Neptunite

inosilicate

Formula: KNa2LiFe2+2Ti2Si6O24
Inosilicate (chain silicate)
Forms a complete solid solution with manganoneptunite (HOM, Dana)
Specific gravity: 3.19 to 3.23
Hardness: 5 to 6
Streak: Cinnamon brown
Colour: Neptunite appears to be jet black in colour, but it is actually deep red, as can be seen in very thin slivers. It also appears reddish brown when heavily included with crossite (an amphibole intermediate between the riebeckite group and the glaucophane group). (SM3)
Solubility: Insoluble in acids (Dana)
Environments:

Plutonic igneous environments
Pegmatites

Neptunite is most commonly associated with syenitic rocks. It alters to crossite (an amphibole intermediate between the riebeckite group and the glaucophane group). (Dana)

At the type locality, the Narsaarsuk pegmatite, Greenland, Denmark, neptunite is found in nepheline syenite associated with eudialite, arfvedsonite and aegirine. (HOM Dana)

At the Kola Peninsula, Russia, neptunite is associated with nordite-(La), lomonosovite, sodalite and ussingite. (HOM)

At the Dallas Gem mine, San Benito county, California, USA, neptunite occurs in natrolite veins cutting a glaucophane schist inclusion in a serpentinite body, associated with natrolite, benitoite and joaquinite-(Ce). (HOM, AM57.85) Neptunite here is about ten times more plentiful than benitoite. (SM3)

Common impurities: Ca

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