Images
Formula: KNa2LiFe2+2Ti2Si6O24
Inosilicate (chain silicate)
Forms a complete solid solution with manganoneptunite
(HOM, Dana)
Crystal System: Monoclinic
Specific gravity: 3.19 to 3.23 measured, 3.24 calculated
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)
Common impurities: Ca
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)
Localities
At the type locality, the Narsaarsuk pegmatite, Greenland, Denmark, neptunite is found in
nepheline syenite associated with
eudialyte, 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, nearly black neptunite occurs in
natrolite
veins cutting a glaucophane
schist
inclusion in a serpentinite body, associated with white
natrolite, blue benitoite and
joaquinite-(Ce).
(HOM, AM57.85)
Neptunite here is about ten times more plentiful than benitoite.
(SM3)
Distinctive X-shaped penetration twins of neptunite have been found here. For a discussion of the applicable twin
law see (R&M 97.2.165-172).
Back to Minerals