BASSANITE FROM SULPHATE ROCKS OF THE GORODYSHCHENSKE GYPSUM DEPOSIT

Abstract

The article is devoted to the study of bassanite, a mineral of the sulphate group, a hemihydrate of calcium sulphate. We discovered it in the gypsum stratum of the Pre-Carpathian region ‒ at the Horodyshchenske gypsum deposit, located near the village of Zhabokruky (the region of Ivano- Frankivsk). This is the southwestern flank of the Upper Transnistrian gypsum-bearing province, which is associated with the Tyraska suite of the Upper Tortonian (Miocene). Primary sedimentary gypsums have undergone dehydration and transformed into anhydrites in most of the Pre-Carpathian evaporite basin. The appearance of bassanite, which is paragenetically associated with gypsum and anhydrite, is often recorded during such transformations. Bassanite is a rare mineral in Ukraine, so it has been poorly studied. Its finds have been described in the Bakhmutska Basin (Donbas) and the Mountainous Crimea; the mineral has also been found in inclusions in halite from evaporite strata of the Carpathian region. Using macroscopic and microscopic analyses, we studied the morphological features of bassanite and its spatial interaction with other minerals (gypsum, anhydrite, calcite), and determined the patterns of its spatial arrangement in the gypsum mass. The mineral is mainly confined to the macrocrystalline variety of gypsum. It forms elongated-prismatic crystals, less often needle-shaped, tabular, conical and wedge-shaped segregations. We also found bundle-like crystal growths confined to grain junctions or microcracks. Areas of intensive dehydration of gypsum with the formation of bassanite are characterized by secondary structures (in particular, fibrous) and textures (in particular, lepidoblastic and microlepidoblastic). Studies have shown that the formation of bassanite directly depends on the directions with weakened resistance in the rock, which are most favourable for the growth of oriented crystals. Existing problems in the study of bassanite and directions for its further research are identified.