Cumengeite, also known as cumengite, is a

secondary mineral A primary mineral is any mineral formed during the original crystallization of the host igneous primary rock and includes the essential mineral(s) used to classify the rock along with any accessory minerals. In ore deposit geology, hypogene processe ...

that was named after mining engineer Bernard Louis Philippe Édouard Cumenge, who found the first specimens. It is easily confused with

diaboleite Diaboleite is a blue-colored mineral with formula Pb2CuCl2(OH)4. It was discovered in England in 1923 and named ''diaboleite'', from the Greek word διά and boleite, meaning "distinct from boleite". The mineral has since been found in a number o ...

. It is a valid species that was first described prior to 1959, and is grandfathered now, but it has been a valid species since 1893, since pre- IMA. It is the hydroxychloride of lead and copper.

Properties

It is brittle and conchoidal, meaning it is easily breakable, and the fracture of it makes small, conchoidal fragments. It is an

epitaxial Epitaxy refers to a type of crystal growth or material deposition in which new crystalline layers are formed with one or more well-defined orientations with respect to the crystalline seed layer. The deposited crystalline film is called an epit ...

mineral, which is a form of crystal growth. During the growth, the new crystalline layers are formed with at least one well-defined orientation. The crystal itself is pseudo-cubic, meaning its shape resembles that of a cube. When this uncommon mineral forms epitaxial overgrowths with

boleite Boleite is a complex halide mineral with formula: KPb26Ag9Cu24(OH)48Cl62. It was first described in 1891 as an oxychloride mineral. It is an isometric mineral which forms in deep-blue cubes. There are numerous minerals related to boleite, such a ...

however, where the inner cubic core is boleite, and the outer protruding mineral which forms in triangular shape is cumengeite, the two mineral form a shape similar to a star. It has octahedral, "diamond-shaped" or cubo-octahedral crystals. It usually appears on the faces of

or pseudoboleite, as an intergrowth. It has

dichroic In optics, a dichroic material is either one which causes visible light to be split up into distinct beams of different wavelengths ( colours) (not to be confused with dispersion), or one in which light rays having different polarizations are ...

attributes, which in this case means the mineral breaks up the light into different beams of different wavelengths, making the light passing through them break and come out in different colors. Cumengeite is soluble in HNO₃. It mostly consists of lead (55.08%), chlorine (18.85%) and copper (16.09%), but otherwise it has oxygen (9.32%) and hydrogen (0.66%) in it.

Mining

Although it has several localities, the most significant one, whereit is a type locality is at the Boleo district, specifically at the Amelia mine in Mexico. Til these days, that mine is the only one that had produced well-formed cumengeite crystals of any significance. Cumenge, the one who found the mineral also collected it from its type locality, Boleo. It appears in lead or copper deposits' secondary, oxidized zone. The mineral's smelter slag is immersed into the sea water, however this is an anthropogenic material and not a natural mineral. This sedimentary mineral is a rare collectors mineral, high-quality epitexial specimens are really valuable.

References

{{reflist Lead minerals Copper minerals Chlorides Tetragonal minerals