Vienlaicgu

Vienlaicgu is a rare silicate mineral first identified in the 19th century in the high-altitude limestone formations of the Pyrenees. The name derives from the Latin words *vien* (meaning "mountain") and *laicgu* (a local term for "silicate crystal"). Its chemical formula is Ca₂(Mn,Mg)(Al,Si)₂O₆, indicating it is a member of the mahoganyite group. The mineral crystallizes in the orthorhombic system and typically occurs as elongated prismatic crystals that are pale green to yellowish, with a vitreous luster.

Geologically, vienlaicgu forms as a secondary mineral in hydrothermal veins where manganese-rich fluids interact with magnesium-bearing host rocks. It is associated with other silicates such as lawsonite, prehnite, and epidote. The mineral’s stability range is between 200 °C and 400 °C under low pressure, making it an effective marker of metamorphic grade in low‑temperature terrains. Structural studies using X‑ray diffraction have revealed a unique channeling of cations, which is responsible for its distinctive optical anisotropy and characteristic pleochroic response.

Historically the mineral has attracted the interest of both collectors and researchers. In the early 1900s, it was prized for decorative purposes due to its delicate color and transparency. More recently, scientific studies have investigated vienlaicgu as a potential reservoir rock for small‑scale geothermal energy production, owing to its porosity and trace‑element contents. The mineral has also been used as a reference standard in laser ablation inductively coupled plasma mass spectrometry to calibrate manganese and calcium isotopic analyses.

In contemporary mineralogy, vienlaicgu remains a subject of limited but ongoing research. Its relative rarity and distinctive physical properties continue to make it a topic of interest in both academic studies of metamorphic petrology and in the private collecting community.