Neptunite, bénitoïte, joaquinite-(Ce) sur natrolite et serpentine (USA) 2.jpg
Neptunite crystals
(repeating unit)
IMA symbolNpt[1]
Strunz classification9.EH.05
Dana classification70.04.01.01
Crystal systemMonoclinic
Crystal classDomatic (m)
(same H-M symbol)
Space groupCc
Unit cella = 16.427(2), b = 12.478(2)
c = 9.975(1) Å; β = 115.56(1)°; Z = 4
ColorBlack; deep red-brown in thin fragments
Crystal habitPrismatic or tabular
TwinningInterpenetrant on {301}
Cleavage{110} good
Mohs scale hardness5-6
StreakBrown to red brown
DiaphaneityNearly opaque
Specific gravity3.19 - 3.23
Optical propertiesBiaxial (+)
Refractive indexnα=1.69-1.6908, nβ=1.6927-1.7, nγ=1.7194-1.736
Pleochroismx=yellow-orange, y=orange, z=deep red
2V angle36° to 49°
Other characteristicsPiezoelectric

Neptunite is a silicate mineral with the formula KNa2Li(Fe2+, Mn2+)2Ti2Si8O24. With increasing manganese it forms a series with mangan-neptunite. Watatsumiite is the variety with vanadium replacing the titanium in the formula.

It was first described in 1893 for an occurrence in the Narssârssuk pegmatite of West Greenland.[3] It is also found within natrolite veins in glaucophane schist within serpentinite in San Benito County, California, US. It also occurs in Mont Saint-Hilaire, Quebec[5] and in the Kola Peninsula of Russia.[2]

The mineral is named for Neptune, Roman god of the sea because of its association with aegirine from Àgir, the Scandinavian sea-god.[3]

The Gemological Institute of America (GIA) identified an 11.78-carat faceted specimen as neptunite based on Raman spectroscopy.[6]

Neptunite crystal from the New Idria District, Diablo Range, San Benito County, California (Crystal size 2.5 cm)
Neptunite crystal from the New Idria District, Diablo Range, San Benito County, California (Crystal size 2.5 cm)


  1. ^ Warr, L.N. (2021). "IMA–CNMNC approved mineral symbols". Mineralogical Magazine. 85: 291–320.
  2. ^ a b Mineral Handbook
  3. ^ a b c Mindat with location data
  4. ^ Webmineral
  5. ^ Normand, Charles; Tarassoff, Peter (2006). Mineralogy and geology of the Poudrette quarry, Mont SaintHilaire, Quebec (PDF). Mineralogical Association of Canada. Retrieved 12 March 2022.
  6. ^ Cooper, Amy; Allen, Tara (Summer 2013). "Rare faceted neptunite". Gems & Gemology. Gemological Institute of America. 49 (2). Retrieved April 18, 2019.