**1.1 Gypsum: CaSO4•2H2O**

Crystal system: monoclinic, hardness: 2, density: 2.3-2.4 g/cm2

soluble: in water, in HCl and in concentrated solution of H2SO4

contains impurities: Ba, Sr, deposit grains where it crystallizes, bituminous substances habit: platy, columnar, fibrous, needle-like, lenticular; forms massive aggregates and twins swallowtail (figs. 1.,2.,3.,7. and 10.), usually colourless, might be coloured by Fe compounds particular varieties:


Fig. 1. Platy gypsum (Petunia Bukta, Spitsbergen) phot. J. Jaworska

Fig. 2. Fibrous gypsum (Germany) phot. J. Jaworska

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

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 &

contains impurities: Ba, Sr, deposit grains where it crystallizes, bituminous substances habit: platy, columnar, fibrous, needle-like, lenticular; forms massive aggregates and twins swallowtail (figs. 1.,2.,3.,7. and 10.), usually colourless, might be coloured by Fe compounds



of water.

Versfeld, 1994).

particular varieties:

usually colourless

**1.1 Gypsum: CaSO4•2H2O** 

Crystal system: monoclinic, hardness: 2, density: 2.3-2.4 g/cm2 soluble: in water, in HCl and in concentrated solution of H2SO4


embedded sand grains built-in during the fast crystal growth.

Fig. 1. Platy gypsum (Petunia Bukta, Spitsbergen) phot. J. Jaworska

Fig. 2. Fibrous gypsum (Germany) phot. J. Jaworska


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

Fig. 4. Alabaser (Ukraine) 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 fragments of older, more lithified gypsum.

Crystallization, Alternation and Recrystallization of Sulphates 469

Recently, the gypsum precipitates from among calcium sulphates; whereas anhydrite crystallizes very rarely – the only locations of its recent crystallization are: the Persian Gulf

coast, lakes: Elton and Inger, Death Valley and Clayton Playa (Nevada).

Fig. 7. Fibrous –spar gypsum in clay-slate (Niwnice, Poland) phot. J. Jaworska

Fig. 9. Biolaminas deformed by crystallizing sulphates (near Ostrówka quarry, Poland)

Fig. 8. Desert rose; phot. J. Jaworska

phot. J. Jaworska

Fig. 5. Selenite (Busko-Zdrój, Poland) phot. J. Jaworska

Fig. 6. Gypsum twins – swallowtail (Dymaczewo Stare, Poland) phot. J. Jaworska
