Astrophyllite: Mineral Information. From Água de Pau volcano, San Miguel Island, Azores District, Portugal. Photo: Serge Lavard
Astrophyllite is a rare mineral. Its crystal structure can be described as small blades extruding in a radiating pattern from the center like a star or snowflake. 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.
Astrophyllite minerals form in igneous rock deposited in cavities and fissures within the Earth’s crust. Igneous rock is formed during the cooling and solidification of liquid magma which can occur in one of two ways; the liquid magma can cool and solidify slowly underground or can be cooled and solidified quickly in the atmosphere or on the surface of the Earth after a volcanic explosion. When igneous rock is released into the atmosphere from a volcanic eruption it is known as pumice. Astrophyllite can be found in large masses of igneous rock and within pegmatite.
Formula: K2NaFe2+7Ti2Si8O28(OH)4FColor is usually golden yellow or yellowish brown, but also greenish brown specimens are found.
Luster is vitreous or submetallic to metallic, can be pearly on cleavage surfaces.
Transparency: crystals are translucent to opaque.
Crystal System is triclinic; bar 1
Crystal Habits include small tabular or bladed crystals often grouped in starlike aggregates. Also found in lamellar massives.
Cleavage is perfect in one direction.
Fracture is uneven.
Hardness is 3.
Specific Gravity is 3.3 - 3.4
Streak is yellowish white.
Associated Minerals are quartz, feldspars, nepheline, micas and aegirine.
Other Characteristics: color is often patchy or heterogenous in distribution.
Notable Occurrences include Kola Penensula, Khibina, Russia; Colorado, USA and Mt St. Hilaire, Quebec, Canada.
Best Field Indicators brittleness, cleavage, color, luster and localities.