Siderophyllite

Siderophyllite
Zinnwaldite (siderophyllite – polylithionite solid solution mineral series)
General
Category	Phyllosilicate minerals
Group	Mica group, trioctahedral mica group, biotite subgroup
Formula	KFe2+2Al(Al2Si2)O10(F,OH)2
IMA symbol	Sid1
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
Identification
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	234

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.⁴

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.³