Frankamenite

(repeating unit)K₃Na₃Ca₅(Si₁₂O₃₀)[F,(OH)]₄·(H₂O)IMA symbolFkm^[1]Strunz classification9.DG.90Crystal systemTriclinicCrystal classPedial (1)
(same H-M symbol)Space groupP1IdentificationColorGreenish blue, bluish gray, greenish gray, lilac grayCrystal habitLath, columnarCleavageGood on (010) and (100)TenacityBrittleMohs scale hardness5.5LusterVitreousStreakPale blueDiaphaneityTransparent to translucentSpecific gravity2.68Optical propertiesBiaxial positiveRefractive indexn_α = 1.536 n_β = 1.539 n_γ = 1.542Birefringence0.0060Dispersionr < v strongReferences^[2][3]

Frankamenite is the fluorine-dominate variation of the rare mineral canasite with a general formula of K₃Na₃Ca₅(Si₁₂O₃₀)[F,(OH)]₄·(H₂O).^[2]

Frankamenite belongs to the triclinic crystal system, with the bases of its structure containing Ca-Na mixed octahedra joined by octagonal tubes SiO₄ of the composition (Si₁₂O₃₀). Frankamenite has six Ca-Na mixed positions distributed amongst these octahedra, reflecting its varying compositions.^[4]

Frankamenite was named for the Russian mineralogist-crystallographer V. A. Frank-Kamentsky (1915–1994), who discovered the mineral.^[3]

Frankamenite occurs in association with the rare mineral charoite, which is found only in the Murun Massif of the Olyokma-Chara Plateau, Aldan Shield, Sakha Republic, Yakutia, Siberia, Russia. Here, metasomatism enriches a syenite massif with potassium when it comes into contact with a limestone at around 200–250 °C. This metamorphic process produces a potassium feldspar metasomatite, the typical geological environment for canasite and, therefore, frankamenite. Frankamenite and charoitein are exclusive to the Sakha Republic in this sort of environment, as mineralogists have yet to discover the minerals elsewhere.^[5]