neptunite

Neptunite is a rare potassium-sodium-lithium silicate mineral. It occurs in alkali-rich igneous rocks, such as nepheline syenites and phonolites, and in related hydrothermal veins. It forms dark, elongated crystals that are black to deep brown with a glassy to submetallic luster. The mineral's fracture is uneven and cleavage is poor or absent. It has a relatively high density for a silicate and a moderate hardness on the Mohs scale. Crystals may be prismatic to tabular and are often found in aggregates.

Chemically, neptunite is described as a complex silicate of sodium, potassium and lithium with a variable composition. In hand specimens it is distinguished by its dark color and occurrence in alkali rock assemblages. It can be mistaken for other dark silicates but is typically associated with natrolite and other alkali silicates.

Neptunite was named after the planet Neptune. It was first described in the late 19th century and is now known from several parts of the world in alkaline igneous suites. The mineral is primarily of interest to scientists and mineral collectors; it has no significant industrial application.