Overview of Colorful Secondary Copper Minerals
Mineral collectors are often very interested in aesthetic and bright colored specimens. Copper is the element which causes various hues of green and blue in many minerals. Colorful secondary copper minerals are thus very popular and nice specimens can be extremely expensive.
Introduction: Origin of Secondary Minerals
The secondary (supergene) minerals are created by weathering and oxidation of primary minerals. Primary copper minerals — mostly sulfides like chalcopyrite, tetrahedrite, bornite — are precipitated from mineral-rich hot solutions (called hydrothermal fluids) deep under the Earths surface. When these primary sulfide minerals get close to the Earths surface (due to erosion and tectonics), they react with oxygen and form huge range of supergene minerals.
The weathered zones close to the surface are called Gossan or Iron Cap, because they are usually composed of limonite and various secondary minerals. Limonite is produced by the oxidation of pyrite and other iron-sulfides. Depending on the overall chemical composition of the original mineral assemblage, various supergene copper minerals are produced.
Malachite and azurite are the most common secondary copper minerals and occur at almost every copper deposit, also cuprite and chrysocolla are quite common. More rare minerals like libethenite, euchroite, tyrolite or olivenite are precipitated in the presence of arsenic. Formation of rare minerals like atacamite, turquoise and dioptase requires special environment like desert climate.
Copper Carbonate Minerals
When the underground water rich in dissolved copper sulfates gets mixed with surface water rich in dissolved carbon dioxide, the various colorful carbonates are precipitated. Malachite and azurite are present at almost all copper deposits at least in small amount.
Malachite
Malachite is probably the most common secondary copper mineral. Most of the green coatings, botryoidal aggregates and tiny green needles associated with copper deposits are malachite. It can even replace some other secondary minerals like native copper, cuprite and azurite. Malachite rarely forms crystals except the fans and sprays of tiny needle crystals but it can form outstanding layered masses which are cut and polished.
Azurite
The deep blue color of azurite is very attractive for mineral collectors, especially the well developed and large azurite crystals are extremely expensive. Azurite is quite common and often occurs together with malachite. Unfortunately, it very rarely forms decent crystals.
Copper Silicate Minerals
Formation of secondary copper silicates is a bit complicated. As you may know, the quartz is not that easily dissolved by water and it requires highly alkaline conditions. As most of the underground water on sulfide deposits is highly acidic, the formation of silicates is unlikely.
Chrysocolla
Unlike most minerals, chrysocolla does not produce visible crystals. It is usually in the form of blue-green masses, coatings and botryoidal aggregates. Chrysocolla is often present in desert environment while it rarely occurs in humid places.
Dioptase
Dioptase is highly aesthetic and popular mineral, both for its scarcity and very unique green-blue color. Formation of dioptase requires very special conditions like warm and dry climate combined with alkaline environment. Best crystals of dioptase were found in Kaokoweld and Tsumeb, Namibia, and on the type locality Altyn-Tube in Kazakhstan.
Copper Phosphate and Arsenate Minerals
If the highly aggresive acidic mine waters react with phosphates in the country rock — like apatite, monazite or xenotime — the secondary copper phosphates may form. Many copper deposits also contain significant amount of arsenic minerals — like tetrahedrite or arsenopyrite. High amount of arsenic is released when these sulfides are oxidized and it readily reacts with copper into the huge range of various copper arsenates. Phosphates and arsenates share the same crystal structure.
Turquoise
The original turquoise was named after its origin in Turkey (the locality is currently in Iran). Turquoise is hydrated copper-aluminum phosphate which occurs mostly in desert environment — like Arizona, Mexico, Iran or Mongolia. Most turquoise specimens and almost all cabochons on the market are fakes created from dyed magnesite, howlite or plastic.
Euchroite
A very rare hydrated arsenate, which is known for its nice color and aesthetic crystals. Decent euchroite specimen is difficult to find. The best specimens come from the type locality Lubietova in Slovakia.
Olivenite
Typical olive green color was the main reason for the olivenite mineral name. This rare copper arsenate forms crusts and tiny needle-like crystals. Olivenite is isostructural with libethenite and may form by partial dehydration of euchroite. Nice olivenite specimens are known from Cornwall, UK.
Tyrolite
This turquoise colored with typical pearly luster was named after its type loclaity in Tyrol, Austria. Tyrolite is a complex hydrated calcium-copper arsenate with two polytypes. Crystals are usually flat and form typical fans and sprays. Very nice tyrolites are known from Farbiste and Novoveska Huta in Slovakia.
Libethenite
Libethenite is a rare copper phosphate and forms typical green dipyramids. Unfortunately, the libethenite crystals are usually very small. The best crystals are probably found in Portugal, the type locality in Lubietova produced very nice specimens too.
Pseudomalachite
Dark green hydrated copper phosphate is called pseudomalachite. It seldom produces crystals of microscopic size, mostly it occurs in the form of botryoidal aggregates, very similar to malachite. Unlike malachite, it does not fizz in the acids. Very nice pseudomalachite aggregates came from Lubietova in Slovakia.
Copper Oxide Minerals
Oxides are a typical example of secondary minerals in a highly oxidized environment close to the surface. Hydrated hydroxides or oxide-hydroxides are formed bellow water table.
Cuprite
Many copper deposits contain significant amount of secondary oxide mineral cuprite. It can create coating on copper minerals or even completely replace them. Cuprite is sometimes replaced by malachite. Most cuprite specimens are just red or red-brown masses but it can rarely form awesome red octahedron shaped crystals. The most valued are translucent gemmy cuprites from Namibia. Cuprite can also form needle-like bright red crystals called chalcotrichite.
Copper Sulfate Minerals
The oxidation of sulfides produces huge amount of various sulfates. Many of these are unstable and react with some other minerals into various phosphates, carbonates, silicates or arsenates. But some copper sulfate minerals also exist.
Chalcanthite
The bright blue crystals of chalcathite are very widespread at many mineral shows. The bad news is that most of these are artificially grown in the lab. Unfortunately, chalcanthite is perfectly soluble in water. Those nice crystals do not survive very long without proper conservation. The natural occurence of chalcanthite is limited to a very dry climate.
Brochantite
Very dark green colored needles of brochantite are very popular. Unlike chalcanthite, this copper sulfate is not soluble in water. It may look quite similar to dark malachite but it does not fizz in the hydrochloric acid. Perfect brochantite specimens came from Namibia and recently from Milpilas, Mexico.
Copper Halide Minerals
Despite the abundant chlorine in the hydrothermal veins, halides as the minerals are not very common. Most of the chlorine is stored in various water soluble halides like a common rock salt. Some rare and water insoluble halides may form in the extreme desert environment.
Atacamite
Dark green crystals of atacamite are very aesthetic and highly popular. This rare halide occurs mostly in Atacama Desert in Chile, as well as its cousin clinoatacamite. It is one of the few halides in nature which form crystallized specimens in a decent size.
