Abstract
In the Dingwall gypsum quarry in Nova Scotia, Canada, operating in 1933–1955, the bedrock anhydrite deposits of the Carboniferous Windsor Group have been uncovered from beneath the secondary gypsum beds of the extracted raw material. The anhydrite has been subjected to weathering undergoing hydration (gypsification), transforming into secondary gypsum due to contact with water of meteoric derivation. The ongoing gypsification is associated with a volume increase and deformation of the quarry bottom. The surface layer of the rocks is locally split from the substrate and raised, forming spectacular hydration relief. It shows numerous domes, ridges and tepee structures with empty internal chambers, some of which represent unique hydration caves (swelling caves, Quellungshöhlen). The petrographic structure of the weathering zone has been revealed by macro- and microscopic observations. It was recognized that gypsification commonly starts from a developing network of tiny fractures penetrating massive anhydrite. The gypsification advances from the fractures towards the interior of the anhydrite rocks, which are subdivided into blocks or nodules similar to corestones. Characteristic zones can be recognized at the contact of the anhydrite and the secondary gypsum: (1) massive and/or microporous anhydrite, (2) anhydrite penetrated by tiny gypsum veinlets separating the disturbed crystals and their fragments (commonly along cleavage planes), (3) gypsum with scattered anhydrite relics, and (4) secondary gypsum. The secondary gypsum crystals grow both by replacement and displacement, and also as cement. Displacive growth, evidenced by abundant deformation of the fragmented anhydrite crystals, is the direct cause of the volume increase. Crystallization pressure exerted by gypsum growth is thought to be the main factor generating volume increase and, consequently, also the formation of new fractures allowing water access to “fresh” massive anhydrite and thus accelerating its further hydration. The expansive hydration is taking place within temperature range from 0 to ~30 °C in which the solubility of gypsum is lower than that of anhydrite. In such conditions, dissolving anhydrite yields a solution supersaturated with gypsum and the dissolution of anhydrite is simultaneous with in situ replacive gypsum crystallization. Accompanying displacive growth leads to volume increase in the poorly confined environment of the weathering zone that is susceptible to upward expansion.
Highlights
Hydration of mineral anhydrite (CaSO4) leading to crystallization of gypsum (CaSO4·2H2O) is a common reaction observed both in the weathering zone and deeper subsurface [1]
The petrological features of the anhydrite hydration, fundamental for the understanding of the process on the microscale level, were not studied so far, and the present paper focuses on this issue
The weathering of anhydrite deposits exposed at the bottom of the Dingwall quarry leads to their hydration and gypsification, which is associated with the significant volume increase and formation of peculiar hydration relief with ridges, tepee structures and domes, as well as unique hydration caves (Quellungshöhlen)
Summary
Hydration of mineral anhydrite (CaSO4) leading to crystallization of gypsum (CaSO4·2H2O) is a common reaction observed both in the weathering zone and deeper subsurface [1] This process has a specific character due to the potential volume changes that accompany mineral transformation. The lack of or smaller volume increase than theoretically predicted is commonly explained by the escape of excess calcium sulphate derived from the dissolution of anhydrite out of the place of dissolution (e.g., [14,15]) In such a case, gypsum crystallization may occur partially or even completely outside the system [3,5]. We attempt to characterize the environmental conditions responsible for the advance of the expansive gypsification at this site and to recognize basic factors which drive and inhibit that process and Minerals 2022, 1c2,o58ntrol the degree of volume increase
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