Arsenudinaite

arsenudinaite

Images

Formula: NaMg4(AsO4)3
Anhydrous arsenate, forms a series with udinaite
Specific gravity: 3.816 calculated
Hardness: 3½:
Streak: White
Colour: Beige, pale brownish, brownish-yellowish or greyish-brownish
Epitaxy: Interesting double epitactic intergrowths were observed: “ribbed” parallel aggregates of small, flattened forsterite crystals epitactically overgrow a larger prismatic anhydrite crystal and crystals of udinaite or arsenudinaite epitactically overgrow forsterite (Mindat).
Environments

Fumeroles

Arsenudinaite is a relatively new mineral, approved in 2018 and to date (September 2025) reported only from the type locality.

Localities

At the type locality, the Arsenatnaya fumarole, Second scoria cone, Northern Breakthrough, Great Fissure eruption, Tolbachik Volcanic field, Milkovsky District, Kamchatka Krai, Russia, minerals of the udinaitearsenudinaite series were found. Strong fumarolic activity occurs here currently, and the active, hot oxidising type fumarole is characterised by an outstanding mineral diversity.
The type specimens of both udinaite and arsenudinaite were collected from 3 to 4 m deep under the day surface, the hottest zone of the Arsenatnaya fumarole. The temperatures in this area measured during collecting were 450 to 470oC. Sublimate incrustations in this zone are up to 4 cm thick and usually mainly consist of anhydrite, diopside and hematite, typically with significant amounts of arsenatevanadate garnets of the berzeliiteschäferite series and svabite. Udinaite and arsenudinaite are associated, besides with these minerals, with one another and with calciojohillerite, tilasite, reznitskyite, ludwigite, rhabdoborite group borates, forsterite, magnesioferrite, fluorapatite, pliniusite, arsenowagnerite, wagnerite, powellite, scheelite, and baryte.
In July 2021 in the upper part of this zone, the As-richest variety of arsenudinaite close to the end-member NaMg4(AsO4)3 was found. It is associated with hematite, fluorophlogopite, sanidine, forsterite, enstatite, calciojohillerite and fluorapatitesvabite series members. (Minerals 12.7.850)
Arsenudinaite from the Arsenatnaya fumarole - Image

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