Phase transitions in the borate minerals from the Klodawa salt dome (central Poland) as indicators of temperature processes in salt diapirs

摘要

The diapiric structures of the Polish Lowlands are tectonically deeply seated down to the autochthonous Zechstein strata at a depth of ca. 6 km. In the process of deep burial and halokinetic diapirism, the salt rocks were subjected to diagenetic and metamorphic transformations, with the temperature being an essential factor. Considering the thermal gradient, a temperature of up to ca. 200 degrees C can be achieved in the salt dome within a depth range from several hundred metres to 6 km, which may lead to transformations of the majority of salt minerals. Phase transitions of two borate minerals boracite and congolite from the Klodawa salt dome provide evidence for higher temperatures in the salt dome rocks. The authigenic euhedral crystals of those borate minerals display their external habitus in ambient temperature in the form of regular symmetry (F43c pseudo-regular polyhedrons), whereas their internal structure is lower: orthorhombic (Pca2(1)) for boracite and rhombohedral (R3(c)) for congolite. The heating and cooling of boracite and congolite crystals show reversible phase transition. At a temperature of ca. 270 degrees C, boracite crystals change their symmetry: orthorhombic <-> cubic. In the case of congolite three reversible phase transitions within a temperature range of 50-339 C can be observed: rhombohedral <-> monoclinic <-> orthorhombic <-> cubic symmetry. Those phase transitions, confirmed experimentally in our study, clearly document at least local occurrences of temperatures exceeding 339 degrees C in the Ktodawa salt dome.