Lattice deformation of blue halite from Zechstein evaporite basin : Kłodawa Salt Mine, Central Poland

Outcrops of natural blue and purple halite crystals have been found in K ł odawa (Poland) salt deposit originating from the Permian (Zechstein) salt formation within tectonic zones. Field works carried out on various levels of K ł odawa Salt Mine indicated differencesbothinintensity ofthe hue and in the size of the outcrops. Their occurrence was connected with the presence of epigenetic sediments rich in potassium. For the samples of blue ha lite, an optical anisotropy (birefringence) was observed for both standard mineralogical thin sections and thick plates, indicating a deviation from cubic symmetry. The blue colouration of the halites, described by ultraviolet – visible spectroscopy, is due to the presence of colour centres. The colour centres have been recently identi- fied as: F, R 1 (F 3 ), R 2 (F 3 ), M and plasmons (Wese ł ucha- Birczy ń ska et al., Vib Spectrosc 60:124 – 128, 2012 ). The trace amount of impurities detected in the blue halites by means of scanning electron microscopy – energy dispersive X-ray spec- troscopy and micro-X-ray fluorescence techniques were con- nected mainly with solid inclusions such as K x Na (1 − x ) Cl, KMgCl 3 ·6H 2 O, KCl, orthorhombic sulphur, quartz and some other phases like pyrite. Crystallographic data obtained by X- ray diffraction experiments for the single crystals of the halite from K ł odawa with different saturation of blue or purple colours, as well as for natural colourless halite, revealed lowering of space-group symmetry to monoclinic, orthorhom- bic, trigonal, tetragonal or even triclinic systems while the space group typical for pure NaCl is Fm -3 m.