Abstract

Pseudocubic crystals of (Fe,Mg,Mn)3B7O13Cl borate, ≤ 1.2 mm in size and yellowish through pale-violet to pale-violet-brown and brownish in color, were found in the Kłodawa salt dome, located within the Mid-Polish Trough (central Poland), an axial part of the Polish basin belonging to a system of Permian-Mesozoic epicontinental basins of Western and Central Europe. The crystals occur in adjacent parts of the Underlying Halite and Youngest Halite units bordering the Anhydrite Pegmatite unit around the PZ–3/PZ–4 (Leine/Aller) boundary. The internal texture of the crystals reflects phase-transitions from rhombohedral congolite (space-group symmetry R3c) through orthorhombic ericaite (Pca21) to a cubic, high-temperature, mainly Fe-dominant analogue of boracite (F43c), indicating formation of the phases under increasing temperature. Currently, the parts of the crystals showing orthorhombic and cubic morphology generally represent congolite and, much less frequently, trembathite paramorphs after higher-temperature structural varieties of (Fe,Mg,Mn)3B7O13Cl and (Mg,Fe,Mn)3B7O13Cl, which are unstable at room-temperature. The phase-transitions R3cPca21 and Pca21F43c recorded in the morphology of the growing crystals took place at temperatures of 230–250 °C and 310–315 °C, respectively, whereas the total range of (Fe,Mg,Mn)3B7O13Cl borate crystallization was determined to be ca. 100 °C to 350–360 °C. The primary congolite crystallized as a result of the metasomatic reaction of Fe and Mn carriers present within the host rock-salts of the Underlying Halite and Youngest Halite with (BO3)3−-bearing brines and fluids released from primary evaporate sediments (probably mainly from Upper Younger Halite with potash impurities of the PZ–3 cyclothem) in diagenetic and metamorphic processes induced by disturbances in halokinetic stability, intensive faulting of the pre-Zechstein basement and decompression, and, connected with movements of evaporates, their upward flow in the crust and the formation of a diapiric structure. The phase-transitions R3cPca21 and Pca21F43c have been forced by inflows of Mg-bearing fluids sourced from relatively thick loads of potash salts (Younger Potash) of the PZ–3 cyclothem, containing inclusions of carnallite, kainite, and kieserite. The brines and fluids have utilized cracks in Pegmatite Anhydrite as convenient pathways for migration in the dome, in contrast to the host rock-salts, which are too solid to be penetrated by the fluids for distances greater than a few meters.

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