Gypsum (CaSO4·2H2O, hydrous calcium sulfate) from Sallent. Gypsum is a colorless, white, gray or red mineral, but always with an obvious white streak. Gypsum is soft; it defines the hardness degree 2 -the minerals that can be scraped with the nail- in the Moh’s scale. Gypsum crystallizes in different shapes: translucent sheets with an excellent exfoliation, colorless crystals with acute arrowhead shape, fibers, compact masses made by tiny, white crystals (variety alabaster) The most frequent gypsum is the so-called secondary gypsum that comes from the hydration of anhydrite (CaSO4 ). Once more, anhydrite was originated from a primary gypsum.
Most of the gypsum in Central Catalonia deposited in the period Eocene, during the first stages of evaporation of the sea that covered this region. This process ended up with the deposit of halite and silvine of the geological formation Cardona. There is gypsum in the borders of this formation, for instance Artés, Avinyó, Igualada, Òdena and Castellterçol, and also below the salt deposit. Furthermore, there is gypsum in Súria and Sallent that comes from the evaporation of continental lagoons. Gypsum is currently extracted in Igualada and Òdena, two villages in the district of Anoia close to the south-west of Bages. Gypsum was also mined in Súria and Artés, always for construction. The clayey stratums of the Artés geological formation are often crossed by narrow veins of white, fibrous gypsum.
[photo Joaquim Sanz / Museum of Geology Valentí Masachs]