Neptunite
Species
Very Rare · KNa₂LiFe++₂Ti₂Si₈O₂₄
Lustrous black prisms, first found in a nepheline syenite pegmatite in Greenland and best known from the natrolite veins cutting serpentinite in California, where they are the constant companion of benitoite. Named after Neptune, because at its first locality it grows with aegirine, named after the Norse sea god Ægir.
Identity
- Formula
- KNa₂LiFe++₂Ti₂Si₈O₂₄
- Chemical class
- Silicate
Crystallography
- Crystal system
- Monoclinic
- Habits
- Prismatic
- Fracture
- Conchoidal
Physical properties
- Mohs hardness
- 5.5
- Density
- 3.23 g/cm³
- Luster
- Vitreous
- Colors
- black, red
- Streak
- brown
- Transparency
- Opaque, Translucent
Geology
- Origins
- Igneous, Hydrothermal, Metamorphic
- Host rocks
- Volcanic rock, Ultramafic rock (peridotite, serpentinite)
- Associated minerals
- Benitoite, Natrolite, Joaquinite
- Found in
- Hydrothermal vein (lode), Pegmatite deposit
- Geography
- USA, Greenland, Russia
Classification
- Rarity
- Very Rare
- Gem quality
- Yes
Sources
- M. Pasero and others, IMA Commission on New Minerals, Nomenclature and Classification, The New IMA List of Minerals, updated September 2026. International Mineralogical Association (CNMNC), 2026.
- Neptunite. Mindat.org (Hudson Institute of Mineralogy).
- J. W. Anthony, R. A. Bideaux, K. W. Bladh, M. C. Nichols, Neptunite. Mineral Data Publishing, Handbook of Mineralogy, 2001.
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