101 contact twins in gypsum experimentally obtained from calcium carbonate enriched solutions: mineralogical implications for natural gypsum deposits

• Published in: Journal of applied crystallography Vol. 56; no. 3; pp. 603 - 610
• Main Authors: Cotellucci, Andrea, Otálora, Fermín, Canals, Àngels, Criado-Reyes, Joaquin, Pellegrino, Luca, Bruno, Marco, Aquilano, Dino, Garcia-Ruiz, Juan Manuel, Dela Pierre, Francesco, Pastero, Linda
• Format: Journal Article
• Language: English
• Published: 5 Abbey Square, Chester, Cheshire CH1 2HU, England International Union of Crystallography 01.06.2023; Blackwell Publishing Ltd
• Subjects: Calcium carbonate; Crystals; Deposits; Elongation; evaporites; Fluid inclusions; Geology; Gypsum; Impurities; Mineralogy; Morphology; Research Papers; swallowtail; Twins; twins; fluid inclusions; gypsum; evaporites; swallowtail
• ISSN: 1600-5767; 0021-8898; 1600-5767
• DOI: 10.1107/S1600576723002674

Gypsum twins are frequently observed in nature, triggered by a wide array of impurities that are present in their depositional environments and that may exert a critical role in the selection of different twin laws. Identifying the impurities able to promote the selection of specific twin laws has relevance for geological studies aimed at interpreting the gypsum depositional environments in ancient and modern deposits. Here, the effect of calcium carbonate (CaCO3) on gypsum (CaSO4·2H2O) growth morphology has been investigated by performing temperature‐controlled laboratory experiments with and without the addition of carbonate ions. The precipitation of twinned gypsum crystals has been achieved experimentally (101 contact twin law) by adding carbonate to the solution, and the involvement of rapidcreekite (Ca2SO4CO3·4H2O) in selecting the 101 gypsum contact twin law was supported, suggesting an epitaxial mechanism. Moreover, the occurrence of 101 gypsum contact twins in nature has been suggested by comparing the natural gypsum twin morphologies observed in evaporitic environments with those obtained in experiments. Finally, both orientations of the primary fluid inclusions (of the negative crystal shape) with respect to the twin plane and the main elongation of sub‐crystals that form the twin are proposed as a fast and useful method (especially in geological samples) to distinguish between the 100 and 101 twin laws. The results of this study provide new insights into the mineralogical implications of twinned gypsum crystals and their potential as a tool to better understand natural gypsum deposits. Identifying the impurities that are able to promote the selection of specific gypsum twin laws has relevance for geological studies aimed at interpreting the gypsum depositional environments in ancient and modern deposits. The results of this study provide new insights into the mineralogical implications of twinned gypsum crystals and should help future research to make a better use of the twin laws observed in gypsum in ancient sedimentary successions as a proxy for the chemistry of the original brine.