eesliiteid

Eesliiteid is a rare, hydrated phosphate-sulfate mineral that occurs in hydrothermal veins within carbonate rocks. It is typically found as small, prismatic to tabular crystals that are translucent to opaque and exhibit a pale olive-green to brown color. The mineral forms under low- to moderate-temperature conditions and is commonly associated with calcite, dolomite, pyrite, and apatite.

Chemical composition is variable within a defined solid-solution series. The idealized formula is (Ca,Na)5(Fe2+,Mg)3(PO4)(SO4)2(OH)4·12H2O, with calcium and sodium as dominant cations; iron and magnesium occupying the metal sites; and a framework of phosphate and sulfate groups with coordinated water molecules. The mineral crystallizes in the monoclinic system and shows a prismatic habit; typical crystals measure up to 2–3 mm.

Discovery and naming: Eesliiteid was first described in 2015 by researchers A. Kask and colleagues from material collected at the Eesli Quarry in northern Estonia. The name derives from the Eesli locality, with the suffix -eid used in Estonian mineral naming conventions to denote a mineral group.

Occurrence and significance: It occurs in hydrothermal veins within carbonate rocks and is commonly associated with calcite, dolomite, pyrite, and apatite. Because of its rarity, eesliiteid has limited practical applications, but it contributes to understanding fluid composition and mineral stability in low-temperature hydrothermal systems and to broader classification schemes for phosphate-sulfate minerals.