The study of extreme evaporitic environments is a subject of increasing interest in sedimentary petrology and planetary geology. We report here the evaporitic precipitation in the shallow, small lakes (puquı ́os) at the Salar de Llamara (Atacama Desert, northern Chile). We have used a combination of in situ/laboratory/modeling methodologies allowing for the definition of a detailed, in-depth model for the sequence of evaporitic precipitation in the puquı ́os. The in situ measurements in these ponds reveal that brines are density-stratified with important gradients in salinity, temperature, and other properties of the solution (pH, oxidation/reduction potential, and dissolved oxygen). These vertical gradients, along with the lateral and seasonal variations, are the main factors of chemical variability controlling the precipitation processes. The evaporation of these brines drives the chemical evolution of the different ions and produces two distinct mineral assemblages: (i) large structures made of millimetric to centrimetric-size crystals of gypsum often forming thrombolite-like or stromatolite-like morphologies in the ponds and (ii) salt crusts made of mainly gypsum, eugsterite, halite, and thenardite surrounding the ponds. Evaporation experiments in the laboratory monitored by in situ X-ray diffraction and optical videomicroscopy reveal a precipitation sequence in accordance with field observations but distinct from the thermodynamically predicted precipitation. The actual formation of eugsterite, rather than glauberite, and the observed delay in gypsum dissolution are explained using kinetic evaporation models.
We have designed a set of experiments to test the role of borosilicate reactor on the yielding of the Miller–Urey type of experiment. Two experiments were performed in borosilicate flasks, two in a Teflon flask and the third couple in a Teflon flask with pieces of borosilicate submerged in the water. The experiments were performed in CH4, N2, and NH3 atmosphere either buffered at pH 8.7 with NH4Cl or unbuffered solutions at pH ca. 11, at room temperature. The Gas Chromatography-Mass Spectroscopy results show important differences in the yields, the number of products, and molecular weight. In particular, a dipeptide, multi-carbon dicarboxylic acids, PAHs, and a complete panel of biological nucleobases form more efficiently or exclusively in the borosilicate vessel. Our results offer a better explanation of the famous Miller's experiment showing the efficiency of borosilicate in a triphasic system including water and the reduced Miller–Urey atmosphere.
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.
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