Single crystals of dithulium disilicate, Tm2Si2O7, were obtained in flux synthesis experiments in the system SiO2–Tm2O3–LiF at ambient pressure. The compound belongs to the group of sorosilicates, i.e. it is based on [Si2O7]-units and crystallizes in the thortveitite (Sc2Si2O7) structure type. The Tm3+ cation (site symmetry .2.) occupies a distorted octa­hedral site, with Tm—O bond lengths in the range 2.217 (4)–2.289 (4) Å. Each of the octa­hedra shares three of its edges with adjacent [TmO6] groups, resulting in the formation of layers parallel to (001). The individual [SiO4] tetra­hedra are more regular, i.e. the differences between the bond lengths between Si and the bridging and non-bridging O atoms are not very pronounced. The layers containing the octa­hedra and the sheets containing the [Si2O7] groups (point group symmetry 2/m) form an alternating sequence. Linkage is provided by sharing common oxygen vertices.

The Meran-Mauls nappe stack is part of the Austroalpine unit in South Tyrol (Italy). There it holds a special position directly in front of the Southalpine Dolomites indenter and west of the Tauern Window. It is situated in the hanging wall of the Southalpine unit, above a NW dipping segment of the Periadriatic fault system, namely the Meran-Mauls fault. Also all other sides are defined by Oligocene-Miocene strike-slip and normal faults. Based on recent mapping the Meran-Mauls nappe stack consists of three nappes separated by NW to NNW dipping shear zones. The lowermost nappe in the southwest is represented by the Schenna (Scena) unit. It is overlain along the Masul shear zone by a nappe consisting of the Hirzer (Punta Cervina) unit and the Pens (Pennes) unit including Triassic (meta)sediments. Separated by the Fartleis fault the St. Leonhard (San Leonardo) unit forms the uppermost nappe. The aim of this study is to describe the individual units and the separating structural elements more properly, based on new structural, petrological, geothermobarometric and geochronological data and to compare these units to other Austroalpine elements in the vicinity. Sillimanite-bearing paragneiss, minor amphibolite and quartzite as well as a distinct marble layer close to its base characterise the Schenna unit. Further, it contains pegmatite dikes, presumably Permian in age. Amphibolite-facies P-T conditions of c. 0.55 ± 0.15 GPa and 600 ± 100°C are thus correlated with a Permian metamorphic imprint. The Masul shear zone mostly consists of mylonitic paragneiss of the Hirzer unit. It is pre-Alpine in age and probably formed during the Jurassic. For the paragneiss of the Hirzer unit upper greenschist- to amphibolite-facies metamorphic conditions of 0.4-0.50 ± 0.15 GPa and 550 ± 70°C are attributed to the Variscan tectonometamorphic imprint. The whole Pens unit represents a shear zone. Due to the occurrence of Permotriassic (meta)-sediments within this shear zone, it is an Alpine structure, as well as the bordering Fartleis fault. Rb/Sr biotite ages yield sometimes partly reset pre-Alpine age values in the whole Meran-Mauls nappe stack, indicating a pervasive anchizonal to lowermost greenschist-facies metamorphic overprint during the Eoalpine tectonometamorphic event. Tectonostratigraphically the Meran-Mauls nappe stack can be attributed to the Drauzug-Gurktal nappe system. The latter forms the uppermost structural element of the Austroalpine nappe stack and thus only shows a weak Eoalpine metamorphic overprint. With respect to its special lithologic composition the Schenna unit can be correlated with the Tonale unit in the southwest and the Strieden-Komplex in the east.

Thortveitite-Type Tm 2Si2O7. -Single crystals of Tm2Si2O7 are obtained via flux synthesis in the system SiO 2-Tm2O3-LiF with a molar ratio of Si:Tm = 4:1 at ambient pressure (sealed Pt tube, 1373 K, 2 h). The compound crystallizes in the monoclinic space group C2/m with Z = 2 (Sc 2Si2O7 type structure, single crystal XRD). The structure is based on [Si 2O7]-units with the Tm 3+ cations occupying a distorted octahedral site. Each octahedron shares three of its edges with adjacent TmO 6 octahedra resulting in the formation of layers. These layers alternate with the sheets containing the [Si 2O7] groups. -(KAHLENBERG*, V.; AICHHOLZER, P.; Acta Crystallogr., Sect. E: Struct. Rep. Online 70 (2014) 7, i34-i35, http://dx.doi.org/10.1107/S1600536814013142 ; Inst. Mineral. Petrogr., Leopold-Franzens-Univ., A-6020 Innsbruck, Austria; Eng.) -J. Schramke 39-006