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Neptunite
Current inventory:  1 gem
 

Chemistry:  KNa₂Li(Fe²⁺,Mn²⁺,Mg)₂Ti₂Si₈O₂₄

Discovered in 1893;   IMA status: Valid (pre-IMA; Grandfathered).
Neptunite was named for Neptune, the Roman god of the sea, because it was discovered with Aegirine, named for Aegir, the Scandinavian god of the sea.

Classification

Mineral Classification:

Silicates

Strunz 8th Ed. ID:

8/F.37-10

8 : Silicates
F : Inosilicates (chain and band) double chains [Si2O6]4-
37 : Neptunite series

Crystal Data

Crystallography:

Monoclinic - Domatic

Crystal Habit:

As prismatic crystals, to 7.5 cm, typically with square cross sections, may be curved or twisted.

Twinning:

Interpenetrant

Physical Properties

Cleavage:

[110] Perfect

Fracture:

Conchoidal

Tenacity:

Brittle

Hardness (Mohs):

5.0 - 6.0

Density:

3.19 - 3.23 (g/cm³)

Luminescence:

None

Radioactivity:

Barely Detectable, GRapi = 60.73 (Gamma Ray American Petroleum Institute Units)

Optical Properties

Color:

Black, Dark Reddish Brown

Transparency:

Opaque; Translucent in very thin fragments

Luster:

Vitreous (glassy)

Refractive Index:

1.690 - 1.736  Biaxial ( + )

Birefringence:

0.0294 - 0.0452

Dispersion:

Strong; r < v

Pleochroism:

X = pale yellow; Y = yellow-orange; Z = red-orange to red-brown

Occurances

Geological Setting:

In natrolite veins cutting a glaucophane schist inclusion in a serpentinite body (San Benito Co., California, USA).

Type Locality:

Narssârssuk pegmatite (Narsarsuk pegmatite), Narssârssuk (Narsarsuk), Igaliku (Igaliko), Narsaq, Kitaa (West Greenland) Province, Greenland

Year Discovered:

1893

View mineral photos:

Neptunite Mineral Photos and Locations

More Information

Mindat.org
Webmineral.com

Neptunite forms shiney black, elongated, prismatic crystals which are usually square in cross section with pointed terminations. It is rarely faceted since it is opaque black except in very small fragments. Very, very small gems of dark red could be faceted from these fragments. Cabochons are also rare. Neptunite is usually available only as beautiful mineral specimens set in a matrix of pure white Natrolite along with rare Benitoite crystals. The main and most well known source of Neptunite is the Benitoite Gem Mine, San Benito County, California, USA.
 

  
Neptunite gems for sale: