KNa2CaFeMg4Si8O22OH2

KNa2CaFeMg4Si8O22OH2 is a complex alkali- and alkaline-earth-containing silicate formula that is commonly interpreted as an end-member composition within the amphibole group of minerals. The arrangement combines potassium and sodium along with calcium in the larger cation sites, and iron and magnesium in the octahedral sites, balanced by a double-chain silicate framework of eight silicon tetrahedra and hydroxyl groups. In conventional notation the formula is often written as KNa2CaFeMg4Si8O22(OH)2.

Structure and classification

Amphiboles are inosilicates characterized by a double-chain silicate structure. In this family, large cations occupy channels in the structure (the A-site) while smaller cations reside in the B and C sites that help coordinate the silicate tetrahedra. The presence of OH groups is typical for amphiboles and contributes to their characteristic cleavage and hydration. The KNa2CaFeMg4Si8O22OH2 formula fits the alkali-calcic, iron- and magnesium-rich end-members of this group.

Occurrence and environments

If realized in nature, compositions like KNa2CaFeMg4Si8O22OH2 would be expected in metamorphic or hydrothermal rocks where alkali metals coexist with calcium and siderophile or Mg-rich components. Amphibole-rich assemblages in ultrabasic to intermediate rocks, as well as high-temperature hydrothermal systems, often host complex substitutions among Na, K, Ca, Fe, and Mg.

Properties and identification

Amphiboles are typically monoclinic with two perfect cleavages near 56 and 124 degrees and a hardness around 5–6 on the Mohs scale. Physical properties such as color, luster, and density vary with the precise Fe/Mg ratio and alkali content. The studied formula would reflect a highly variable solid solution, with potential color ranging from light to dark green or brown depending on iron content.

Notes

As an explicit named mineral, KNa2CaFeMg4Si8O22OH2 is not widely recognized in standard databases; it is best described as a theoretical or end-member composition illustrating the diversity of amphibole chemistries.