More than one hundred rock samples were collected from the weathering zone of the gypsum-anhydrite rock during survey work at the Dingwall Quarry in Canada. This was done in every part of the site, both in the central field and the east, west and south fields. The detailed selection of samples was guided by the recognition of primarily anhydrite gypsification zones divided into initial, advanced and mature (Jarzyna et al. 2021). Thus, rocks built almost exclusively of anhydrite representing initial gypsification (fig. 1), gypsum-anhydrite rocks associated with progressive transformation of anhydrite to gypsum but not completely transformed into the latter mineral (advanced gypsification), and samples completely transformed to gypsum, which in turn indicates mature gypsification, were used for the study.
Description and study of samples
The rock samples had to be characterized primarily in terms of structures and fabrics. For this purpose, the samples were carefully collected from the exposure, taken to the Faculty of Geology, where they were cut with a power saw (fig. 2, 3), and more than 250 rock cubes with dimensions of 4.0 x 2.5 x 1.0 cm (fig. 4) were selected for imaging with a scanning optical microscope (SEM). SEM observations were made in the Geological Faculty (University of Warsaw) Cryo-SEM laboratory (fig. 5). For more than 40 cubes in the preparation laboratory at the Faculty of Geology (University of Warsaw) and at the Polish Geological Institute in Warsaw thin plates with a thickness of about 30 µm were made (fig. 6). The thin sections were observed under a polarized optical microscope also at the Faculty of Geology.
The samples were also subjected to analyses related to the study of mineral composition, mainly with the determination of gypsum and anhydrite. The primary method was a geochemical method using an ICP-EOS/ICP-MS spectrometer at the laboratory of Bureau Veritas Commodities Canada Ltd. formerly known as ACME Laboratories Ltd. in Vancouver, Canada. Equally important, a hydrostatic method was used to quantify the gypsum and anhydrite content of all samples. In addition, the mineral composition of selected samples was determined using the X-ray method, and the presence of bassanite among the gypsum-anhydrite rock was verified using Raman spectrometry. In turn, the physical properties of the studied rocks were determined using helium pycnometry.