Zinnwaldite (siderophyllite – polylithionite solid solution mineral series)
CategorySilicate mineral
(repeating unit)
IMA symbolSid[1]
Strunz classification9.EC.21
Crystal systemMonoclinic
Crystal classPrismatic (2/m)
(same H-M symbol)
Space groupC2/m
Unit cella = 5.36 Å, b = 9.29 Å
c = 10.26 Å; β = 100.1°; Z = 2
ColorBlue green, dark brown, black.
Crystal habitMicaceous foliated; pseudohexagonal crystals
CleavagePerfect basal
Mohs scale hardness2.5
LusterVitreous to dull
StreakGreenish gray
DiaphaneityTransparent to subopaque
Specific gravity3
Optical propertiesBiaxial (-)
Refractive indexnα = 1.582 nβ = 1.625 nγ = 1.625
Birefringenceδ = 0.043
2V angle4° (measured)

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]


  1. ^ Warr, L.N. (2021). "IMA–CNMNC approved mineral symbols". Mineralogical Magazine. 85 (3): 291–320. Bibcode:2021MinM...85..291W. doi:10.1180/mgm.2021.43. S2CID 235729616.
  2. ^ Siderophyllite data on Webmineral
  3. ^ a b Siderophyllite on
  4. ^ a b Siderophyllite in the Handbook of Mineralogy