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Formula: Ba3Nb6(Si2O7)2O12
Sorosilicate (Si2O7 groups), barium- and
niobium- bearing mineral
Crystal System: Hexagonal
Specific gravity: 4.16 measured, 4.25 calculated from the empirical formula
Hardness: 6 to 7
Streak: White
Colour: Brown
Common impurities: Zr,Al,Fe,Ta,Ca,Na,K
Environments
Localities
At the Sevattur carbonatite complex, Krishnagiri District, Tamil Nadu, India,
pyrochlore group minerals are common in the Neoproterozoic
(1 billion to 538.8 million years ago) rocks. This complex is composed of
dolomite-, calcite-, banded- and
blue- carbonatite together with
pyroxenite, albitite
and diverse syenites. There are also alteration assemblages
containing belkovite and baotite
(MM 85.4.588-606).
At Ust'-Biraya, Biraya Fe-REE ore occurrence, Biraya and Bya Rivers confluence area, Vitim Plateau, Irkutsk Oblast,
Russia, the iron – rare earth element deposit comprises a sequence of lens-shaped
carbonatite bodies, accompanied by
fenites, which are between 50 and 300 m in thickness and up to 10 km
long. The fenites developed at the expense of Precambrian (more than
541 million years ago) gneisses, and
schists are crosscut by Permian (298.9 to 251.9 million years ago)
granitoids. The deposit is considered to be of
mid-Paleozoic age (about 450 to 350 million years ago).
Cordylite-(La) was discovered in a vein composed mainly of the
carbonates
aragonite–strontianite,
emmonite, strontium-rich
calcite, iron-rich
dolomite, cordylite-(La)
and cordylite-(Ce).
Also contained in the vein and in direct association with
cordylite-(La) are baryte,
biraite-(Ce), niobium-rich
chevkinite-(Ce),
fergusonite-(Nd),
ancylite-(Ce),
ancylite-(La),
daqingshanite-(Ce),
bastnäsite-(Ce),
hydroxylbastnäsite-(Ce),
carbocernaite,
monazite-(Ce), talc,
thorite, humite,
galena, pyrite and
pyrrhotite. Minerals in the outer zone of the vein near the
fenite include tremolite,
winchite,
ferriallanite-(Ce),
törnebohmite-(Ce), belkovite,
ancylite-(La),
strontianite,
fergusonite-(Ce),
pyrochlore,
nioboaeschynite-(Ce) and
nioboaeschynite-(Nd)
(CM 50.5.1281-1290).
At the type locality, the Vuoriyarvi alkaline-ultrabasic massif, Northern Karelia, Murmansk Oblast, Russia,
belkovite occurs as thin rims on barium-rich
pyrochlore and as brown, transparent crystals with an adamantine
lustre, up to 1.0 mm long, commonly barrel shaped. Other associated minerals include
magnetite, chlorite,
pyrite, pyrrhotite,
apatite, baryte,
alstonite and
nenadkevichite in dolomite
- calcite
carbonatites
(AM 76.1728-1735).
Belkovite is of secondary origin, formed by alteration
of barium-rich pyrochlore
during dolomitisation of calcite
carbonatites in
pyroxenites
(HOM).
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