Gypsum Crystals: The Importance and the Role of Calcium Sulphate in Past and Modern Environments

This Special Issue gathers together a diverse set of studies that collectively advance our understanding of gypsum (CaSO₄·2H₂O) and evaporite systems, encompassing sedimentology, mineralogy, geochemistry, experimental crystallography, and planetary mineralogy. Evaporite formation represents a complex interplay of climatic, tectonic, and geochemical factors. Elucidating the processes controlling gypsum deposition, transformation, and growth is crucial for reconstructing past environments, interpreting modern analogues, and exploring extraterrestrial settings. The contributions in this Special Issue highlight the value of integrating multiple approaches, from field studies and laboratory experiments to theoretical models and planetary exploration, to unravel these multifaceted processes.

This Special Issue begins with a critical reassessment of the classical evaporite paradigm in “A Challenged Evaporite Paradigm?” (contribution 1). Traditional models posit that large salt and gypsum deposits form via solar evaporation in restricted basins, with deformation and diapirism controlled primarily by gravity-driven processes. By reviewing observations from various disciplines, the authors identify numerous discrepancies that remain unexplained by this framework, particularly the pervasive salinity patterns observed in subsurface crusts and mantle analogues. Their synthesis underscores the need to integrate microscale crystal growth mechanisms with basin-scale geodynamic and hydrological processes. This contribution sets the stage for a more holistic understanding of evaporite systems, emphasizing that no single paradigm fully captures the diversity of natural occurrences and that novel frameworks must reconcile all observational evidence.

Extending the focus beyond Earth, “Gypsum on Mars: A Detailed View at Gale Crater” (contribution 2) examines gypsum and other Ca-sulphate minerals on Mars using in situ data from the Curiosity rover. The study documents associations of gypsum with bassanite, anhydrite, jarosite, starkeyite, and kieserite across stratigraphic sections of Aeolis Palus and the lower slopes of Mount Sharp. The observed dehydration of gypsum to bassanite and, in some cases, to anhydrite reflects limited equilibration under cold and arid conditions. The results reveal the heterogeneity of Martian sulphate assemblages, indicating that local observations cannot be universally extrapolated to the planet as a whole. These findings demonstrate the utility of gypsum as a proxy for reconstructing paleoenvironmental conditions, including episodic groundwater acidity and aqueous activity, and highlight the importance of comparative planetary mineralogy in understanding evaporite processes under diverse environmental constraints.

From a laboratory and experimental viewpoint, “Gypsum Crystals Formed by the Anhydrite–Gypsum Transformation at Low Temperatures: Implications for the Formation of the Geode of Pulpí” (contribution 3) investigates the nucleation and growth dynamics of gypsum under controlled conditions. Using anhydrite dissolution in airtight, low-temperature reactors (15–25 °C), the study reproduces crystal habits observed in nature, including single and twinned selenite crystals with the 100 contact twinning law. Growth rates ranged from 3.8 to 35.3 μm/day, highlighting the sensitivity of crystal formation to low supersaturation. These experiments provide a mechanistic basis for interpreting the giant selenite crystals of Pulpí and illustrate the relevance of laboratory simulations for understanding natural crystal growth, nucleation, and diagenetic transformations in sedimentary evaporitic environments.

Complementing these experimental insights, in “Gypsum: From the Equilibrium to the Growth Shapes-Theory and Experiments” (contribution 4), the authors integrate theoretical and experimental approaches to elucidate the relationships between equilibrium crystal shapes, growth morphologies, and twinning phenomena. The authors demonstrate the role of adhesion energy (β_adh) in dictating homo- and hetero-taxy, contact and penetration twinning, and crystal nucleation on pre-existing substrates. They also unify classical (CNT) and non-classical nucleation (NCNT) theories to interpret crystal size distributions and induction times under different solution conditions. This framework bridges fundamental crystallography and experimental observations, offering predictive insights into gypsum morphology and growth, with implications for sedimentological interpretation, industrial crystallization, and environmental reconstructions.

This Special Issue also emphasizes paleoenvironmental and sedimentological perspectives through “Short-Term Climate Oscillations During the Messinian Salinity Crisis: New Insights from Gypsum Lithofacies of the Crati Basin (Lattarico, Calabria, Southern Italy)” (contribution 5). This study provides a detailed petrographic and mineralogical analyses of Messinian gypsum deposits, including laminar, nodular, and clastic facies formed during the second stage of the Messinian Salinity Crisis (5.60–5.55 Ma). Multi-scale observations reveal the influence of short-term climatic fluctuations, tectonic activity, and basin hydrodynamics on facies distribution, from shallow-water evaporitic environments to deep-basin settings affected by slope failures. These findings highlight the sensitivity of gypsum deposition to both environmental and tectonic controls and contribute to a refined understanding of Mediterranean Messinian evaporite formation.

Collectively, the contributions of this Special Issue address multiple scales of evaporite research—from microscale crystallography and nucleation kinetics to basin-scale facies architecture and planetary mineralogy. Several cross-cutting themes emerge: (i) the importance of revisiting and challenging classical paradigms of evaporite formation, (ii) the critical role of laboratory experiments and theoretical modelling in elucidating crystal growth and phase transformations, (iii) the use of gypsum as a sensitive proxy for reconstructing past environmental and aqueous conditions, and (iv) the integration of observational, experimental, and modelling approaches to bridge micro- and macro-scale processes.

Despite these new studies, a significant knowledge gap remains. The precise mechanisms controlling large-scale gypsum deposition under varying hydrological, climatic, and tectonic regimes are not fully understood. Similarly, the interplay between dehydration, recrystallization, and twinning under low-temperature or low-supersaturation conditions warrants further investigation. On a planetary scale, extrapolating site-specific observations such as those from Gale Crater to broader Martian contexts remains a challenge, underscoring the need for additional in situ analyses, comparative planetary studies, and laboratory simulations. Furthermore, coupling geodynamic processes with evaporite basin evolution and chemical sedimentology continues to present unresolved questions critical for understanding massive gypsum and mixed-salt accumulations.

Looking forward, we believe that future research should prioritize high-resolution temporal and spatial reconstructions of evaporite systems, integrating field studies, laboratory experiments, and theoretical modelling. Investigations should aim to quantify nucleation and growth rates, elucidate the mechanisms of phase transformations, and explore the influence of climatic and tectonic variability on deposition and diagenesis. Interdisciplinary approaches combining geochemistry, mineralogy, sedimentology, and planetary science are essential to improve predictive models and to explore the implications of evaporite processes for planetary habitability and resource potential.

In summary, this Special Issue highlights the richness and complexity of gypsum and evaporite research. By integrating terrestrial, experimental, and planetary perspectives, the collected studies provide critical insights into crystal growth, mineralogical transformations, depositional processes, and environmental reconstructions. The contributions underscore the progress achieved, reveal persistent challenges, and set the stage for future investigations aimed at unravelling the dynamics of gypsum and evaporite systems on Earth and beyond.