Amethyst is a purple variety of quartz that has been cherished as a gemstone for many years and plays an important economic role in northern Uruguay. Geodes are hollow rock formations that often contain quartz crystals, including amethyst. In Uruguay, amethyst geodes have been uncovered within cooled lava flows that trace back to the time when the supercontinent Gondwana split apart approximately 134 million years ago. Nevertheless, how these geodes formed has puzzled scientists.
Amethyst is a purple variety of quartz that has been cherished as a gemstone for many years and plays an important economic role in northern Uruguay. Geodes are hollow rock formations that often contain quartz crystals, including amethyst. In Uruguay, amethyst geodes have been uncovered within cooled lava flows that trace back to the time when the supercontinent Gondwana split apart approximately 134 million years ago. Nevertheless, how these geodes formed has puzzled scientists. To investigate this, a research team led by the University of Göttingen employed advanced techniques. They found that the amethyst geodes formed at surprisingly low crystallization temperatures, ranging from 15 to 60 °C. Based on their findings, the researchers proposed a novel model to explain the geodes’ formation. This research was documented in the journal Mineralium Deposita.
Amethyst has been mined for over 150 years in the Los Catalanes District of Uruguay, which is where this study was conducted. This area is celebrated for its deep violet-colored gems and impressive geodes, some of which can exceed 5 meters in height. The deposits in this region have been recognized as one of the top 100 geological heritage sites worldwide, underscoring their scientific and natural significance. However, the limited understanding of how these geodes originated has made it difficult to locate new deposits, heavily relying on miners’ expertise. To tackle this issue, researchers performed extensive geological surveys across more than 30 active mines, examining the minerals within the geodes, the water they contain, and surrounding groundwater. Utilizing advanced methods such as nucleation-assisted microthermometry of initial one-phase fluid inclusion and triple-oxygen-isotope geochemistry, they gained new insights into the formation of these valuable geodes. Along with discovering the geodes formed at unexpectedly low crystallization temperatures, the researchers also found that the mineralizing fluids had a low salinity and isotopic composition consistent with water derived from the natural weather cycle, likely sourced from groundwater in nearby rocks.
“The precision and accuracy of these new techniques allowed us to estimate with confidence the temperature and composition of the mineralizing fluids,” stated Fiorella Arduin Rode, the lead author and a PhD researcher at Göttingen University’s Geoscience Centre. “Our findings support the idea that these amethysts crystallized at low temperatures from groundwater-like fluids.” The study suggests a model in which minerals like amethyst crystallize within volcanic cavities in a dark rock called basalt, influenced by regional temperature differences in the Earth’s crust. Arduin Rode added, “Understanding the conditions for amethyst formation—including the temperature and composition of the mineralizing fluid, the silica source, the timing of mineralization, and its connection to the host rocks—is vital for uncovering the process. This could greatly enhance exploration methods and contribute to sustainable mining practices in the future.”
Financial support for this research was provided by Research Grants — Doctoral Programmes in Germany, 2021/22 — 57552340 — Deutscher Akademischer Austauschdienst (DAAD).
