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Formula: Na2Mg(AsO4)F
Arsenate
Crystal System: Orthorhombic
Specific gravity: 3.377 calculated for the holotype and 3.226 calculated for the cotype
Streak: White
Colour: Colourless or pale pinkish
Environments
Evseevite is a relatively new mineral, approved in 2019 and to date (October 2023) reported only from the
type locality.
Localities
The type locality, the Arsenatnaya fumarole, Second scoria cone, Northern Breakthrough, Great Fissure
eruption, Tolbachik Volcanic field, Milkovsky District, Kamchatka Krai, Russia, is the hottest fumarole, with
temperatures reaching 500°C at the deepest measured areas. Evseevite specimens were collected in 2018 from
hot zones 1.5 to 2 m below the surface
(AM 108.1993).
Evseevite is represented by two chemical varieties. The variety close to the end-member, the holotype, is
associated with sanidine,
hematite, tenorite,
aegirine, cassiterite,
sylvite, halite,
johillerite, badalovite,
calciojohillerite,
hatertite, arsmirandite,
yurmarinite, axelite,
polyarsite, aphthitalite,
potassic-magnesio-fluoro-arfvedsonite,
litidionite,
ferrisanidine and tridymite.
The P- and S-enriched variety, the cotype, is associated with hematite,
fluorophlogopite, svabite,
fluorapatite, tilasite,
calciojohillerite,
forsterite, cassiterite,
belomarinaite and
aphthitalite.
Evseevite occurs as prismatic, acicular or hair-like crystals up to 0.7 mm long combined in clusters up to
0.5 mm, brushes or crusts up to 2 × 2 mm
(MM 87.839-848).
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