CHEMISTRY - KNa2Li(Fe,Mn,Mg)2Ti2Si8O24
CRYSTALLOGRAPHY - Monoclinic
CRYSTAL GROWTH AND HABITS - Neptunite typically forms prismatic crystals with square cross sections. Crystals may be curved.
COLOR AND OTHER OPTICAL PROPERTIES - Black, nearly opaque
HARDNESS - 5 - 6
SPECIFIC GRAVITY - 3.19 -3.24
LUSTER - Vitreous
STREAK - Reddish brown
BREAKABILITY - Neptunite has a very good cleavage along {110} with a conchoidal fracture. It is brittle.
OCCURRENCE - Neptunite is found in veins cutting a glaucophane rich schist at San Benito County, CA.
ASSOCIATED MINERALS - Aegirine, Eudialyte, Arfvedsonite, Natrolite, Benitoite.
NAME - Neptunite was named after the Roman god of the sea - Neptune because of its close association with Aegirine at the type locality. Aegirine is the Scandinavian god of the sea.