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Formula: La2Fe2+(CO3)(Si2O7)
Sorosilicate (Si2O7 groups), biraite group,
lanthanum-bearing mineral
Crystal System: Monoclinic
Specific gravity: 4.682 calculated
Hardness: 5
Streak: White
Colour: Brown to dark brown
Common impurities: (HCO2)-
Environments
Biraite-(La) is a new mineral, approved in 2020 and to date (November 2024) reported only from the type locality.
Localities
At the type locality, Mochalin Log, Kyshtym, Chelyabinsk Oblast, Russia, three new isostructural minerals,
alexkuznetsovite-(La),
alexkuznetsovite-(Ce) and biraite-(La) were discovered
in polymineralic nodules. The new minerals form anhedral grains up to 0.3 mm × 0.4 mm
(alexkuznetsovite-(La)), 0.5 mm × 0.9 mm
(alexkuznetsovite-(Ce)) and 0.2 mm × 1.2 mm (biraite-(La))
embedded in granular aggregates consisting of the rare earth element minerals
allanite-(Ce),
allanite-(La),
bastnäsite-(Ce),
bastnäsite-(La),
fluorbritholite-(Ce),
perbøeite-(Ce),
perbøeite-(La),
percleveite-(Ce),
percleveite-(La),
törnebohmite-(Ce) and
törnebohmite-(La). All three new species are brown to dark brown,
translucent in thin fragments, with white streak, vitreous lustre and Mohs’ hardness of ~5. Calculated specific
gravities are 4.713 (alexkuznetsovite-(La)), 4.687
(alexkuznetsovite-(Ce)) and 4.682 (biraite-La
(MM 85.5.772-783).
Biraite-(La) is formed in polymineralic nodules by metasomatism of
primary
bastnäsite-(La) -
bastnäsite-(Ce) of probable alkaline
pegmatitic origin
(HOM).
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