Neptunite is a rare lithium titanium silicate mineral that is popular as mineral specimens but rarely available as faceted gems. The chemical formula of Neptunite is KNa2Li(Fe2+, Mn2+)2Ti2Si8O24 (Potassium Sodium Lithium Iron Manganese Titanium Silicate and its molecular weight is 907.69 gm. Neptunite forms shiny black, elongated, prismatic crystals which are usually square in cross section with pointed terminations. The hardness is 5.0 – 6.0 and the specific gravity 3.19 – 3.23 (g/cm3). It is opaque and black except in very small fragments. Thin fragments may be dark, blood red to dark, brownish red. Neptunite is usually available only as beautiful mineral specimens set in a matrix of pure white Natrolite along with blue Benitoite crystals and rarely with orangish brown Joaquinite-(Ce) crystals. Although there are several locations throughout the world to find Neptunite crystals, the most well-known source of Neptunite is at the now-closed Benitoite Gem Mine, San Benito County, California, USA.

Neptunite is one of the unusual minerals that exhibit the piezoelectric effect. Piezoelectricity is the ability of some mineral crystals to generate a voltage in response to applied mechanical stress such as an external pressure. Piezoelectricity was discovered in 1880 by French physicists, brothers Jacques and Pierre Curie.

Neptunite was named in 1893 by Swedish mineralogist Gustav Flink (1848-1931) for Neptune, the Roman god of the sea. This name was given because of Neptunite’s close association at its type locality at Narssârssuk, Greenland with the mineral Aegirine, named for Ægir, god of the sea and king of all sea creatures in Norse mythology. Aegirine was named in 1834 by Norwegian priest and mineralogist Hans Morten Thrane Esmark (1801-1882).

Neptunite distribution: from Narssârssuk, Iglunguak, and the Ilímaussaq intrusion, Greenland. In the Inagli massif, 30 km west of Aldan, Yakutia, and in the Khibiny and Lovozero massifs, Kola Peninsula, Russia. From the Khan-Bogdinskii granitic massif, Gobi, Mongolia. In the Dara-i-Pioz massif, Alai Range, Tien Shan, Tajikistan. From Barnavave, near Carlingford, County Louth, Ireland. At Mont Saint-Hilaire, Quebec, and Seal Lake, Labrador, Newfoundland, Canada. In the USA, splendid crystals from the Benitoite Gem mine and Mina Numero Uno, San Benito County, California, and at Point of Rocks, Colfax County, New Mexico. From near Woodsreef, New South Wales, Australia.

Category: Phyllosilicate
Chemical Formula: KNa2Li(Fe2+,Mn2+)2Ti2Si8O24
Potassium Sodium Lithium Iron Manganese Titanium Silicate
Molecular Weight: 907.69 gm
Composition: Potassium 4.31 % K 5.19 % K2O
Sodium 5.07 % Na 6.83 % Na2O
Lithium 0.76 % Li 1.65 % Li2O
Titanium 10.55 % Ti 17.60 % TiO2
Manganese 3.03 % Mn 3.91 % MnO
Iron 9.23 % Fe 11.87 % FeO
Silicon 24.75 % Si 52.96 % SiO2
Oxygen 42.30 % O
  100.00 % 100.00 % = TOTAL OXIDE


Crystallography: Monoclinic – Domatic
Crystal Habit: As prismatic crystals, to 7.5 cm, with {110} prominent, typically with square cross sections, may be curved or twisted.
Twinning: Interpenetrant on {301}.


Cleavage: Perfect on {110}
Fracture: Conchoidal
Tenacity: Brittle
Mohs Hardness: 5.0 – 6.0
Density: 3.19 – 3.23 (g/cm3)
Luminescence: None
Radioactivity: Not Radioactive
Other: Piezoelectric


Color: Black; deep blood red to red-brown in thin fragments
Transparency: Opaque to nearly opaque
Luster: Vitreoius
Refractive Index: 1.690 – 1.736  Biaxial ( + )
Birefringence: 0.029 – 0.045
Dispersion: r < v; strong to extreme
Pleochroism: Visible; X = pale yellow; Y = yellow-orange; Z = red-orange to red-brown