Overview
Astrophyllite is a rare potassium-iron-titanium silicate whose name, from the Greek for star and leaf, captures its most striking habit: bladed, bronze-to-golden crystals that radiate outward in star-like sprays. Set against dark feldspar or smoky quartz, these metallic copper-gold blades make astrophyllite one of the most ornamental of the rare alkaline-rock minerals, and polished slabs are popular in the lapidary and metaphysical trade. It is the namesake of the astrophyllite group, a family of complex titanosilicates, and was first described in 1854 from an island in southern Norway.
Composition & structure
Astrophyllite is a hydrous potassium-sodium iron titanium silicate, typically written K2NaFe7Ti2Si8O26(OH)4F or, in classic form, (K,Na)3(Fe,Mn)7Ti2Si8O24(O,OH)7. Manganese commonly substitutes for iron, and several related group members differ by which cations dominate. Structurally it is a layered titanosilicate built from sheets of silica tetrahedra linked by titanium and iron, giving it character intermediate between chain and sheet silicates. This layering produces its perfect mica-like cleavage and its tendency to form thin, flexible-looking blades, though it is more brittle than true mica.
| Formula | K2NaFe7Ti2Si8O26(OH)4F |
| Crystal system | Triclinic |
| Mohs hardness | 3 to 4 |
| Lustre | Submetallic, pearly to greasy on cleavage |
| Colour | Golden-yellow to bronze, brown, sometimes with greenish tints |
| Type locality | Laven (Laaven) Island, Langesundsfjord, Norway |
Formation & occurrence
Astrophyllite is a hallmark mineral of peralkaline igneous rocks, those unusually rich in sodium and potassium relative to aluminium. It crystallises in nepheline syenites, alkali granites and their associated pegmatites, where the magma is enriched in titanium, zirconium and rare elements. In these settings it grows alongside aegirine, arfvedsonite, nepheline, eudialyte, feldspar and zircon. Because such peralkaline complexes are themselves uncommon, astrophyllite is confined to a relatively small number of classic alkaline massifs worldwide.
Identification & similar species
Astrophyllite is identified by its bronze-gold colour, submetallic to pearly lustre, low hardness of 3 to 4 and its diagnostic radiating, bladed crystal sprays. The micaceous cleavage and golden sheen can suggest phlogopite mica or the related mineral lamprophyllite, but astrophyllite's distinctly bronze, almost metallic colour and its star-like aggregates are usually decisive. It is softer and more brittle than the superficially similar bronze mica varieties, and its restriction to alkaline rocks alongside aegirine and eudialyte provides a strong contextual clue.
Notable localities & collecting
Beyond the Norwegian type locality in the Langesundsfjord district, the finest specimens come from a handful of famous alkaline complexes: the Khibiny and Lovozero massifs of the Kola Peninsula in Russia, Mont Saint-Hilaire in Quebec, the Pikes Peak batholith of Colorado, and the alkaline rocks of Greenland. Most collector material on the market today is bronze astrophyllite from peralkaline granites cut into spheres, cabochons and slabs to display the radiating blades. Specimens are best handled with care because the mineral is soft and its perfect cleavage makes the thin blades easy to chip.