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Formula: Cu3(VO4)2
Anhydrous vanadate, monoclinic
paramorph of triclinic
mcbirneyite
Crystal System: Monoclinic
Specific gravity: 4.749 calculated
Hardness: 2 to 3
Streak: Reddish brown
Colour: Dark red with a brownish tint to black
Environments
Pseudolyonsite was approved in 2009; to date (January 2023) it has been reported only from the type locality.
Localities
At the type locality, the Tolbachik Volcanic field, Milkovsky District, Kamchatka Krai, Russia, pseudolyonsite
was found among other fumarolic minerals in the Yadovitaya fumarole. The scoria cones formed during the Tolbachik
Fissure Eruption in 1975; in 2009, 34 years after the eruption, there were still several gas vents with temperatures of
200 to 480oC at the top parts of the cones. The rocks there are altered by HF- and HCl-
rich gases and are often covered by a fluoride-rich crust. Pseudolyonsite, together with most other minerals
discovered on Tolbachik, occurs in cavities from several centimetres to 1.5 m in size under the surface crust.
Pseudolyonsite is one of the rarest minerals here. It is commonly present as an overgrowth on acicular
piypite. The pseudolyonsite crystals occur as needles that are
5 to 20 µm across and up to 0.5 mm in the length, which sometimes produce parallel intergrowths, sprays or openwork
clusters up to 2 mm in size. The mineral’s dark red crystals are clearly visible on the overall green background of
copper sulphates. Pseudolyonsite was most likely deposited at a
temperature ranging from 200 to 300oC. It appears that it occurred during one of the latest stages of
deposition.
Closely associated minerals are piypite,
palmierite, lyonsite and
hematite. Pseudolyonsite overgrows aggregates of
piypite, and it is itself rarely overgrown by minute crystals of
piypite, palmierite and
hematite. Other spatially related minerals are
magnetite, aphthitalite,
langbeinite, filatovite,
lammerite, vergasovaite,
rutile and native gold
(EJM 23.3.475-481).
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