7.4 x 6.4 x 3.8 cm. Ferrowyllieite is a very rare phosphate found at only three localities worldwide and this rich specimen is from the Type Locality in South Dakota. Lustrous, laminated, black crystal sections to about 2.2 cm are embedded in feldspar matrix on all sides of this piece. Not pretty to look at, but this is a very highly representative and rich example of the rare species from the locale. — Photo: Robert M. Lavinsky · CC BY-SA 3.0
Ferrowyllieite
Ferrowyllieite is an IMA-recognized mineral species in the phosphate class, with the chemical formula NaNaFe²⁺(Fe²⁺Al)(PO₄)₃.
It crystallizes in the monoclinic crystal system (space group P2_1/n). It is a member of the Alluaudite-Wyllieite structural group.
Key properties: Mohs hardness 4; specific gravity 3.601; Dirty olive-green streak; Oily to submetallic luster; optically biaxial.
IMA symbol Fwyl, IMA status: approved, named in 1979.
A primary phosphate in zoned granite pegmatites.
It is found associated with Arrojadite, muscovite, schorl, plagioclase, quartz (Victory mine, South Dakota, USA).
As a mineral with dominant ferrous iron, and its relation to wyllieite.