ASTROPHYLLITE

Description:

Astrophyllite is a rare titanium mineral found in some unusual granites and syenites. 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. Its name translated means star sheets : heavy, soft and fragile, astrophyllite typically forms as bladed, radiating stellate aggregates. Astrophyllite is usually opaque to translucent, but may be transparent in thin specimens. Its color is usually golden yellow or yellowish brown, but also greenish brown specimens are found. It produces a variable luster that can be nearly metallic in one specimen to vitreous in another.

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 used in jewellery.

Astrophyllite is found in cavities and fissures in unusual felsic ingenous rocks and 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 on Laven Island, Norway. Interestingly, kupletskite was not known until 1956, over a hundred years later.

Astrophyllite is found in a few localities, such as Mont-Saint-Hilaire, Quebec, Canada; Pikes Peak, Colorado, USA; Narsarsuk and Kangerdluarsuk, Greenland; Brevig, Norway, the Kola Peninsula, Russia.

Characteristics:

Chemistry: (K,Na)3(Fe,Mn)7Ti2(SiO3)8(O,OH)7, Potassium Sodium Iron Manganese Titanium Silicate Hydroxide

Hardness (Mohs scale): 3-4

Density: 3.3 - 3.4

Class: Silicates

Crystal System: triclinic

Crystal Habit: small tabular or bladed crystals often grouped in starlike aggregates.

Cleavage is perfect in one direction.

Fracture is uneven.

Streak is yellowish white.