Neptunite Locality : Dallas Gem Mine (Benitoite Mine; Benitoite Gem Mine; Gem Mine), Dallas Gem Mine area, San Benito River headwaters area, New Idria District, Diablo Range, San Benito Co., California, USA Size : (Crystal size 2.5 cm) — Photo: Didier Descouens · CC BY-SA 4.0
Neptunite
A black potassium-lithium-iron-manganese titanium silicate — famous for superb crystals at California's Benitoite Gem Mine.
Intro adapted from Wikipedia (CC BY-SA).
Neptunite is an IMA-recognized mineral species in the silicate class, with the chemical formula KNa₂LiFe²⁺₂Ti₂Si₈O₂₄.
It crystallizes in the monoclinic crystal system (space group Cc). It is a member of the Neptunite structural group.
Key properties: Mohs hardness 5–6; specific gravity 3.19–3.23; Cinnamon-brown streak; Vitreous luster; refractive index 1.705; optically biaxial.
IMA symbol Npt, grandfathered status — known before the IMA’s founding, named in 1893.
Found in natrolite veins cutting a glaucophane schist inclusion in a serpentinite body (San Benito Co., California, USA).
It is found associated with Eudialyte, arfvedsonite, aegirine (Narss^a rssuk, Greenland); natrolite, benitoite, joaquinite-(Ce) (San Benito Co., California, USA); nordite-(La).
The name refers to Neptune, sea-god of Roman mythology; so named because of its close association at the type occurrence with aegirine, the name of which derives from Àgir, the Scandinavian sea-go d.