Inclusions in anhydrite crystals from blue halite veins in the Kłodawa Salt Dome (Zechstein, Poland)

The occurrence of both the blue and violet halites is one of the most interesting phenomena in nature. Despite numerous laboratory and field works, their origin in natural evaporitic environments has not been satisfactorily explained. In the Kłodawa Salt Dome (Zechstein, Central Po land), blue or vio let halites occur relatively frequently. Their accumulations differ in size and intensity of colours. In this paper, petrological features of anhydrite crystals derived from one of the largest outcrops of the blue halite at the Kłodawa Salt Mine are presented. Anhydrite is one of solid inclusions encountered in blue-coloured halite crystals. Special attention was paid to fluid inclusions present in this anhydrite. The microthermometric measurements showed two directions of homogenisation, i.e., towards the liquid phase (LG→L, LL→L) ortowards the gas phase (LG→G). In the former case, the temperatures ranged from 174 to 513°C, whereas in the latter one, the values from 224 to 385°C were measured. The composition of inclusions is relatively variable. We can observe transparent and opaque daughter minerals as well as CO2 in the liquid phase accompanied by a variable amount of methane or hydrogen sulphide. These features of inclusions indicate that anhydrite crystals and, thus, blue halite were formed under the influence of hydrothermal conditions. Observations in the mine workings combined with petrological studies enable to conclude that blue colouration of halite crystals is controlled by three factors: a high temperature, reducing conditions and defects in halite lattice related to tectonic stress.