Multiple oxide containing niobium, tantalum and titanium, commonly metamict, euxenite group
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)

Plutonic igneous environments
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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