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Crystal symmetry and chemical composition of yukonite: TEM study of specimens collected from Nalychevskie hot springs, Kamchatka, Russia and from Venus Mine, Yukon Territory, Canada

O. Nishikawa^1,*, V. Okrugin², N. Belkova^3,, I. Saji³, K. Shiraki⁴ and K. Tazaki⁴

¹ Mineral Industry Museum, Faculty of Engineering and Resource Science, Akita University, Tegata, Akita 010-8502, Japan
² Institute of Volcanology and Seismology, Far East Division, Russian Academy of Science, Petropavlovsk-Kamchatskii 683006, Russia
³ Graduate School of Natural Science and Technology, Kanazawa University, Kakuma, Kanazawa 920-1192, Japan
⁴ Department of Earth Sciences, Kanazawa University, Kakuma, Kanazawa 920-1192, Japan

^* E-mail: nisikawa{at}lfp03.mine.akita-u.ac.jp

Yukonite, a rare arsenic-bearing hydrous mineral, the crystal symmetry and variety in chemical composition of which have so far been insufficiently studied, has been found in the modern deposit of Nalychevskie hot springs in Central Kamchatka, Russia. This is the first finding of yukonite in Eastern Eurasia and Siberia. Yukonite specimens from Nalychevskie hot springs and from Venus Mine in Yukon Territory, Canada, have been investigated using an analytical transmission electron microscope (TEM). Yukonite is a crystalline substance with extremely thin (~5 nm) platy morphology. Yukonite from Venus Mine forms brittle aggregates in which grains are irregularly bent and randomly distributed. At Nalychevskie hot springs, yukonite occurs as single plates, coexisting with some amorphous material of similar composition. Intensity distributions in electron diffraction indicate that most plates of yukonite at Nalychevskie hot springs have orthorhombic symmetry, but some are hexagonal with a_hex = 11.3 Å. The orthorhombic cell is C-centred with a_orth = 3a_hex, b_orth = a_hex. High-resolution images of edge-on mounts indicate that the periodicity normal to the planes is d₀₀₁ = 11.2 Å. Yukonite from Nalychevskie hot springs contains anomalously high Si relative to that in yukonite from Venus Mine and that reported previously. Strong negative correlation between As and Si indicates that Si substitutes for As in the structure.

KEYWORDS: yukonite, arsenic mineral, TEM, crystal symmetry, chemical composition