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Siderophyllite

Siderophyllite
Category	Silicate mineral
Chemical formula	KFe⁺⁺Al(Al₂Si₂)O₁₀(F,OH)₂
Identification
Color	Blue green, dark brown, black.
Crystal habit	Micaceous foliated
Crystal system	Monoclinic - Prismatic 2/m
Cleavage	Perfect basal
Fracture	Conchoidal
Mohs Scale hardness	2.5
Luster	Vitreous to dull
Refractive index	nα = 1.582 nβ = 1.625 nγ = 1.625
Optical Properties	Biaxial (-)
Birefringence	δ = 0.043
Streak	Greenish gray
Specific gravity	3
Diaphaneity	Transparent to subopaque
References	^[1]^[2]^[3]

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

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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.

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