Astrophyllite

Infobox mineral
name = Astrophyllite
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formula = (K,Na)₃(Fe⁺⁺,Mn)₇ Ti₂Si₈O₂₄(O,OH)₇
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color = Golden brown to yellow; rarely greenish
habit = Tabular to bladed, radiating, stellate aggregates; lamellar masses
system = Triclinic
twinning =
cleavage = Perfect; into inflexible, inelastic plates
fracture = Uneven
mohs = 3-4
luster = Submetallic to vitreous
refractive = 1.67-1.75, biaxial positive
opticalprop =
birefringence = 0.050
pleochroism = None
streak = Yellowish brown or white
gravity = 3.2-3.4
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fusibility = Easily to a dark magnetic glass
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solubility = Soluble in hydrochloric and sulfuric acid, but with difficulty
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Astrophyllite is a very rare, brown to golden yellow hydrous potassium iron titanium silicate mineral. Belonging to the astrophyllite group, astrophyllite may be classed either as an inosilicate, phyllosilicate or an intermediate between the two. It forms an isomorphous series with kupletskite, 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 as "bladed", radiating "stellate" aggregates. It is this crystal habit that gives astrophyllite its name; from the Greek words "astron" meaning "star" and "phyllon" meaning "leaf." Its high, submetallic lustre and dark colour contrast sharply with the light coloured (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 left "in 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 in jewellery where it is fashioned into cabochons.

Found in cavities and fissures in unusual felsic igneous rocks, astrophyllite is associated with feldspar, mica, titanite, zircon, nepheline and aegirine. Common impurities include magnesium, aluminium, calcium, zirconium, niobium, and tantalum. It was first discovered in 1854 at its type locality; Laven Island, Norway. Interestingly, 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, USA; Narsarsuk and Kangerdluarsuk, Greenland; Brevig, Norway; and the Kola Peninsula, Russia.

Along with kupletskite, astrophyllite grouped with: caesium-kupletskite, niobophyllite, zircophyllite, hydroastrophyllite, magnesiumastrophyllite, and niobokupletskite.

See also

*List of minerals