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The Mineral NEPTUNITE

  • Chemistry: KNa2Li(Fe, Mn)2Ti2Si8O24, Potasium Sodium Lithium Iron Manganese Titanium Silicate
  • Class: Silicates
  • Subclass: Inosilicates
  • Uses: mineral specimens.
  • Specimens

Neptunite was only discovered in the early 1900's and is of rare occurence. Its most famous specimens come from the wonderful deposits at San Benito County in California. This site contains natrolite veins in a serpentinite rock. Embedded in the pure white natrolite are the sapphire blue benitoite and the black prismatic crystals of neptunite. The assemblage is a classic among collectors and specimens with all three minerals are much sought after. Neptunite is named after the Roman god of the sea, Neptune. Crystals are well formed and typically show multiple secondary faces. The flashes of red that appear to come right from the center of the crystal are a nice added touch to this rare and interesting mineral.

PHYSICAL CHARACTERISTICS:

  • Color is black to reddish black.
  • Luster is vitreous.
  • Transparency crystals are translucent to opaque.
  • Crystal System is monoclinic; 2/m
  • Crystal Habits include prismatic, elongated crystals with nearly square cross-sections. The terminations are modified and slanted dome faces. Also found massive and in disseminated grains.
  • Cleavage is perfect in two direction at 80 and 100 degree angles.
  • Fracture is splintery.
  • Hardness is 5.5
  • Specific Gravity is approximately 3.23 (slightly above average)
  • Streak is reddish-brown.
  • Associated Minerals are benitoite, natrolite, serpentine, aegerine and joaquinite.
  • Other Characteristics: an internal red reflection can sometimes be seen through the prismatic crystal faces.
  • Notable Occurrences include the famous San Benito Co., California site as well as Narsarssuk Greenland and the Kola penninsula, Russia.
  • Best Field Indicators are crystal habit, internal reflection, streak, associations, color, and cleavage.

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