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Formula: Cu11Mg2Cl18(OH)8.16H2O
Hydrated chloride
Crystal System: Monoclinic
Specific gravity: 2.57 measured, 2.563 calculated
Hardness: 3
Streak: Light green
Colour: Bright green
Solubility: Slowly dissolves in water at room temperature; in humid air feodosiyite is unstable and alters to a
bluish friable aggregate of hydrous copper and magnesium chlorides after several months.
Environments
Feodosiyite is a relatively new mineral, approved in 2015 and to date (March 2025) reported only from the
Type locality.
Localities
At the type locality, the Glavnaya Tenoritovaya fumarole, Second scoria cone, Northern Breakthrough, Great Fissure
eruption, Tolbachik Volcanic field, Milkovsky District, Kamchatka Krai, Russia, feodosiyite occurs as
sublimates on basaltic
scoria near a volcanic
fumarole vent. The gas temperature in this zone during
collecting was about 100°C. Feodosiyite is closely associated with
belloite, avdoninite,
sylvite, carnallite,
chlorothionite and
dioskouriite. Other associated minerals include
eriochalcite, halite,
mitscherlichite, sanguite,
chrysothallite,
romanorlovite,
mellizinkalite, flinteite,
kainite, gypsum and
sellaite; hematite,
tenorite and chalcocyanite are
earlier, sublimate minerals.
Feodosiyite occurs as tabular or prismatic crystals to 0.1 mm, combined in interrupted incrustations overgrowing
basalt scoria
(AM 104.1867).
Feodosiyite from the Glavnaya Tenoritovaya
Fumarole - Image
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