Siderophyllite
| Siderophyllite | |
|---|---|
|
Zinnwaldite (siderophyllite – polylithionite solid solution mineral series) | |
| General | |
| Category | Silicate mineral |
| Formula (repeating unit) | KFe2+2Al(Al2Si2)O10(F,OH)2 |
| Strunz classification | 9.EC.21 |
| Crystal symmetry |
Monoclinic prismatic H-M symbol: (2/m) Space group: C 2/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 |
| Crystal system | Monoclinic |
| 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]