Thin-section view in cross-polarized light showing large agrellite grains (light-blue prisms) in a matrix of greenish aegirine-augite and a fine granular mixture of miserite and feldspar. (Photo by Gordon Medaris.)

Formula: NaCa2Si4O10F Triclinic


Rock sample showing agrellite prisms up to 1 cm long in granitic rock. (Photo by Gordon Medaris.)

Agrellite is a very rare mineral found in mafic, sodium-rich and silicon-poor igneous and metamorphic rocks.

MARATHON COUNTY: Agrellite was found as light-gray prisms as much as 1 cm. in maximum dimension in a leucocratic dike exposed during road construction along I-39 just north of Wausau, Marathon County (Medaris and Fournelle, 2007). This is only the second known occurrence in the world of this mineral.