Eristiins

Eristiins are a rare group of silicate minerals defined by their elongated prismatic crystals and blue-green color. The term eristiin derives from the Eristi region, where the first material fitting the description was documented. In mineralogical literature, eristiins are described as hydrated aluminosilicates with prominent pleochroism and a distinctive optical anisotropy that makes them attractive to collectors and researchers alike.

Chemically, eristiins are dominated by aluminum and silica with substitutions by magnesium and iron. The estimated formula is (Na,K)2(Mg,Fe)Al2Si4O12·nH2O. They crystallize in the monoclinic system, with a Mohs hardness of about 6.0–6.5, a vitreous luster, and transparency ranging from translucent to transparent. They exhibit weak to moderate cleavage along {010} and {001} and strong pleochroism from pale blue to emerald-green under polarized light.

Geologically, eristiins form in serpentinite-hosted hydrothermal veins within ultramafic rocks and are typically associated with talc, magnesite, and chrysotile. Formation temperatures are estimated around 200–350°C during low-grade metamorphism or metasomatic alteration near fracture zones. The deposits are localized, leading to limited supply and high specimen value.

Discovery and taxonomy: Eristiins were first described in 1984 by geologist Arin Tseli. The name eristiin was later formalized to describe a mineral group with subvarieties by 1992.

Applications and significance: In addition to mineral collecting, eristiins are studied for their crystal chemistry and potential uses in optical components due to their color and birefringence. Ongoing research focuses on stability ranges and trace-element signatures that could illuminate regional metamorphic histories.