Neptunite - Encyclopedia

Neptunite de Californie

    Class : Silicates
    Subclass : Inosilicates
    Crystal system : Monoclinic
    Chemistry : KNa2Li(Fe,Mn)2Ti2Si8O24
    Rarity : Rare

Neptunite is a rare silicate that has been found in various nepheline syenites and especially in natrolite fissures intersecting serpentinite (California). It is a mineral which owes its name to Neptune, the Roman god of the Sea, because it was discovered in association with aegyrine, which itself takes its name from the Scandinavian god of the Sea. This mineral occurs in monoclinic prisms with square section, terminated by numerous pyramidal faces. Translucent to opaque, its sheen is vitreous and its color black with very dark reddish internal reflections. It has a perfect cleavage according to {110}. It is a rare mineral sought after by collectors, especially when it is associated with benitoite.

Neptunite in the World

Neptunite was found in crystals up to 7.5 cm (the best known), frequently clustered on natrolite, in mines in San Benito County (California). It is associated with exceptional crystals of benitoite. Smaller crystals (maximum 2 cm) exist in the nepheline syenites of Narssârssuak (Ilimaussaq complex, Greenland). Neptunite is also reported from Mont St-Hilaire (Canada) and Seal Lake (Canada).

Neptunite in France

This mineral is not present in the French underground.


Rare twin by interpenetration on {301}.

Fakes and scams

No known fake for this mineral species.

Hardness : 5 to 6
Density : 3.19 to 3.23
Fracture : Conchoidal
Streak : Brown

TP : Opaque to translucent
IR : 1.690 to 1.736
Birefringence : 0.029 to 0.045
Optical character : Biaxial +
Pleochroism : Visible
Fluorescence : None

Solubility : Insoluble

Magnetism : None
Radioactivity : None