Sulphates are one of the principal groups of evaporitic rocks, Gypsum tends to become into anhydrite when buried because of dehydration and the opposite process also takes place when anhydrite is affected by weathering and superficial waters. One of the most important problems found while quarrying gypsum rock is the presence of anhydrite; the drilling machines can be damaged because of this hardness and when an anhydrite body appears, the exploitation must be stopped at the moment. A geoelectrical classification of calcium sulphate rocks has been elaborated comparing the resistivity values obtained from theoretical models, laboratory tests, and field examples. A Gypsum-Anhydrite-Lutite system has been elaborated using Hashin-Shtrikman bounds. The lower bound fits with the data obtained experimentally for a matrix presence below 60% and the upper bound for the rest. With this ternary system it is possible to interpret the composition of calcium sulphate rocks from ERT profiles. The electrical value of pure gypsum and anhydrite rocks has been defined (1000 and 5000 ohm.m respectively). The most important component in determining the electrical resistivity of the bulk rock is the quantity of lutitic matrix while the significance of gypsum and anhydrite presence only is important in the purest rocks.


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