Barite (BaSO₄) and celestite (SrSO₄) are the end-members of a nearly ideal solid solution. Most of the exploitable deposits of celestite occur associated with evaporitic sediments which consist of gypsum (CaSO₄·2H₂O) or anhydrite (CaSO₄). Barite, despite having a broader geological distribution is rarely present in these deposits. In this work, we present an experimental study of the interaction between gypsum crystals and aqueous solutions that bear Sr or Ba. This interaction leads to the development of dissolution-crystallization reactions that result in the pseudomorphic replacement of the gypsum crystals by aggregates of celestite or barite, respectively. The monitoring of both replacement reactions shows that they take place at very different rates. Millimeter-sized gypsum crystals in contact with a 0.5 M SrCl₂ solution are completely replaced by celestite aggregates in less than 1 day. In contrast, only a thin barite rim replaces gypsum after seven days of interaction of the latter with a 0.5 M BaCl₂ solution. We interpret that this marked difference in the kinetics of the two replacement reactions relates the different orientational relationship that exists between the crystals of the two replacing phases and the gypsum substrate. This influence is further modulated by the specific crystal habit of each secondary phase. Thus, the formation of a thin oriented layer of platy barite crystals effectively armors the gypsum surface and prevents its interaction with the Ba-bearing solution, thereby strongly hindering the progress of the replacement reaction. In contrast, the random orientation of celestite crystals with respect to gypsum guarantees that a significant volume of porosity contained in the celestite layer is interconnected, facilitating the continuous communication between the gypsum surface and the fluid phase and guaranteeing the progress of the gypsum-by-celestite replacement.

Carbonate biological hard tissues are valuable archives of environmental information. However, this information can be blurred or even completely lost as hard tissues undergo diagenetic alteration. This is more likely to occur in aragonitic skeletons because bioaragonite often transforms into calcite during diagenesis. For reliably using aragonitic skeletons as geochemical proxies, it is necessary to understand in depth the diagenetic alteration processes that they undergo. Several works have recently investigated the hydrothermal alteration of aragonitic hard tissues during short-term experiments at high temperatures (T > 160 ^∘C). In this study, we conduct long-term (4 and 6 months) hydrothermal alteration experiments at 80 ^∘C using burial-like fluids. We document and evaluate the changes undergone by the outer and inner layers of the shell of the bivalve Arctica islandica, the prismatic and nacreous layers of the hard tissue of the gastropod Haliotis ovina, and the skeleton of the coral Porites sp. combining a variety of analytical tools (X-ray diffraction, thermogravimetry analysis, laser confocal microscopy, scanning electron microscopy, electron backscatter diffraction and atomic force microscopy). We demonstrate that this approach is the most adequate to trace subtle, diagenetic-alteration-related changes in aragonitic biocarbonate structural hard materials. Furthermore, we unveil that the diagenetic alteration of aragonitic biological hard tissues is a complex multi-step process where major changes occur even at the low temperature used in this study, well before any aragonite into calcite transformation takes place. Alteration starts with biopolymer decomposition and concomitant generation of secondary porosity. These processes are followed by abiogenic aragonite precipitation that partially or totally obliterates the secondary porosity. Only subsequently does the transformation of the aragonite into calcite occur. The kinetics of the alteration process is highly dependent on primary microstructural features of the aragonitic biomineral. While the skeleton of Porites sp. remains virtually unaltered for the entire duration of the conducted experiments, Haliotis ovina nacre undergoes extensive abiogenic aragonite precipitation. The outer and inner shell layers of Arctica islandica are significantly affected by aragonite transformation into calcite. This transformation is extensive for the prismatic shell layer of Haliotis ovina. Our results suggest that the majority of aragonitic fossil archives are overprinted, even those free of clear diagenetic alteration signs. This finding may have major implications for the use of these archives as geochemical proxies.

Epitactic crystal growth plays a main role in the development of mineral processes and in the synthesis of advanced materials. Celestite (SrSO₄) forms epitactic overgrowths on anhydrite (CaSO₄) (100), (010) and (001) surfaces upon interacting with Sr-bearing aqueous solutions. Two populations of differently oriented celestite crystals related by symmetry operators of substrate are identified on (001)_Anh and (100)_Anh anhydrite substrates by SEM observations and synchrotron X-ray diffraction analysis. Substrate-induced twins arise after the coalescence of individuals belonging to these populations. Progressing growth results in a marked morphological evolution of epitactic celestite, whose crystals undergo sustained branching and loss of co-orientation that result in the formation of sheaf-like aggregates, on (100)_Anh, and swan-like aggregates, on (001)_Anh. We relate this evolution to celestite growth in a Ca-rich environment due to continued anhydrite dissolution and incorporation of small amounts of Ca into celestite structure. This incorporation would induce lattice strain which would be released through the formation of dislocations. The regular arrangement of these dislocations in small-angle boundaries would result in progressive splitting, driving the evolution from celestite single crystals to aggregates. Sharp compositional gradients in the boundary layer could explain the anisotropic development that leads to the formation of the swan-like celestites.

Struvite (MgNH4PO4·6H2O), a magnesium phosphate mineral, is a promising recoverable phosphorus (P) and nitrogen (N) source and is employed as a slow-release agricultural fertilizer. However, struvite is unstable in air and transforms into two distinct crystalline phases, newberyite (MgHPO4·3H2O) and dittmarite (MgNH4PO4·H2O), before breaking down into an amorphous phase. The transformation products of struvite have been indicated in the literature. However, published studies quantifying these processes have focused on struvite decomposition in solutions, and investigations in air have been conducted only at high temperatures, far from the conditions under which this mineral would be stored for use as a fertilizer. Furthermore, the kinetics are unknown, including transformation rates and mechanisms. Thus, we present complementary X-ray diffraction (XRD), vibrational spectroscopic, and light to electron microscopic data from which we determined the mechanisms for the transformation at 22–60 °C and breakdown of struvite in air. Our results revealed that its transformation into both newberyite and dittmarite follows a coupled dissolution-reprecipitation mechanism, with the reaction kinetics dominated by the diffusion of H2O and NH3 out of the struvite structure. The reaction pathways proved to be temperature dependent, with newberyite being the only transformation product at room temperature, even after 10 months. This transformation resulted from the loss of ammonia and three of the six water molecules. On the contrary, at higher temperatures, dittmarite was the major transformation product of struvite, formed through the release of five of the six water molecules while retaining the ammonia. At 60 °C and after 3 months, dittmarite breaks down into an amorphous magnesium phosphate phase. This study provides comprehensive insights into the kinetics and the underlying mechanisms governing the transformation of struvite at atmospheric conditions. Our findings have significant implications for the long-term storage of struvite and its subsequent utilization as a slow-release fertilizer.