File:Azurite-malachite-tenorite in quartz monzonite (Continental Pit, Butte, Montana, USA) 1.jpg

English: Blue = azurite (Cu3(CO3)2(OH)2, copper hydroxy-carbonate)

Green = malachite (Cu2CO3(OH)2, copper hydroxy-carbonate) Black = tenorite (CuO, copper oxide)

A mineral is a naturally-occurring, solid, inorganic, crystalline substance having a fairly definite chemical composition and having fairly definite physical properties. At its simplest, a mineral is a naturally-occurring solid chemical. Currently, there are over 5600 named and described minerals - about 200 of them are common and about 20 of them are very common. Mineral classification is based on anion chemistry. Major categories of minerals are: elements, sulfides, oxides, halides, carbonates, sulfates, phosphates, and silicates.

The carbonate minerals all contain one or more carbonate (CO3-2) anions.

Malachite and azurite are attractive, richly colored copper hydroxy-carbonate minerals. Malachite has a nice green color - its formula is Cu2CO3(OH)2. Azurite has a dark, rich blue color - its formula, Cu3(CO3)2(OH)2, is very close to malachite. The blue color of azurite is from Cu+, while the green color of malachite is from Cu+2. Azurite & malachite almost invariably occur together, and are telling indicators of copper in the field, even in very small quantities. Blue azurite tends to crystallize first, and can convert to green malachite. Some azurite-malachite specimens are solid enough to be cut and polished as semi-precious stone.

The sample seen here is on an ore pile at the Continental Mine (= Continental Pit) in Butte, Montana. The town is known as the “Richest Hill on Earth” and "The Mining City". The Butte Mining District has produced gold, silver, copper, molybdenum, manganese, and other metals.

The area's bedrock consists of the Butte Quartz Monzonite (a.k.a. Butte Pluton), which is part of the Boulder Batholith. The Butte Quartz Monzonite ("BQM") formed 76.3 million years ago, during the mid-Campanian Stage in the Late Cretaceous. BQM rocks have been intruded and altered by hydrothermal veins containing valuable metallic minerals - principally sulfides. The copper mineralization has been dated to 62-66 million years ago, during the latest Maastrichtian Stage (latest Cretaceous) and Danian Stage (Early Paleocene). In the supergene enrichment zone of the area, the original sulfide mineralogy has been altered.

The Continental Mine targets a low-grade copper and molybdenum deposit on the eastern side of the Continental Fault, a major Basin & Range normal fault in the Butte area with about 3500 feet of offset. The mine's rocks consists of disseminated copper sulfides plus copper- and molybdenum-bearing hydrothermal veins that intrude the BQM. Minerals at the site include chalcopyrite, molybdenite, malachite, azurite, tenorite, and cuprite. The latter four minerals are secondary copper minerals, produced by alteration of the primary copper sulfides.

When I visited in 2010, the Continental Mine was making 50,000 to 52,000 tons of ore each day. This mine can operate down to an ore grade of 0.1% copper. Most of the mineralization is disseminated copper, but veins are also present. Two stages of mineralization occurred in the Butte area - a porphyry copper system and a main stage system with large veins. The bottom of the porphyry copper system is ~ less than 12,800 feet below the surface. Veins peter out at 5600 to 5800 feet below the surface. At the Continental Mine, veins are small - they're veinlets less than 6 inches wide.

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Photo gallery of malachite: www.mindat.org/gallery.php?min=2550

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Photo gallery of azurite:

www.mindat.org/gallery.php?min=447