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Formula: K[B5O7(OH)2].H2O
Pentaborate
Crystal System: Monoclinic
Specific gravity: 2.13 measured, 2.112 calculated for the empirical formula
Hardness: 2½
Streak: White
Colour: Colourless
Luminescence: Non-fluorescent under both UV and an electron beam
Environments
Yarzhemskiite is a relatively new mineral, approved in 2018 and to date (March 2025) reported only from the
type locality.
Localities
At the type locality, Borehole no. 800, Chelkar salt dome, Aksai Valley, Shalkar District, Aktobe Region,
Kazakhstan, yarzhemskiite was found in core of the borehole as separate crystals or grains having
irregular outlines embedded in a
halite–sylvite rock. Other
associated minerals are carnallite,
polyhalite, gypsum,
strontioginorite,
satimolite and quartz.
Yarzhemskiite was formed as a result of diagenesis or of the post-diagenesis processes in
boron-bearing
evaporite rocks. It is likely that the formation of
yarzhemskiite and associated borates accompanies the recrystallisation
of the halite–sylvite rock.
Crystals of yarzhemskiite are thick tabular, slightly elongate (short prismatic) or equant, commonly coarse,
with a sculptured and/or cavernous surface. Well-formed crystals are up to 0.5 mm × 0.7 mm × 1 mm (typically much
less) whereas grains having irregular outlines are up to 1 mm × 1.5 mm × 2 mm. The crystals typically contain
inclusions of halite, sylvite and
quartz. Yarzhemskiite is transparent and colourless, with a white
streak and vitreous lustre
(MM 84.335–342).
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