| Astrophyllite | |
|---|---|
 | |
| General | |
| Category | Inosilicates Astrophyllite group |
| Formula | (K,Na)3(Fe++,Mn)7Ti2Si8O24(O,OH)7 |
| IMA symbol | Ast[1] |
| Strunz classification | 9.DC.05 |
| Crystal system | Triclinic |
| Crystal class | Pinacoidal (1) (sameH-M symbol) |
| Space group | A1 |
| Identification | |
| Color | Golden brown to yellow; rarely greenish |
| Crystal habit | Tabular to bladed, radiating, stellate aggregates; lamellar masses |
| Cleavage | Perfect on [001] imperfect on [100] |
| Fracture | Uneven |
| Mohs scale hardness | 3–4 |
| Luster | Greasy, pearly, sub-metallic |
| Streak | Yellowish brown or white |
| Diaphaneity | Translucent to opaque |
| Specific gravity | 3.2–3.4 |
| Optical properties | Biaxial positive |
| Refractive index | nα = 1.680 nβ = 1.700 nγ = 1.730 |
| Birefringence | 0.050 |
| Pleochroism | Strong: X= deep red-orange Y= orange-yellow Z= lemon-yellow |
| References | [2][3][4] |
Astrophyllite is a very rare, brown to golden-yellowhydrouspotassiumirontitaniumsilicate mineral. Belonging to the astrophyllite group, astrophyllite may be classed either as aninosilicate,phyllosilicate, or an intermediate between the two. It forms anisomorphous series withkupletskite, to which it is visually identical and often intimately associated. Astrophyllite is of interest primarily to scientists and collectors.
Heavy, soft and fragile, astrophyllite typically forms asbladed, radiatingstellate aggregates. It is thiscrystal habit that gives astrophyllite its name, from theGreek wordsastron meaning "star" andphyllon meaning "leaf". Its great submetallic gleam and darkness contrast sharply with the light (felsic) matrix the mineral is regularly found within. Astrophyllite is usually opaque to translucent, but may be transparent in thin specimens.
As the crystals themselves possess perfect cleavage, they are typically leftin situ, the entire aggregate often cut into slabs and polished. Owing to its limited availability and high cost, astrophyllite is seldom seen in an ornamental capacity. It is sometimes used injewellery where it is fashioned intocabochons.
Found in cavities and fissures in unusual felsicigneous rocks, astrophyllite is associated withfeldspar,mica,titanite,zircon,nepheline, andaegirine. Common impurities includemagnesium,aluminium,calcium,zirconium,niobium, andtantalum. It was first discovered in 1854 at its type locality;Laven Island,Norway. Kupletskite was not known until 1956, over a hundred years later.
Astrophyllite is found in a few scarce, remote localities:Mont-Saint-Hilaire,Quebec,Canada;Pikes Peak,Colorado,US;Narsarsuk andKangerdluarsuk,Greenland; Brevig, Norway; and theKola Peninsula,Russia.
