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
\|re\|er}}	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 K Na₂Li(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^[5] and in the Kola Peninsula of Russia .^[1]