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Zinnwaldite (siderophyllite – polylithionite solid solution mineral series)
Category Silicate mineral
(repeating unit)
Strunz classification 9.EC.21
Crystal system Monoclinic
Crystal class Prismatic (2/m)
(same H-M symbol)
Space group C2/m
Unit cell a = 5.36 Å, b = 9.29 Å
c = 10.26 Å; β = 100.1°; Z = 2
Color Blue green, dark brown, black.
Crystal habit Micaceous foliated; pseudohexagonal crystals
Cleavage Perfect basal
Fracture Conchoidal
Mohs scale hardness 2.5
Luster Vitreous to dull
Streak Greenish gray
Diaphaneity Transparent to subopaque
Specific gravity 3
Optical properties Biaxial (-)
Refractive index nα = 1.582 nβ = 1.625 nγ = 1.625
Birefringence δ = 0.043
2V angle 4° (measured)
References [1][2][3]

Siderophyllite is a rare member of the mica group of silicate minerals with formula KFe2+2Al(Al2Si2)O10(F,OH)2.

The mineral occurs in nepheline syenite pegmatites and granite and aplite greisens. It is associated with microcline and astrophyllite at Pikes Peak, Colorado. It is also found in the alkali pegmatites of Mont Saint-Hilaire, Quebec.[3]

It was first described in 1880 for an occurrence near Pikes Peak, Colorado. The name derives from the Greek sideros, iron, and phyllon, leaf, in reference to its iron rich composition and perfect basal cleavage.[2]