The Lauzière Massif, located in the French Alps, is renowned for its lithological diversity and its wealth of rare minerals, evidence of the complex geological processes that have shaped the region. Among the minerals of interest, bazzite stands out for its unique composition and rarity. This beryllium silicate, often identifiable by its intense blue color, has a hexagonal crystalline structure and contains rare earth elements, including scandium. Its study offers a valuable opportunity to better understand the interactions between metamorphism, hydrothermalism, and rare element geochemistry. This article focuses on the bazzites of Kersy Forest, an emblematic locality of the Lauzière Massif, by examining their geological context, their mineralogical and chemical properties, their genesis, and their scientific and economic implications.

Photo : Bazzite from Kersy Forest, Bonneval, Massif de la Lauzière, Savoie, France © Pascale and Daniel Journet

The bazzite crystals discovered in the Kersy Forest have a particularly aesthetic and well-developed morphology. They appear as hexagonal prisms, often elongated and sometimes slightly squat, with sharp and smooth terminations. Their size generally varies from a few millimeters to more than a centimeter, and they are distinguished by an intense azure blue color, characteristic of this mineral. The crystals exhibit intense pleochroism.

The bazzites of the Kersy Forest adopt a typical but slightly variable chemical composition depending on their local geological environment. Their general formula, Be₃Sc₂Si₆O₁₈, reflects their belonging to the beryl group, with scandium playing a central role in partially replacing aluminum in the crystal structure. This substitution, essential to define bazzite, is responsible for its characteristic blue hue, accentuated by the presence of minor chromophoric elements. Indeed, traces of iron (Fe²⁺ or Fe³⁺) can be detected, subtly influencing the nuances of the coloring. Furthermore, the hexagonal crystalline structure of this cyclosilicate confers great stability to the scandium and beryllium ions, despite local variations in silicon or impurities such as magnesium, sodium or lithium, observed in certain samples.

The study of the Kersy Forest bazzites is of significant scientific importance, as it provides information on the mechanisms of concentration of rare elements in metamorphic and hydrothermal environments. Rare earths, essential to many modern technologies, such as batteries, electronic devices and renewable energies, are at the heart of current economic and geopolitical concerns. Although bazzite itself does not represent an exploitable source of rare earths due to its rarity, it is a promising indicator for the exploration of potential deposits.

In addition, understanding the conditions of formation of bazzites contributes to the modeling of the geological processes responsible for the concentration of strategic resources in the Earth’s crust, paving the way for more targeted approaches in mineral exploration.

Photo : Bazzite from Bois de Kersy, Bonneval, Massif de la Lauzière, Savoie, France © Orso Martinelli