Amphiboles (silicates): Neptunite
Diagnostic card.
In the photo there is an elongated-prismatic crystal. Below: Neptunite in the natrolite from San Benito County (California).
Na 4 Fe Ti Si 4 O 12
Singonia monoclinic
Hardness 5,5
Specific weight 3.23
Cleavage is perfect
Cracked shell
Color brownish black
Color in powder brown
Glitter glass
Crystals of neptunite are prismatic, with easily manifested cleavage, shining, united in beautiful druses. The color is black, with internal reflexes of light brown-red tonality.
The mineral may also contain other elements (potassium, lithium, manganese); If manganese is more than iron, this is called manganeptunite.
Diagnostic signs.
It does not dissolve in hydrochloric acid, it easily melts.
Origin.
Occurs in alkaline pegmatites, in nepheline syenites, in altered serpentinites (together with natrolite and benitoite).
Place of Birth.
Samples of good quality came from a single mine in the upper reaches of the San Benito River in the same district of California. Minerals are also mined in New South Wales (Australia), Narsarsuke (Greenland) and the Kola Peninsula (Russia). Manganeptunite comes from Mont-Saint-Hilaire in Quebec (Canada).
Neptunite. San Benito, California, USA. Photo: © А.А. Evseev.
Neptunite on the monitor. Dallas Gem Mine, San Benito County, California, USA.
Sample: Museum of Terra mineralia, Germany. Photo: © D. Tonkacheev.
Neptunite. San Benito, California, USA. Min. Museum named after. A.E. Fersman of the Russian Academy of Sciences. Photo: © А.А. Evseev.
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