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]


  1. ^ Warr, L.N. (2021). "IMA–CNMNC approved mineral symbols". Mineralogical Magazine. 85 (3): 291–320. Bibcode:2021MinM...85..291W. doi:10.1180/mgm.2021.43. S2CID 235729616.
  2. ^ a b Anthony, John W.; Bideaux, Richard A.; Bladh, Kenneth W.; Nichols, Monte C. (2005). "Neptunite" (PDF). Handbook of Mineralogy. Mineral Data Publishing. Retrieved 14 March 2022.
  3. ^ a b c Neptunite,
  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.