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Formula:   KNa2LiFe2TiSi8O24   mineral photo

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Crystal:   Monoclinic  
Hardness:   5-6  
Spec. Gr.:   3.23  
Streak:   Cinnamon-brown  
Cleavage:   (110)  
Location:   San Benito River, San Benito County, California  

First discovered in Greenland, Neptunite's most important source is the Benitoite Gem mine high in the mountains northwest of Coalinga, California. It occurs in shiny black prismatic crystals, which actually are red-brown in thin sections, and whose length is usually about eight times its width. The typical matrix is snow-white natrolite, which can be etched away with hydrochloric acid. Crystals are frequently doubly terminated with the longest known specimen being 7.5 cm. Neptunite sometimes forms interesting twins with the angle between the c-axis near 40 degrees. Some crystals are S shaped from uneven growth on the a and m faces. Neptunite crystals are quite brittle and care must be taken for preservation. Black neptunite crystals mixed with blue benitoite crystals on white natrolite matrix, are very attractive display specimens.

Bibliography: Wise, William S., Gill Robert H., Benitoite Gem Mine.

Mineralogical Record, V8, No. 6, pg. 442-452.

University of California, Santa Barbara—Department of Earth Science
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