Esseneite from Xenoliths in Dacite Lavas: Crystal Structure and Genesis

Resumo

Esseneite from ultramafic xenoliths in dacite flow of the Ten’-01 paleovolcano at the Lena–Vilyui watershed, East Yakutia has been studied. The empirical formula of the mineral has been obtained using electron microprobe analysis: \({\text{C}}{{{\text{a}}}_{{0.99}}}{\text{Fe}}_{{0.52}}^{{3 + }}{\text{M}}{{{\text{g}}}_{{0.32}}}{\text{Fe}}_{{0.06}}^{{2 + }}{\text{T}}{{{\text{i}}}_{{0.05}}}{\text{Mn}}_{{0.01}}^{{2 + }}{\text{A}}{{{\text{l}}}_{{0.71}}}{\text{S}}{{{\text{i}}}_{{1.34}}}{{{\text{O}}}_{6}}\). Its crystal structure has been refined on the basis of the single crystal X-ray diffraction data, (R = 0.0152). The resulting crystal chemical formula is: \(^{{Ca}}{\text{C}}{{{\text{a}}}^{M}}{{\left( {{\text{Fe}}_{{0.48}}^{{3 + }}{\text{M}}{{{\text{g}}}_{{0.33}}}{\text{T}}{{{\text{i}}}_{{0.05}}}{\text{A}}{{{\text{l}}}_{{0.14}}}} \right)}^{T}}({\text{S}}{{{\text{i}}}_{{1.28}}}{\text{A}}{{{\text{l}}}_{{0.68}}}{\text{Fe}}_{{0.04}}^{{3 + }}){{{\text{O}}}_{6}}\). The monoclinic unit-cell parameters are: a = 9.7610(12), b = 9.8223(8), c = 5.3360(5) Å, β = 105.92(1), V = 441.89(8) ų, Z= 4, space group C2/c. The distribution of atoms over positions in the crystal structures of the Ca–Fe–Al clinopyroxenes and coordination polyhedron distortions, which reflect the formation conditions of the minerals, have been analyzed. The Yakutia esseneite has been formed in a highly oxidizing environment at 1200–950°C and approximate pressure of 2 kbar.