Mechanical control of soft mineralogical phases on the global strength of the rock: the case of anhydrite and gypsum

The mechanical response of polymineralic rocks is the result of the combination of the individual strengths of all the mineralogical phases that are included in it. But which percentage of a softer mineral is sufficient to soften the mechanical strength of the entire rock? And, on the other hand, which percentage of a stronger material is enough to increase the strength of a soft rock? And do these percentages depend on the microstructure?

Gypsum-anhydrite rocks have the requisites to answer these questions, due to the high mechanical contrast between anhydrite and gypsum mineralogical phases and to the high heterogeneity of microstructures that are usually present even in a single rock mass because of the low temperatures of the phase transition between gypsum and anhydrite.

For these reasons, the present study investigated the strength and creep response under uniaxial compression of Triassic sulphates from the Italian Western Alps. The samples considered may be clustered in three main groups, depending on the occurring microstructural organization of gypsum and anhydrite mineralogical phases: i) pure gypsum, ii) gypsum with relicts of anhydrite at the nuclei of the crystals and iii) anhydrite with gypsum bordering the rims among the crystals.

Results of mechanical tests showed that even a low percentage of anhydrite present at the nuclei of the gypsum crystals strongly controls the mechanical response, causing an increase in the uniaxial strength from 20-25 MPa to 50-70 MPa. On the other hand, the presence of small quantities of gypsum at the rim of anhydrite crystals implies a decrease of mechanical strength of up to 50% with respect to the values expected for pure anhydrite.

Unlike the results about strength, creep strain rate data in gypsum showed a high predictability, suggesting that time-dependent deformation is mainly controlled by mechanisms occurring at the rim of crystals (e.g., pressure solution).