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Dominant species
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Neptunite
Neptunite
Chemical
Formula
KNa2Li(Fe2+,Mn2+)2Ti2Si8O24
Species
Silicates
Crystal
System
Monoclinic
Mohs
Scale
5-6
Specific
Gravity
3.19-3.23
Color
Black; deep red-brown in thin fragments
Streak
Brown to red brown
Luster
Vitreous
Refractive
Index
n = 1.690 - 1.691 n = 1.693 - 1.700 n = 1.719 - 1.736
Diaphaneity
Opaque
Cleavage
Perfecton
Fracture
Conchoidal
Crystal Habit:Prismatic or tabular
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 Narssarssuk pegmatite of West Greenland. It is also found within natrolite veins in glaucophane schist within serpentinite in San Benito County, California, USA. It also occurs in Mont Saint-Hilaire, Quebec and in the Kola Peninsula of Russia.

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