Neptunite
Image: <a href="//commons.wikimedia.org/wiki/User:Archaeodontosaurus" title="User:Archaeodontosaurus">Didier Descouens</a> (CC BY-SA 4.0)
Mineral NameNeptunite
Chemical FormulaKNa₂Li(Fe²⁺,Mn²⁺)₂Ti₂Si₈O₂₄
Mohs Hardness5
Crystal Systemmonoclinic
Lustervitreous
Streak Colorreddish brown
Cleavagegood on {110}
Fractureuneven
Specific Gravity3.2
Colorsblack, reddish-brown
Transparencytranslucent
Type LocalityDallas Gem Mine (Benitoite Mine; Benitoite Gem Mine; Gem Mine), Dallas Gem Mine area, San Benito River headwaters area, New Idria District, Diablo Range, San Benito Co., California, USA
Mineral GroupSilicates
Usescollector, gemstone (rare)
Associated Mineralsbenitoite, natrolite, joaquinite, serpentine
Rarityuncommon
Description
This image features a sharp, prismatic, black neptunite crystal, approximately 2.5 cm long, exhibiting a distinct vitreous luster. It is prominently set against a white, likely natrolite or albite, matrix.
Geological Context
Neptunite typically forms in hydrothermally altered serpentinite, often found in natrolite veins within areas of blueschist metamorphism. Its most famous occurrence is in the unique geological environment of the Benitoite Gem Mine.
