neptunite

{{Infobox mineral | name = Neptunite | category = Phyllosilicate minerals | boxwidth = | boxbgcolor = | image = Neptunite, bénitoïte, joaquinite-(Ce) sur natrolite et serpentine (USA) 2.jpg | imagesize = 260 | caption = Neptunite crystals | formula = KNa2Li(Fe2+,Mn2+)2Ti2Si8O24 | IMAsymbol = Npt | dana = 70.04.01.01 | strunz = 9.EH.05 | system = Monoclinic | class = Domatic (m) (same H-M symbol) | symmetry = Cc | unit cell = a = 16.427(2), b = 12.478(2) c = 9.975(1) Å; β = 115.56(1)°; Z = 4 | color = Black; deep red-brown in thin fragments | habit = Prismatic or tabular | twinning = Interpenetrant on {301} | cleavage = {110} good | fracture = Conchoidal | tenacity = Brittle | mohs = 5–6 | luster = Vitreous | refractive = nα = 1.69–1.6908, nβ = 1.6927–1.7, nγ = 1.7194–1.736 | opticalprop = Biaxial (+) | 2V = 36° to 49° | birefringence = 0.0294–0.0452 | pleochroism = x = yellow-orange, y = orange, z = deep red | streak = Brown to red brown | gravity = 3.19–3.23 | density = | melt = | fusibility = | diagnostic = | solubility = | diaphaneity = Nearly opaque | other = Piezoelectric | references = }}

Neptunite is a silicate mineral with the formula KNa2Li(Fe2+, Mn2+)2Ti2Si8O24. With increasing manganese it forms a series with mangan-neptunite. is the variety with vanadium replacing the titanium in the formula.