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The four twin laws of calcite (from Bruno et al., 2010). P-parent, T-twin. 

The four twin laws of calcite (from Bruno et al., 2010). P-parent, T-twin. 

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Marbles in Pohorje occur in lenses and smaller bodies in the southern and southeastern part of the massif. Marbles are very pure, predominantly calcitic and rarely calcitic-dolomitic, containing a maximum of 5 % of non-carbonate mineral phases. The latter comprise pyroxenes (diopside), amphiboles (tremolite), olivines (forsterite) in places replace...

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... (CaCO3, space group R¯ 3c) has dimen- sions of the hexagonal structural unit cell a=4.99 Å and c=17.06 Å. Twinning in calcite has been known for more than 150 years. Four different kinds of twins occur in naturally grown calcite fig. 1), which include all possible twins that may form either during crystal growth or by deforma- tion ( Wenk et al., 1983;Bruno et al., 2010). These twin laws are expressed by the twin planes c = {00¯ 11}, r = {10¯ 14} , e = {01¯ 18}, and f = {01¯ 12} which coincide with the original composition planes. The main deformation twin law of calcite is on e-planes (Weiss & Turner, 1972;Barber & Wenk, 1979;Bueble & Schmahl, 1999), for which the shear displacement is in positive sense, in the direction < 0¯ 221 > (fi g. 2a). The twinned (lower) layers of a crystal, which has positive e-axis upwards, are displaced in a sense opposed to the positive e-axis of the untwined layers. During deformation twin- ning the e-axis moves through an angle of 52.5 ° while the plane of the carbonate groups, which is perpendicular to the e-axis, must be rotated through the same angle ( Barber & Wenk, 1979). Minor deformation twining in calcite may be ob- served also on r-planes and f-planes (Paterson & Turner, 1970), where the sense of r-twinning is positive (Weiss & Turner, 1972) and r-plane is the usual slip plane ( Barber & Wenk, 1979). Dolomite (Ca 0.5 Mg 0.5 CO 3 , space group R¯ 3) has lower symmetry than calcite due to the alternat- ing layers of Ca and Mg atoms arranged parallel to the basal plane. Ca can exist in excess up to 0.25 apfu (atoms per formula unit) in non-stoichi- ometric dolomite, which are maximum substitu- tions the formula support (dos Santos et al., 2017). Additionally, Mg ions in dolomite may be partly substituted by Fe ions, producing isostructural mineral ankerite, or rarely Mn ions giving exotic mineral kutnahorite. For stoichiometric dolomite the lattice parameters of the hexagonal structur- al unit cell are a=4.81 Å and c=16.01 Å. Twin laws, also applying to deformation twinning, are de- pendent on symmetry characteristics (e.g. Dana et al., 1951). As expected from its lower symmetry, dolomite possesses fewer twin forms than calcite, and only one type of deformation twinning is known ( Barber & Wenk, 1979;Chang et al., 1998). It is expressed by f = {01¯ 12} twin plane which oc- curs within an f compositional plane. The shear displacement is in the < 0¯ 111 > direction ( Barber & Wenk, 1979) and is negative in sense (Turner et al., 1954; fi g. 2b). The f-plane is a usual slip plane for dolomite (Barber, 1977), and the resulting slip has positive sense ( Barber & Wenk, 1979). Com- plete absence of e-twinning in dolomite which is the pervasive mechanism of mechanical twin- ning in calcite, is explained by dolomite compo- sition, since the {01¯ 18} planes contain both Ca and Mg atoms, while the f planes do not. A shear on the e-planes would bring like species of cations into "closer-than-allowed" proximity ( Bradley et al., ...

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... In the past, much attention was given to metamorphic rocks from Pohorje Mts. (Germovšek, 1954;Hinterlechner-Ravnik, 1971, 1973Janák et al., 2004Janák et al., , 2005Janák et al., , 2006Janák et al., , 2009Janák et al., , 2015Jarc & Zupančič, 2009;Jarc et al., 2010;Jeršek et al., 2013;Mrvar, 2013;Vrabec et al., 2010a, b;Vrabec et al., 2018). Here, the majority of medium to high-grade marbles are located in the eastern and southern parts of the massif between Oplotnica and Dravinja brooks and in the surroundings of Šmartno, where they are placed among gneisses, mica-schists and amphibolites (Hinterlechner-Ravnik & Moine, 1977;Mioč, 1978). ...
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Common rock-forming and accessory minerals in marbles from various localities in Slovenia were studied using scanning electron microscopy with energy dispersive spectroscopy (SEM/EDS). Minerals and their chemical composition were identified in order to verify the variability of mineral assemblages in marbles from different localities in Slovenia. The analysis showed that marbles from Košenjak are the most mineralogically diverse, followed by Pohorje and finally Strojna marbles. Common rock-forming minerals calcite and dolomite are more abundant in Pohorje marbles where calcite contains higher levels of magnesium but no strontium and iron as compared with Strojna and Košenjak marbles. Accessory minerals like quartz, mica, titanite, apatite, rutile, zircon, chlorite group minerals, kaolinite and iron oxides/hydroxides were found in marbles from all localities. Clinopyroxene, amphibole, epidote and smectite group minerals, talc, tungsten-bearing ilmenorutile, psilomelane and bismuth oxides/carbonates, were observed only in marbles from Pohorje, while tourmaline and allanite group minerals, thorite or huttonite, chalcopyrite and synchysite group minerals were detected in marbles from Košenjak and Strojna. Variations in mineral assemblages in marbles from different locations are likely a consequence of different sedimentary environment and conditions and metamorphic grade of marble. These differences indicate that marbles from Košenjak and Strojna are genetically different from those from Pohorje and probably reflect mineral composition of the protolith. Thus, they enable rough distinction between more distant locations, but not between individual sub-localities.
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This study presents geochronological and geochemical data from newly dated Permian granitic orthogneisses associated with the Eclogite-Gneiss unit (EGU) from the southernmost part of the Austroalpine nappe stack, exposed within the Pohorje Mountains (Slovenia). LA-ICP-MS zircon U–Pb ages of two samples of the augen-gneisses are 255 ± 2.2 Ma and 260 ± 0.81 Ma, which are interpreted as the age of magmatic crystallization of zircon. In contrast, all round zircons from leucogneisses give Cretaceous ages (89.3 ± 0.7 Ma and 90.8 ± 1.2 Ma), considered as the age of UHP/HP metamorphism. The round zircons overgrew older euhedral zircons of Permian and rare older ages tentatively indicating that these rocks are of latest Permian age, too. Zircon εHf(t) values of the four orthogneiss samples are between − 13.7 and − 1.7 with an initial 176Hf/177Hf ratio ranging from 0.282201 to 0.282562; T CDM is Proterozoic. The augen-gneisses show geochemical features, e.g. high (La/Lu)N ratios and strong negative Eu anomalies, of an evolved granitic magma derived from continental crust. The leucogneisses are more heterogeneously composed and are granitic to granodioritic in composition and associated with eclogites and ultramafic cumulates of oceanic affinity. We argue that the Permian granitic orthogneisses might be derived from partial melting of lower crust in a rift zone. We consider, therefore, that segment of the EGU is part of the distal Late Permian rift zone, which finally led to the opening of the Meliata Ocean during Middle Triassic times. If true, the new data also imply that the Permian stretched continental crust was potentially not much wider than ca. 100 km, was subducted and then rapidly exhumed during early Late Cretaceous times.