cations, 72 oxygens) (La1.828Nd0.715Ce0.095Ca0.209Sc0.153)σ3.000 Mn2+2.998 (Cu0.941Mn3+0.058)σ0.999 (Mn3+14.183Fe3+5.814Al2.298 Ti0.195Sc0.604Mg0.933Co0.085Mn4+1.973)σ26.085 Si11.915 O72. Thus, the structural and chemical data are mutually consistent showing all crystallographic positions fully occupied. There are no indications for the presence of hydrogen or other light elements, which were not analysed. Stavelotite-(La) is the first REE silicate mineral containing only large amounts of Mn (with some Fe) as additional cations.
