Colemanite

specimen number:
1052552

location:
Pacific Mine, Calico, San Bernardino Co., California, USA

description:
An interesting old specimen from California, an open pocket on the front of this matrix is lined with colorless crystals of colemanite. In good condition. The specimen comes with an antique label as shown at the end of the video and dates to the very early 20th century.

The Pacific Mine, located in the Calico District of the Borate area in San Bernardino County, California, was a key producer of borate minerals during the late 19th century. Geologically, the deposit is hosted within the Miocene-aged Daggett Lithofacies of the Pickhandle Formation, where borate mineralization occurred as layered nodules and crystalline masses within lacustrine shale and limestone. These deposits formed in an arid, closed-basin lacustrine environment where boron-rich volcanic fluids concentrated in ancient lake beds. Historically, the mine was operated by the Pacific Coast Borax Company between approximately 1884 and 1907. It was primarily worked for colemanite, a calcium borate mineral that occurs here as magnificent, colorless to white, glassy monoclinic crystals often found in large cavities or geode-like nodules. The discovery of these high-grade colemanite deposits at the Pacific Mine and the nearby Borate camp effectively ended the era of scraping borax from desert dry lakes, as underground mining proved far more efficient. Production ceased when even larger and more easily accessible deposits were discovered in the Kramer District, but the Pacific Mine remains a classic locality for collectors due to the exceptional luster and clarity of its colemanite specimens.

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