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Formula: CaCe(Fe3+AlFe2+)[Si2O7][SiO4]O(OH)
Sorosilicate (Si2O7 groups), allanite group,
cerium-bearing mineral
Crystal System: Monoclinic
Specific gravity: 4.22 measured, 4.16 calculated
Hardness: 6
Streak: Brown
Colour: Black
Common impurities: Al
Environments
Pegmatites
Metamorphic environments
Localities
At the type locality, the Neprimetnyi pegmatite, Mount Ulyn Khuren, Khaldzan Buragtag massif, Myangad District,
Khovd Province, Mongolia, ferriallanite-(Ce) is of metasomatic origin and represents a major product of the
pseudomorphism of rare-earth-element-rich minerals of the
eudialyte group. It occurs as an aggregate of subhedral grains from
0.3 x 0.5 to 1 x 2 mm within masses of zircon,
quartz and kainosite-(Y)
and is associated with aegirine,
fergusonite-(Y) beta,
yttrium-rich and normal ilvaite,
hingganite-(Ce), neodymium-rich
allanite-(Ce), magnetite,
fayalite and fluorite
(CM 40.1641-1648)
At the Bastnäs Mines, Riddarhyttan, Skinnskatteberg, Västmanland County, Sweden,
quartz banded hematite ore
occurs adjacent to a magnetite
skarn ore, the latter replacing a
dolomite layer. The cerite
ore was worked in two small mines, where it formed a restricted layer in the
skarn zone, composed mainly of
cerite-(CeCa), tremolitic
amphibole, allanite-(Ce),
bastnäsite-(Ce),
törnebohmite-(Ce) and talc.
Sulphide minerals, dominantly chalcopyrite,
bismuthinite and
molybdenite, are closely associated with the lanthanide minerals, and
usually are interstitial to them.
Three samples were analysed.
Sample 1 consists of coarse-grained ferriallanite-(Ce) forming euhedral crystals up to 8 mm long, together
with bastnäsite-(Ce) –
bastnäsite-(La)
and quartz. Bismuthinite,
chalcopyrite and
molybdenite
occupy smaller voids between the ferriallanite-(Ce) crystals.
Sample 2 consists of rounded aggregates of fine-grained cerite-(CeCa)
with lesser amounts of bastnäsite-(Ce), surrounded by margins of
anhedral,
irregular ferriallanite-(Ce) grains. Törnebohmite-(Ce) and a
new sorosilicate mineral species are closely associated with the ferriallanite-(Ce). Patches of
fluocerite-(Ce), partly altered to
cerianite and
bastnäsite-(La), occur sporadically in the
cerite-bearing assemblage.
Sample 3 consists of euhedral to subhedral crystals of ferriallanite-(Ce) up to 1 mm long, together with
cerite-(CeCa) crystals, in massive
bismuthinite with subordinate
chalcopyrite and
molybdenite
(CM 41.1233-1240).
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