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Neptunite, bénitoïte, joaquinite-(Ce) sur natrolite et serpentine (USA) 2.jpg
Neptunite crystals
Category Phyllosilicate
(repeating unit)
Strunz classification 9.EH.05
Dana classification
Crystal system Monoclinic
Crystal class Domatic (m)
(same H-M symbol)
Space group Cc
Unit cell a = 16.427(2), b = 12.478(2)
c = 9.975(1) Å; β = 115.56(1)°; Z = 4
Color Black; deep red-brown in thin fragments
Crystal habit Prismatic or tabular
Twinning Interpenetrant on {301}
Cleavage {110} good
Fracture Conchoidal
Tenacity Brittle
Mohs scale hardness 5-6
Luster Vitreous
Streak Brown to red brown
Diaphaneity Nearly opaque
Specific gravity 3.19 - 3.23
Optical properties Biaxial (+)
Refractive index nα=1.69-1.6908, nβ=1.6927-1.7, nγ=1.7194-1.736
Birefringence 0.0294-0.0452
Pleochroism x=yellow-orange, y=orange, z=deep red
2V angle 36° to 49°
Other characteristics Piezoelectric
References [1][2][3]

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.[2] 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 and in the Kola Peninsula of Russia.[1]

The mineral is named for Neptune, Roman god of the sea because of its association with aegirine from Àgir, the Scandinavian sea-god.[2]

The Gemological Institute of America (GIA) identified an 11.78-carat faceted specimen as neptunite based on Raman spectroscopy.[4]

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