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By Reviewed By Andreas Zabczyk Jul 16, 2010 Updated Nov 04, 2019

Boleite Gemstones

Boleite in Matrix
Mexican Boleite in Matrix

Boleite was first discovered in Boleo, Mexico in 1891, in a region of the Baja peninsula that has become well known for rare minerals. Boleite is such an unusual and complex mineral that the definition of its chemical composition and crystal structure have undergone several revisions since its first discovery.

The first specimens of boleite were discovered by Eduoard Cumenge, a French mining engineer. Another rare halide mineral from the same location, with a similar structure and a slightly different chemical composition, was named cumengite in honor of Cumenge.

There are several features of boleite that make it of particular interest to collectors. For one thing, it has an unusual chemical composition, being a hydrated lead copper silver chloride hydroxide. It contains significant quantities of these metals, with about 49% lead, 14% copper and 9% silver. Secondly, it has a fascinating crystal structure, since boleite is always pseudocubically twinned. That is, each cube-like crystal of boleite is actually composed of three rectangular box-like crystals oriented at right angles to each other. Boleite also has a very attractive indigo color and some very rare transparent crystals have been cut as gemstones.

Boleite in Matrix
Boleite in Matrix

Like other halide minerals, such as fluorite, boleite is quite soft, with a Mohs hardness rating of only 3 to 3.5. It has a vitreous to pearly luster. It is an unusually dense mineral, with a specific gravity of 5.05. Only a few minerals, such as hematite and cassiterite, have a higher density. Boleite's refractive index is also very high at 2.03-2.05; higher than sapphire, sphene and demantoid garnet.

In addition to cumengite, there are a number of other minerals associated with boleite, including pseudoboleite, chloroxiphite, paralaurionite, matlockite, chlorargyrite, bideauxite, atacamite, mendipite and diaboleite.

In addition to the deposit in Baja, Mexico, boleite has been found in Broken Hill in New South Wales, Australia; in the Mammoth District of Arizona in the United States; and in the Mendip Hills of Somerset, England.

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