Images
Formula: CaY(Al2Fe2+)[Si2O7][SiO4]O(OH)
Sorosilicate (Si2O7 groups), allanite group of
the epidote supergroup,
yttrium-bearing mineral
Crystal System: monoclinic
Specific gravity: 3.5 to 4.2 measured
Hardness: 5½ to 6½
Streak: Greyish brown
Colour: Black
Environments
Plutonic igneous environments
Pegmatites
Metamorphic environments
Allanite-(Y) is often slightly radioactive due to minor uranium and/or thorium impurities, hence it is often
metamict (virtually amorphous owing to the breakdown of the original crystal structure by internal bombardment with
alpha particles emitted by radioactive atoms within the mineral).
Localities
At the Julianna pegmatite, DSS Piława Górna Quarry, Piława Górna, Dzierżoniów County, Lower Silesian Voivodeship,
Poland, pilawite-(Y) occurs as white, translucent, brittle crystals
up to 1.5 mm in size. The assemblage crystallised in the sequence:
keiviite-(Y) →
gadolinite-(Y) to
hingganite-(Y) +
hellandite-(Y) →
pilawite-(Y) → allanite-(Y) →
epidote/zoisite
(MM 79.1143-1157).
Allanite-(Y) was originally described from the Zaaiplaats tin mine, Potgietersrust district, Transvaal,
South Africa. It occurs in an explosion breccia in a
cassiterite-bearing
pipe in granite, in hydrothermally altered
arkoses, and in a
granitic
pegmatite
(Dana).
At the type locality, the Åskagen Quarry, Persberg ore district, Filipstad, Värmland County, Sweden,
Allanite-(Y) occurs as an accessory phase in the blocky zone of the
niobium - yttrium - fluorine
granitic
pegmatite. It forms rims together with
iimoriite-(Y),
gadolinite-(Y) and
allanite-(Nd) around altered crystals of
thalénite-(Y). Allanite-(Y) replaced
primary
thalénite-(Y) during an episode of early post-magmatic hydrothermal
activity. Allanite-(Y) forms euhedral crystals up to 1 mm in size
(MM 89.1.172-182)
Allanite-(Y) from the Åskagen Quarry -
Image
At the Longs Peak - St Vrain batholith near Jamestown, Jamestown District, Boulder county,
Colorado, USA, centimetre to decimetre sized mineralised pods and veins consist of zoned
mineral assemblages dominated by fluorbritholite-(Ce)
in a core 10 cm thick, with monazite-(Ce),
fluorite and minor quartz,
uraninite and sulphides. The core is surrounded by a
typically millimetre thick rim of allanite-(Ce), with
minor monazite-(Ce) in the inner part of the rim.
Bastnäsite-(Ce),
törnebohmite-(Ce) and
cerite-(CeCa) appear in an intermediate zone between core and
rim, often just a few hundreds of microns wide
(R&M 96.3.252-253).
Back to Minerals