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Neptunite

Neptunite
Category	Silicate mineral
Chemical formula	KNa₂Li(Fe²⁺,Mn²⁺)₂Ti₂Si₈O₂₄
Identification
Color	Black, red
Crystal habit	Prismatic or tabular
Crystal system	Monoclinic
Twinning	Interpenetrant on {301}
Cleavage	{110} good
Fracture	Conchoidal
Tenacity	Brittle
Mohs Scale hardness	5-6
Luster	Vitreous
Refractive index	α=1.69-1.6908, β=1.6927-1.7, γ=1.7194-1.736
Optical Properties	Biaxial (+) 2V = 36° to 49°
Birefringence	0.0294-0.0452
Pleochroism	x=yellow-orange, y=orange, z=deep red
Streak	Brown to red brown
Specific gravity	3.19 - 3.23
Diaphaneity	Nearly opaque
Other Characteristics	Piezoelectric

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.

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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, USA. The mineral is named for Neptune, Roman god of the sea because of its association with aegirine from Àgir, the Scandinavian sea-god. It also occurs in Mont Saint-Hilaire, Quebec and in the Kola Peninsula of Russia.