PHYSICAL PROPERTIES:
Chemistry: KNa2Li(Fe,
Mn)2Ti2Si8O24
Composition: Potasium Sodium Lithium
Iron Manganese Titanium Silicate
Class: Silicates
Subclass: Inosilicates
Crystal system:monoclinic; 2/m
Fracture: splintery
Hardness: 5.5
Specific gravity: 3.23
Luster: vitreous
Streak: reddish brown
Cleavage: perfect at 80 and 100 degree
angles
Color:black to reddish black
Transparency: crystals are translucent to opaque
Associated Minerals: aegerine,
benitoite, joaquinite, natrolite and serpentine.
COMPOSITION:
A soda and potash, iron manganese titanosilicate (17.8% Ti02,
9.8% Na20, 5.1% K2O, 11.6%
FeO, 3.8% MnO, 5 1.9% SiO2).
DISTINGUISHING CHARACTERISTICS:
Neptunite was only discovered during this century. It's most famous
specimens come from San Benito County, California. This site is not
only noted for black prismatic crystals of Neptunite. But the very rare
sapphire blue benitoite as well. These minerals are embedded in a pure
white natrolite veins found in a serpentine rock formation. These three
rare minerals being found together, so far, is limited to this location.
Specimens with all three of these minerals are very popular among collectors.
Another interesting characteristic of Neptunite, is the appearance of
an internal red reflection through the prismatic crystal faces.
The perfect cleavages at about 80 degrees and 100 degrees distinguish
it from the amphiboles, and it is more fusible than most similar pyroxenes.
The red brown translucency and streak are very characteristic.
ENVIRONMENT:
Nepheline syenite cavities and in serpentine veins with natrolite and
benitoite.
CRYSTAL DESCRIPTION:
Crystals are well formed and typically show multiple secondary
faces. The best crystals are from California over an inch in length
and about 3/8 to 1/4 inch across. They are elongated prismatic crystals,
with nearly square cross sections. The terminations are modified and
have slanted dome faces. Also found in massive and granular forms.
TESTS:
Fuses readily to a black nonmagnetic shiny glass sphere which crushes
to a brown powder.
LOCALITIES:
Neptunite is a rare mineral of particular interest to
the collector because of the unique occurrence in San Benito County,
California, where it has formed on the walls of natrolite veins with
well crystallized benitoite in a green serpentine. Smaller and duller
crystals were found in Narsarssuak, Greenland, in pockets in a nepheline
syenite along the coast. Similar crystals have been found in the Kola
Peninsula, Russia.
USES:
mineral specimens.
FACTS & HISTORY:
Neptunite is named after the Roman god of the sea, Neptune.
It is usually freed from its white natrolite coating by a hydrochloric
acid bath.
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Neptunite Specimens
