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The chemical formula of mineral Cumengite is indicated by the formula Pb21Cu20Cl42(OH)40 or Lead Copper Chloride Hydroxide. Cumengite is actually a Halide mineral. Cumengite is actually considered as the most desirable piece of halide mineral collection due to its extremely interesting crystal habit, rarity, unusual chemistry and fascinatingly beautiful color under polarizing light microscope. Sometimes the name of the mineral species Cumengite is spelled as cumengeite. The mineral Cumengite was named after Edward Cumenge, a French mining engineer. Mineral Cumengite was first discovered in the year 1893 at Baja California, Mexico. Cumengite is known to crystallize in the tetragonal system of crystal formation. In optical mineralogy, this tetragonal system comprises crystals having three axes, which are all in a position perpendicular to one another. Two axes are usually found having the same or equal length.

 

            Cumengite is considerably rare halide mineral. Mineral Cumengite is commonly of great interest to most mineral collectors of rare halides. Cumengite is very closely related to another interesting halide mineral called boleite. Cumengite and Boleite come from the same locality and they are formed through the same geological process in nature. They both have very similar chemical composition except that Cumengite has no silver component. They also exhibit the same interesting indigo blue color and both form interesting crystal shapes. But all of the above is not the only reason for the close relationship of the two minerals. They are actually as close as how two minerals can appear as possible. This is because Cumengite crystals are actually forming or growing on the cube faces of the Boleite mineral crystals. Some other minerals are found having random growth on other minerals but in the case of Cumengite, it seems that Cumengite has found enough similar structures of boleite that appear very similar to its own. That is why Cumengite seems to grow continuously on Boleite crystal surface and thus finishing the crystal growth of the whole piece of specimen. The relationship between these two minerals is called epitaxial overgrowth. There are also several other cases on this mineral relationship but the case of Cumengite and boleite is the most amazing. When fully developed piece of specimen is found, it can be noticed that all of the six boleite cube faces are capped with a four sided Cumengite pyramid. The result is actually a three dimensional six pointed star with indigo blue color. But unfortunately, there has been no complete specimen found existing. Only partial stars are found and they are considered very valuable.

 

            Mineral Cumengite is commonly found exhibiting an indigo blue color with violet tints that could appear more splendidly exhibited when viewed with the aid of petrographic polarizing light microscope used in optical mineralogy. Cumengite is also most commonly found exhibiting vitreous luster when the mineral is closely evaluated in reflected light of polarized microscope for mineralogists. It has numerous cleavage planes found and it usually includes four good cleavages to distinct directions and one poor basal cleavage in other direction all of which can be seen more clearly exhibited when mineral specimen is evaluated with the aid of polarized microscopes for mineralogists. Cumengite is also found showing uneven fracture when it is evaluated under several adjustments on the aperture diaphragm of polarizing microscope used in optical mineralogy. They are also considered brittle, a property that is commonly displayed by glasses and most nonmetallic minerals. The specific gravity measure of the mineral is commonly found as 4.6 grams per cubic centimeters, which is considered rather heavy for translucent minerals. The hardness measure of mineral Cumengite when it is evaluated using the Mohs scale method is commonly 2.5. Cumengite is commonly found leaving a blue streak when mineral specimen is rubbed on a white porcelain streak plate.

 

            Cumengite is most commonly used as mineral specimen and it is often found exhibiting nice and splendid microscope images under polarized microscopes used in optical mineralogy. Cumengite is also considered a very minor ore of lead and it usually shows fascinating appearance when viewed under an ore polarizing microscope. Most Cumengite mineral crystals are found transparent to translucent in appearance. They commonly appear translucent on thin edges. The crystal habit of mineral Cumengite as described in optical mineralogy commonly includes pseudo-octahedral crystals, which can be exhibited more clearly visible when mineral is viewed under polarizing microscopes. But more commonly, Cumengite is found as epitaxial overgrowths on boleite crystals as mentioned. The star shaped aggregate that can be formed is actually very amazing and fascinating when viewed under petrographic polarizing light microscope.

 

            Cumengite is most often found exhibiting a very high surface relief when it is closely evaluated under polarizing light microscope for mineralogists. Mineral Cumengite is most commonly found showing uniaxial negative figure when it is evaluated between crossed nicols of petrographic polarizing microscopes used in optical mineralogy. The refractive indices found exhibited by mineral Cumengite when it is evaluated in plane polarized light of polarizing microscope is usually ranging from 2.041 to 1.926. The maximum birefringence exhibited by the mineral when viewed in plane light under polarized microscope is commonly 0.115. Cumengite is a pleochroic mineral. It shows dichroism in shades of darker blue with green tint and pure blue colors when examined under polarizing microscopes used in optical mineralogy. There is no specific data on the toxicity and health danger for this mineral Cumengite. However, the specimen of this mineral species should be treated with great care and use of sensible precaution is advised upon handling them. Mineral Cumengite is not radioactive.

 

The best field indicators of mineral Cumengite usually include color, density, locality, crystal habit and its amazing association with other interesting mineral. Mineral Cumengite is most commonly found associated with several other interesting minerals especially boleite and also pseudoboleite. Both minerals, Cumengite and Boleite come from the same type of locality. They are both found at Boleo, Baja California, Mexico. Both of them is also a results of the of the oxidation process of igneous copper ore bodies. Mineral Cumengite is commonly found in the oxidized zone of sedimentary copper ores associated with mineral boleite. The occurrence of mineral Cumengite is limited only to Boleo, Santa Rosalia, Baja California, Mexico.



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Thursday, January 29th, 2009 at 4:05 am
Category:
The Halides Mineral Class
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