Neptunite

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.

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	[1][2][3]

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)