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

Abstract

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.

Effect of Different Evaporation Rates on Gypsum Habit: Mineralogical Implications for Natural Gypsum Deposits

“…In the range 1.97 < AR < 2.51, the aspect ratio of both 100 and 101 penetration twinning laws are measured and thus this range does not allow to distinguish between the two. Therefore, the AR parameter might be a fast and useful tool to distinguish between 100 and 101 penetration twins, in addition to those already described in Cotellucci et al 31 The curved habit of gypsum, also known as "bent habit", has already been reported in the literature. 34,35 To our knowledge, two different experimental growth conditions can promote the curvature in gypsum crystals: diffusion-controlled experiments in the presence of impurities, and evaporation experiments in the presence of impurities.…”

“…We propose that 100 and 01 penetration twins have been observed in nature but inappropriately defined as “swallowtail twins”, which is the terminology commonly used to identify gypsum twinned crystals with a shape similar at first glance similar. Recently, straightforward and reliable methods have been proposed to geometrically and univocally recognize the different twinning laws of gypsum . Therefore, we strongly suggest improving the identification of the different gypsum twinning laws observed in nature, as they can serve as a proxy for the chemistry of the original water from which gypsum precipitated, , hence contributing to the interpretation of the gypsum depositional environment in ancient deposits.…”

“…From the evaporation of the G1–G4 solutions, both single and twinning habits of gypsum crystals have been obtained. The related twinning laws have been identified by measuring the extinction angle with the optical microscope using crossed polarizers as described in Cotellucci et al…”

“…Therefore, in addition to the extinction angle, both the subcrystals and fluid inclusion growth directions are useful tools to distinguish the 100 from the 101 twinning laws. 31 At the slowest evaporation rate (G1 solution: 0.022 < ER < 0.030 g/h), single crystals [001] elongated, 100 penetration twins, and 101 penetration twins have been detected (Figure 2A−C). The precipitation frequencies of 100 and 101 gypsum twinning laws are as follows: 45.4 ± 1.8% for the 100 penetration twinning law and 54.6 ± 1.8% for the 101 penetration one.…”

“…However, it is important to acknowledge that more rigorous analyses are available for distinguishing between these two twinning laws. 31 Previous crystal growth experiments have associated the curved habit of gypsum with specific impurities dissolved in the solution. 34,35 Moreover, our findings suggest that high ER can also promote the curvature, and a homoepitaxial mechanism may be involved in the formation of the asymmetric curved habit.…”