Siderophyllite

(repeating unit)KFe²⁺₂Al(Al₂Si₂)O₁₀(F,OH)₂IMA symbolSid^[1]Strunz classification9.EC.21Crystal systemMonoclinicCrystal classPrismatic (2/m)
(same H-M symbol)Space groupC2/mUnit cella = 5.36 Å, b = 9.29 Å
c = 10.26 Å; β = 100.1°; Z = 2IdentificationColorBlue green, dark brown, black.Crystal habitMicaceous foliated; pseudohexagonal crystalsCleavagePerfect basalFractureConchoidalMohs scale hardness2.5LusterVitreous to dullStreakGreenish grayDiaphaneityTransparent to subopaqueSpecific gravity3Optical propertiesBiaxial (-)Refractive indexn_α = 1.582 n_β = 1.625 n_γ = 1.625Birefringenceδ = 0.0432V angle4° (measured)References^[2][3][4]

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]

References