Amethyst is a violet variety of quartz that has been used as a gemstone for many centuries and is a key economic resource in northern Uruguay. Geodes are hollow rock formations often with quartz crystals, such as amethyst, inside.

Amethyst geodes in Uruguay have been found in cooled lava flows, which date from the original breakup of the supercontinent Gondwana around 134 million years ago. However, their formation has remained a mystery. A research team led by the University of Göttingen has now investigated, using cutting-edge techniques. The researchers discovered that the amethyst geodes formed at unexpectedly low crystallization temperatures of just 15 to 60 °C.

Taken with their other results, researchers were able to propose a new model to explain their formation. The research is published in the journal Mineralium Deposita.

Amethyst has been mined for over 150 years in the Los Catalanes District of Uruguay, where the research was carried out. This is an area renowned for the deep violet color and high quality of its gems, as well as magnificent giant geodes sometimes over 5 m high. The deposits here have been recognized as one of the top 100 geological heritage sites in the world, highlighting their scientific and natural value.

However, limited knowledge of how these geodes formed has made locating them challenging, relying largely on miners' experience. To address this, researchers conducted extensive geological surveys across more than 30 active mines, analyzing geode minerals, geode-hosted water, and groundwater. Using advanced techniques like nucleation-assisted microthermometry of initial one-phase fluid inclusion and triple-oxygen-isotope geochemistry, the team uncovered new insights into how these prized geodes formed.

As well as finding that the amethyst geodes formed at unexpectedly low crystallization temperatures, the researchers also showed that the mineralizing fluids had low levels of salinity and a proportion of isotopes consistent with water originating from the natural weather cycle, which probably came from groundwater held 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," said Fiorella Arduin Rode, lead author and Ph.D. 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 proposes a model where mineral phases like amethyst crystallize within volcanic cavities in a dark rock known as basalt, influenced by regional variations in temperature in the Earth's crust.

Arduin Rode adds, "Understanding the conditions for amethyst formation—such as the temperature and composition of the mineralizing fluid, as well as the silica source, the timing of the mineralization, and its relationship with the host rocks—is crucial for unraveling the process. This could significantly improve exploration techniques and lead to sustainable mining strategies in the future."