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Formula: TlPbSbS3
Sulphosalt, thallium- and antimony-
bearing mineral
Crystal System: Monoclinic
Specific gravity: 6.696 calculated for the empirical formula
Hardness: 3 to 4
Streak: Black
Colour: Black
Environments
Markwelchite is a new mineral, approved in 2024 and to date (October 2024) reported only from the type locality.
Localities
At the type locality, Jas Roux, La Chapelle-en-Valgaudemar, Gap, Hautes-Alpes, Provence-Alpes-Côte d'Azur, France,
the deposit is hosted in dolomitic
metalimestones of Triassic age (251.902 to 201.4 million years ago) and
appears as several sedimentary lenses embedded in anatexites. These
lenses protrude and form small cliffs parallel to the valley. Samples studied consist of boulders weathered out of the
cliffs which contained small veinlets of primary sulphosalts
accompanied by a variety of secondary minerals.
Markwelchite is associated with stibnite,
twinnite, guettardite,
pierrotite, realgar and
protochabournéite. The formation of markwelchite is linked to
the activity of thallium - lead -
antimony rich fluids during the hydrothermal evolution of the Jas Roux
deposit. The markwelchite occurs as black anhedral crystals up to 40 μm in size; it is opaque
with a metallic lustre and a black streak
(MM 88.503–509).
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