Formula:(Ca,Ce,Na)(Nb,Ta,Ti)2(O,OH,F)6
Multiple oxide containing niobium, tantalum and titanium, commonly metamict,
euxenite group
Crystal System: Orthorhombic
Specific gravity: 4.78 (calculated)
Hardness: 4 to 4½
Streak: Greyish brown
Colour: Black, dark brown, lemon-yellow to yellow-brown
Solubility: Sparingly attacked by sulphuric acid (AM 32.373)
Environments
Plutonic igneous environments
Pegmatites
Carbonatites
Metamorphic environments (only at the Dark Star Claim, Montana)
Fersmite is a rare accessory mineral in nepheline syenite and carbonatites, and
also in miarolitic cavities in an albite pegmatite dike (HOM).
At the type locality, Vein Number 37A, Buldym Lake, Vishnevye Mountains, Chelyabinsk Oblast, Russia, fersmite is found in a
syenite pegmatite, in veins composed of
microcline and plagioclase. It is an
alteration product of pyrochlore, intergrown with
columbite-(Fe). Associated minerals include
columbite, plagioclase,
microcline, biotite,
pyrochlore, hornblende,
apatite, titanite, and
quartz; accessories include pyrite,
magnetite, muscovite,
zircon and xenotime (AM 32.373, Dana, HOM).
At the Dark Star Claim, Ravalli county, Montana, USA, fersmite occurs in marble,
intergrown with columbite-(Fe), associated with
monazite, ancylite,
baryte, quartz and
apatite (Dana, HOM, AM 44.1-8). Rare-earth minerals are usually of
granitic or pegmatitic origin, but here they occur in a metamorphosed
limestone probably of sedimentary origin, representing a unique occurrence
(AM 44.1-8).
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