Fengchengite
A rare eudialyte-group mineral with a vacancy-rich complex formula.
Intro adapted from Wikipedia (CC BY-SA).
Fengchengite is an IMA-recognized mineral species in the silicate class, with the chemical formula Na₁₂ ₃Ca₆Fe³⁺₃Zr₃Si(Si₂₅O₇₃)(H₂O)₃(OH)₂.
It crystallizes in the hexagonal crystal system (space group R-3m). It is a member of the Eudialyte structural group.
Key properties: Mohs hardness 5; specific gravity 2.9; White streak; Vitreous luster; optically uniaxial.
IMA symbol Fcg, IMA status: approved, named in 2007.
Found in agpaitic nepheline syenite and pegmatite, crystallized directly from a REE-, U-, Th-, and Zr-rich alkaline magma enriched in volatile components.
It is found associated with Microcline, orthoclase, albite, nepheline, sodic amphiboles, aegirine, lamprophyllite, Na-poor rinkite, loparite, catapleiite, zircon, titanite.
The name refers to Fengcheng City, China, near the type locality.