**1. Introduction**

464 Advances in Crystallization Processes

[97] P. Sulapha, S.F. Wong, T.H. Wee, S. Swaddiwudhipong, Carbonation of concrete containing mineral admixtures, J. Mater. Civ. Eng. 2003, 15(2): 134-143. [98] Monkman, Sean,, Shao, Yixin, Assessing the carbonation behavior of cementitious

[99] Marlova P. Kulakowski, Fernanda M. Pereira, Denise C.C. Dal Molin Carbonation-

induced reinforcement corrosion in silica fume concrete, Constr. Build. Mater. 2009,

materials, J. Mater. Civ. Eng. 2006, 18(6): 768-776.

23(3): 1189-1195.

Sulphates as well as silicates and carbonates are one of the most common minerals on the Earth's surface. They cover about 25% of continents surface (Blatt et al. 1980; Ford &Williams, 1989). Their recent sedimentary environments are the terrains of: the southern Mediterranean coast – coastal salt lakes of Marocco, Libya, Tunisia and Egypt, Gulf of Kara Bogaz (Caspian Sea), Persian Gulf – coastal sabkhas of UAE (special Abu Dhabi Emirate) and Qatar, Texas and California (Death Valley), salt lakes of South and Central Australia and salt lakes, salinas and salares of South America.

Annual total world production of gypsum in 2010 exceeded 146 million metric tones (http://minerals.usgs.gov/minerals/pubs/commodity/).

First of all, the sulphates are represented by two kinds of calcium sulphate - gypsum (CaSO4•2H2O) and anhydrite (CaSO4); mainly the first one creates deposits that are of economical value; it is used in the construction industry as bond material and to control the bonding speed, in casting and modelling and also in medicine (surgery and stomatology), during the production of paper. Its properties influence the parameters and quality of materials which it consist in. In construction/building industry the semi-hydrated gypsum is used as a result of frying in temperatures about 160°C (150-190°C) with sufficient amount of added water, the material bonds and hardens – the reaction is exothermic and the gypsum's volume increases of about 1%. Bassanite (CaSO4 ½ H2O), calcium sulphate semihydrate, is also known.

Rarely we can find the sulphates of: strontium (celestine), barium (barite), potassium (e.g. polyhalite), sodium (e.g. mirabilite, glauberite), magnesium (e.g. epsomite, kieserite), copper (e.g. brochantite, chalcanthite) and others. Most of gypsum and anhydrite on Earth are of evaporate origin, they are formed in specific order as a result of precipitation of the calcium sulphate inside the gradually drying sea basin (deep or shallow), lake, by the coastal lagoons, bays or sabkhas (indications of hot and arid climate). They are also the products of volcanic exhalations or low temperature hydrothermal processes, as well as of oxidation of sulphide deposits. The sulphates are also found above the salt mirror of diapirs, where they form the secondary deposit as the harder soluble residuum after the salt leaching – they constitute the main component of so-called gypsum or anhydrite-gypsum cap-rock.

Crystallization, Alternation and Recrystallization of Sulphates 467

Fig. 3. Columnar to needle-like gypsum (Polkowice, Poland) phot. J. Jaworska

Primarily, gypsum that crystallizes in the evaporite basins forms usually medium or coarse grains; sometimes the lamination occurs, reflecting the changes in the basin (water composition, water level). Among the gypsum laminas, biolaminae appear; they are formed in the neritic zones and can be either deformed by periodical droughts (mudcraks) or ruptured by crystallizing sulphates (teepee-like structures, see fig. 9). In deeper zones of the basin, sabre-like gypsum (fig. 14.) can crystallize; these are elongated gypsum crystals, 20-30 cm long, distorted in one direction due to the demersal current activity (they constitute the perfect indicators of paleocurrents). Selenite gypsum is an exceptional feature; it forms under stable conditions at the depth of few to several m (figs. 16. and 17.) and reaches the dimensions of 3.5-4 m usually, but even up to 10 m. In deeper zones, laminated gypsum forms; sometimes with the ripplemark remains or even turbidites and slump structure with

Fig. 4. Alabaser (Ukraine) phot. J. Jaworska

fragments of older, more lithified gypsum.

The average precipitation rate of sulphates (gypsum and anhydrite) in the evaporite basin is ca. 0.5-1.2 mm/year and requires the evaporation of few to few tens cm high (2 m) column of water.

Probably, the oldest documented sulphate pseudomorphs are 3. 45 billion years old and come from West Australia (Pilbara), cm-size growth and interpreted to replace gypsum (Barley et al., 1979; Buick & Dunlop, 1990); only slightly younger are pseudomorphs after swallowtail gypsum – 3.4 billion years old – from S. Africa, Kaapvall Craton (Wilson & Versfeld, 1994).
