Neptunite

specimen number:
2072131

location:
Dallas Gem Mine, San Benito Co., California, USA

description:
Classic California specimen with lustrous prismatic crystals of neptunite measuring to 1.0 cm in size across the top of matrix with white natrolite. In good condition with minimal damage present.

The Dallas Gem Mine, located in the remote Diablo Range of San Benito County, California, is world-famous as the type locality for benitoite and a premier source of high-quality neptunite. Geologically, the deposit is situated within a tectonic inclusion of blueschist-facies metamorphic rock, specifically a glaucophane schist, trapped within a massive serpentinite body along the San Andreas Fault zone. The mineralization occurred during low-temperature, high-pressure hydrothermal events where fluids rich in barium, titanium, and lithium reacted with the host rock to form rare silicate minerals. Historically, the site was discovered in 1907 by James Couch while prospecting for copper and diamonds, but it was later identified by mineralogist George Louderback as a unique barium titanium silicate. The mine operated sporadically through the 20th century under various owners, including the Dallas Mining Company, and remains a site of intense interest for its unique geochemical signature that is found in very few other places on Earth.

For mineral collectors, the neptunite from the Dallas Gem Mine is considered the finest in the world, characterized by its deep, near-black color and brilliant vitreous luster. These crystals typically form as sharp, elongated monoclinic prisms that can reach several centimeters in length, often appearing in spectacular contrast against a snow-white matrix of fibrous natrolite. Unlike neptunite found in Greenland or Russia, the San Benito specimens frequently occur in aesthetic association with the vivid blue, triangular crystals of benitoite and brownish-yellow joaquinite. Because the natrolite matrix is typically removed using hydrochloric acid to reveal the embedded crystals, prepared specimens often look like dark, architectural towers rising from a white base. These pieces are highly prized for their sharp geometric form and the remarkable rarity of the mineral species found within this small, localized geological environment.

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