Gypsum whiskers in Messinian evaporites identified by μ-XRD2

Abstract

The gypsum mineralogy of hair-like, so-called whisker crystals was determined by micro X-ray diffractometry equipped with a focusing X-ray optic and a two-dimensional detector (μ-XRD2). Gypsum whiskers typically form by efflorescence on surfaces that are in contact with water. The delicate whiskers studied here formed within a Messinian evaporite mostly consisting of anhydrite. They project from alabastrine gypsum into former cavities. The gypsum resulted from hydration of anhydrite. Whiskers were finally engulfed by biogenic, native sulphur, which filled the cavities in the course of microbial alteration of the host lithology. The sulphur protected the delicate whisker crystals from destruction, including a period of exposure to weathering on a dumping site of a former mine where the rocks have been sampled. To the best of our knowledge, this is the first report of gypsum whiskers in evaporites. More significantly, this study reveals that μ-XRD2 has great potential for sedimentary petrology, allowing the in situ identification of minute mineral phases that cannot be identified with conventional techniques.