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This paper presents the results of mineralogical and crystallographic studies in the first Russian finding of a mineral belonging to the fluoride class, viz., knasibfite, which was found in fumarolic deposits discharged during the Great Tolbachik Fissure Eruption. We investigated its crystal structure to corroborate the fact that knasibfite is orthorhombic, sp. gr. Imm2; a = 17.181 (2), b = 5.537 (4), c = 9.205 (7) Å, V = 875 (1) Å3. The structural features that can be seen in this mineral classify it as belonging to a special group of compounds that has inherited its structure from a high temperature modification or from the medium where it crystallized. [ABSTRACT FROM AUTHOR]

Unusual variety of forsterite strongly enriched by pentavalent constituents, phosphorus and arsenic (up to 12.9 wt% P2O5 and up to 16.0 wt% As2O5) was found in sublimates of the Arsenatnaya fumarole, Tolbachik volcano, Kamchatka, Russia, in association with As- and P-depleted forsterite, anhydrite, diopside, hematite, spinel, haüyne, fluorapatite, svabite, berzeliite, calciojohillerite, and arsenudinaite. This forsterite is the P-richest natural olivine which also demonstrates first case of As incorporation in an olivine-type silicate. P- and As-rich forsterite is orthorhombic, space group Pbnm, unit-cell parameters are: a = 4.7762(2), b =10.2643(4), c = 5.9646(2) Å and V = 292.41(2) Å3. Its crystal structure was solved on single crystal and refined to R = 2.88%. The crystal chemical formula is M(1)(Mg0.83[vac]0.17)M(2)(Mg0.95[vac]0.05)[Si0.56P0.32As0.12]O4. The substitution scheme for the incorporation of pentavalent constituents in forsterite is: VIMg2+ + 2IVSi4+ ↔ VI[vac]0 + 2IVT5+, in which T5+ = P, As and [vac] = vacancy. No sign of vacancies in the tetrahedrally coordinated T site is found. [ABSTRACT FROM AUTHOR]