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.

Epitaxial growth is a long-standing crystallization phenomenon of great technological interest. Here we present the use of a new methodology approach making full use of the concept of adhesion energy...

Understanding the selection mechanisms of CaCO 3 polymorphs (vaterite, aragonite, and calcite) is pivotal for elucidating both inorganic and biogenic carbonate formation. It is peculiar that different polymorphs originate from the same organism; in addition, these polymorphs can even be epitaxially related. Here, we ask why some mollusks and gastropods develop calcite (Cc) layers at the contact with aragonite (Ar) in the outer portion of their shell, while others do not. To establish the most likely epitaxial relationships between calcite and aragonite, here, we investigated at the empirical level (and 0 K) the (001) Ar /(00.1) Cc , (110) Ar /(10.0) Cc , (010) Ar / (01.2) Cc , and (100) Ar /(11.0) Cc interfaces. Upon analyzing the (001) Ar /(00.1) Cc epitaxial and relaxed interface, we found that a hexagonal (space group, P6 3 22) CaCO 3 polymorph is generated, corresponding to the phase recently identified during the molecular dynamics study on the high-temperature (∼600 K) aragonite− calcite transition. This polymorph, showing a symmetric intermediate between aragonite (orthorhombic) and calcite (rhombohedral), develops as a nanometric phase and exhaustively explains the observed epitaxy, (001) Ar /(00.1) Cc . We propose that the growth of calcite at room pressure and temperature in the outer portion of the shell of mollusks and gastropods is strictly associated with the formation of this hexagonal CaCO 3 polymorph. Moreover, to intensify the growth process of carbonate polymorphs, we can envisage that the findings have implications for understanding the sluggish mechanisms of aragonite−calcite transformation under ambient conditions.

Calcium oxalates are naturally occurring biominerals and can be found as a byproduct of some industrial processes. Recently, a new and green method for carbon capture and sequestration in stable calcium oxalate from oxalic acid produced by carbon dioxide reduction was proposed. The reaction resulted in high-quality weddellite crystals. Assessing the stability of these weddellite crystals is crucial to forecast their reuse as solid-state reservoir of pure CO2 and CaO in a circular economy perspective or, eventually, their disposal. The thermal decomposition of weddellite obtained from the new method of carbon capture and storage was studied by coupling in-situ high-temperature X-ray powder diffraction and thermogravimetric analysis, in order to evaluate the dehydration, decarbonation, and the possible production of unwanted volatile species during heating. At low temperature (119–255 °C), structural water release was superimposed to an early CO2 feeble evolution, resulting in a water-carbon dioxide mixture that should be separated for reuse. Furthermore, the storage temperature limit must be considered bearing in mind this CO2 release low-temperature event. In the range 390–550 °C, a two-component mixture of carbon monoxide and dioxide is evolved, requiring oxidation of the former or gas separation to reuse pure gases. Finally, the last decarbonation reaction produced pure CO2 starting from 550 °C.

3D-epitaxy of the {010} pinacoid of deposited gypsum (CaSO4·2H2O) on the substrate {10.4} calcite rhombohedron is described, in order to give a theoretical background to the replacement of calcite by...