A New Chladniite Variety from Volcanic Exhalations. The Genetic Crystal Chemistry of Chladniite

Abstract

Abstract A new, unusual, Fe-free, and As5+-rich variety of chladniite, a rare phosphate of the fillowite group, was found in exhalations of the active Arsenatnaya fumarole, Tolbachik Volcano, Kamchatka, Russia. It belongs to the high-temperature (500–750°С) paragenesis with calciojohillerite, fluorapatite, metathénardite, diopside, enstatite, forsterite, and hematite. This genetic type is novel for fillowite-group minerals. The crystal structure of Tolbachik chladniite is determined, R1 = 4.32%. It is trigonal, R–3, a = 14.9831(2) Å, c  = 42.8050(7) Å, and V = 8322.1(3) Å3. The structural formula is M13(Na0.56$$\square $$0.44)$$_{2}^{{M21}}$$(Na0.81Ca0.19)$$_{2}^{{}}$$$$_{{}}^{{M31}}$$(Na0.97$$\square $$0.03)$$_{6}^{{M12}}$$(Ca0.64Na0.36)$$_{6}^{{M1}}$$MnM2(Mg0.54Ca0.46)M3-9Mg30M10(Mg0.94Ca0.06)$$_{6}^{{M11}}$$Mg6(P33.53As2.47)O144 (Z = 3). The crystal chemical features of chladniite and their correlation with formation settings are discussed for all chladniite findings known in meteorites and terrestrial objects of different genetic types.