Oxynatromicrolite
Oxynatromicrolite is an IMA-recognized mineral species in the halide class, with the chemical formula (Na,Ca,U)₂(Ta,Nb)₂O₆(O,F).
It crystallizes in the cubic crystal system (space group Fd-3m). It is a member of the Pyrochlore-microlite structural group.
Key properties: Mohs hardness 5; specific gravity 6.58; Yellowish brown streak; Greasy to waxy luster; optically isotropic.
IMA symbol Onmic, IMA status: approved, named in 2013.
Found in a rare-element granitic pegmatite, crystallized from a highly evolved felsic magma or transitional magmatic-hydrothermal fluids enriched in volatile, flux and lithophile elements.
It is found associated with Quartz, albite, potassium feldspar, muscovite, kaolinite, tantalite-Mn, stibiotantalite, pollucite, spodumene, montebrasite, Hf-rich zircon, tourmaline.
The name refers to a member of the microlite group with prefixes to indicate essential oxygen (oxy) in the Y site and dominant sodium (natro) in the A site.