Neptunite

Neptunite
Neptunite crystals
General
Category	Phyllosilicate
Formula (repeating unit)	KNa2Li(Fe2+,Mn2+)2Ti2Si8O24
Strunz classification	9.EH.05
Dana classification	70.04.01.01
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
Identification
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

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. 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.

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

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

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