Covellite
Encyclopedia
Copper sulfide
Copper sulfides describe a family of chemical compounds and minerals with the formula CuxSy. Both minerals and synthetic materials comprise these compounds. Some copper sulfides are economically important ores....
mineral with the formula
Chemical formula
A chemical formula or molecular formula is a way of expressing information about the atoms that constitute a particular chemical compound....
CuS. This indigo blue mineral is ubiquitous in copper ores, it is found in limited abundance and is not an important ore of copper itself, although it is well known to mineral collectors.
The mineral is associated with chalcocite in zones of secondary enrichment (supergene
Supergene (geology)
In ore deposit geology, supergene processes or enrichment occur relatively near the surface. Supergene processes include the predominance of meteoric water circulation with concomitant oxidation and chemical weathering. The descending meteoric waters oxidize the primary sulfide ore minerals and...
) of copper sulfide deposits. Commonly found with and as coatings on chalcocite
Chalcocite
Chalcocite, copper sulfide , is an important copper ore mineral. It is opaque, being colored dark-gray to black with a metallic luster. It has a hardness of 2½ - 3. It is a sulfide with an orthorhombic crystal system....
, chalcopyrite
Chalcopyrite
Chalcopyrite is a copper iron sulfide mineral that crystallizes in the tetragonal system. It has the chemical composition CuFeS2. It has a brassy to golden yellow color and a hardness of 3.5 to 4 on the Mohs scale. Its streak is diagnostic as green tinged black.On exposure to air, chalcopyrite...
, bornite
Bornite
Bornite is a sulfide mineral with chemical composition Cu5FeS4 that crystallizes in the orthorhombic system .-Appearance:Bornite has a brown to copper-red color on fresh surfaces that tarnishes to various iridescent shades of blue to purple in places...
, enargite
Enargite
Enargite is a copper arsenic sulfosalt mineral with formula: Cu3AsS4. It takes its name from the Greek word enarge, "distinct." Enargite is a steel gray, blackish gray, to violet black mineral with metallic luster. It forms slender orthorhombic prisms as well as massive aggregates...
, pyrite
Pyrite
The mineral pyrite, or iron pyrite, is an iron sulfide with the formula FeS2. This mineral's metallic luster and pale-to-normal, brass-yellow hue have earned it the nickname fool's gold because of its resemblance to gold...
, and other sulfides, it often occurs as pseudomorphic replacements after other minerals. Despite the very rare occurrence as a volcanic sublimate, the initial description was at Mount Vesuvius by Nicola Covelli (1790–1829).
Composition
Covellite (CuS) belongs to the binary copper sulfides group, which has the formula CuxSy and can have a wide-ranging copper/sulfur ratio from 1:2 to 2:1 (Cu/S)., However this series is by no means a continuous one and the width of the homogeneity range of covellite CuS is narrow. Materials rich in sulfur CuSx where x~ 1.1- 1.2 do exist but they exihibit "superstructures" a modulation of the hexagonal ground plain of the structure spanning a number of adjacent unit cells. This indicates that several of covellite's special properties are the result of molecular structure at this level. Even though the peculiar monovalent nature of copper in sulfides has been known since the 1980s there are many places in the later literature, even in standard texts, where the divalent copper idea pops up again.As described for the mineral pyrite
Pyrite
The mineral pyrite, or iron pyrite, is an iron sulfide with the formula FeS2. This mineral's metallic luster and pale-to-normal, brass-yellow hue have earned it the nickname fool's gold because of its resemblance to gold...
, the assignment of formal oxidation states (or charges) to the atoms that constitute covellite is deceptive. The formula might seem to suggest the description Cu2+, S2−. In fact the atomic structure shows that copper and sulfur each adopt two different geometries. However photoelectron spectroscopy, magnetic and electrical properties all indicate the absence of Cu2+ (d9) ions. In contrast to the oxide CuO the material is not a magnetic semiconductor but a metallic conductor with weak Pauli-paramagnetism. Thus, the mineral is better described as consisting of Cu+ and S− rather than (Cu2+ and S2−). As happens in the pyrite
Pyrite
The mineral pyrite, or iron pyrite, is an iron sulfide with the formula FeS2. This mineral's metallic luster and pale-to-normal, brass-yellow hue have earned it the nickname fool's gold because of its resemblance to gold...
structure the non-closed shell S− undergoes pairing to form S22−, but interestingly this only holds for 2/3 of the sulfur atoms. The other 1/3 remains unpaired and together with Cu atoms forms hexagonal layers reminiscent of the boron nitride (graphite structure). Thus a description Cu+3S−S22− would seem appropriate with a delocalized hole in the valence band leading to metallic conductivity. Later bandstructure calculations however indicate that the hole is more localized on the sulfur pairs than on the unpaired sulfur. This means that Cu+3S2−S2− with a mixed sulfur oxidation state -2 and -1/2 is more appropriate. There is a precedence for that in the (synthetic) pyrite CuS2 that should also be seen mostly as Cu+S2− producing a Pauli-paramagnetic metallic conductor rather than a semiconductor with strong magnetic properties. (For references see: copper monosulfide).
Formation
Covellite is commonly found as a secondary copper mineral in deposits. It is rarely a primary mineral in copper deposits, and is even less likely to be found as a volcanic sublimate. Covellite is known to form in weatheringWeathering
Weathering is the breaking down of rocks, soils and minerals as well as artificial materials through contact with the Earth's atmosphere, biota and waters...
environments near the surface in deposits where copper is the primary sulfide. As a primary mineral, the formation of covellite is restricted to hydrothermal conditions.
Covellite's unique crystal structure is related to the conditions of its formation. Its distinct framework is produced during oxidative formation conditions. Attempts to synthesize covellite indicate it undergoes a complex oxidation process, and that its formation depends on the state and history of the associated sulfides it was derived from. Experimental evidence shows ammonium metavanadate
Ammonium metavanadate
Ammonium metavanadate, NH4VO3, is a yellow crystalline solid which is water soluble inorganic acid that acts as insulin mimic. It functions as a catalyst to certain reactions and is known to have toxic effects in certain species. It has been found in slags and fly ash from coal burning which has...
(NH4VO3) to be a potentially important catalyst for covellite's solid state transformation from other copper sulfides.
Geologic occurrence
Covellite's occurrence is widespread in the United States. In Silver Bow County, MontanaSilver Bow County, Montana
-National protected areas:*Beaverhead National Forest *Deerlodge National Forest -Demographics:As of the census of 2000, there were 34,606 people, 14,432 households, and 8,933 families residing in the county. The population density was 48 people per square mile . There were 16,176 housing units at...
, covellite has been found in veins at depths of 1,150 meters, as the primary mineral. Covellite formed as clusters in these veins
Vein (geology)
In geology, a vein is a distinct sheetlike body of crystallized minerals within a rock. Veins form when mineral constituents carried by an aqueous solution within the rock mass are deposited through precipitation...
reaching one meter across in Leonard mines, Montana. As a secondary mineral, covellite also forms as descending surface water in the supergene
Supergene (geology)
In ore deposit geology, supergene processes or enrichment occur relatively near the surface. Supergene processes include the predominance of meteoric water circulation with concomitant oxidation and chemical weathering. The descending meteoric waters oxidize the primary sulfide ore minerals and...
enrichment zone oxidizes and redeposits covellite on hypogene sulfides (pyrite and chalcopyrite). Locally, findings of covellite have been discovered in salt domes and at the McCellan copper mine in Foard County, Texas
Foard County, Texas
As of the census of 2000, there were 1,622 people, 664 households, and 438 families residing in the county. The population density was 2 people per square mile . There were 850 housing units at an average density of 1 per square mile...
. An unusual occurrence of covellite was found replacing organic debris in the red beds of New Mexico
New Mexico
New Mexico is a state located in the southwest and western regions of the United States. New Mexico is also usually considered one of the Mountain States. With a population density of 16 per square mile, New Mexico is the sixth-most sparsely inhabited U.S...
.
It has also been reported from the Calabona mine, Alghero
Alghero
Alghero , is a town of about 44,000 inhabitants in Italy. It lies in the province of Sassari in northwestern Sardinia, next to the sea.-History:The area of today's Alghero has been settled since pre-historic times...
, Sardinia
Sardinia
Sardinia is the second-largest island in the Mediterranean Sea . It is an autonomous region of Italy, and the nearest land masses are the French island of Corsica, the Italian Peninsula, Sicily, Tunisia and the Spanish Balearic Islands.The name Sardinia is from the pre-Roman noun *sard[],...
; at
Bor
Bor
Bor may refer to:* Matej Bor, the pen name of the Slovene poet Vladimir Pavšič *Bor , a location in Afghanistan*Bor , a town in Plzeň Region , Czech Republic...
, Serbia
Serbia
Serbia , officially the Republic of Serbia , is a landlocked country located at the crossroads of Central and Southeast Europe, covering the southern part of the Carpathian basin and the central part of the Balkans...
; from Leogang, Salzburg
Salzburg
-Population development:In 1935, the population significantly increased when Salzburg absorbed adjacent municipalities. After World War II, numerous refugees found a new home in the city. New residential space was created for American soldiers of the postwar Occupation, and could be used for...
, Austria
Austria
Austria , officially the Republic of Austria , is a landlocked country of roughly 8.4 million people in Central Europe. It is bordered by the Czech Republic and Germany to the north, Slovakia and Hungary to the east, Slovenia and Italy to the south, and Switzerland and Liechtenstein to the...
; at Dillenburg
Dillenburg
Dillenburg is a town in Hesse's Gießen region in Germany. The town was formerly the seat of the old Dillkreis district, which is now part of the Lahn-Dill-Kreis....
, Hesse
Hesse
Hesse or Hessia is both a cultural region of Germany and the name of an individual German state.* The cultural region of Hesse includes both the State of Hesse and the area known as Rhenish Hesse in the neighbouring Rhineland-Palatinate state...
and Sangerhausen
Sangerhausen
Sangerhausen is a town in Saxony-Anhalt, Germany, capital of the district of Mansfeld-Südharz, without being part of it.It is situated southeast of the Harz, approx. 35 km east of Nordhausen, and 50 km west of Halle...
, Saxony
Saxony
The Free State of Saxony is a landlocked state of Germany, contingent with Brandenburg, Saxony Anhalt, Thuringia, Bavaria, the Czech Republic and Poland. It is the tenth-largest German state in area, with of Germany's sixteen states....
, Germany
Germany
Germany , officially the Federal Republic of Germany , is a federal parliamentary republic in Europe. The country consists of 16 states while the capital and largest city is Berlin. Germany covers an area of 357,021 km2 and has a largely temperate seasonal climate...
; from Kedabek, Caucasus Mountains
Caucasus Mountains
The Caucasus Mountains is a mountain system in Eurasia between the Black Sea and the Caspian Sea in the Caucasus region .The Caucasus Mountains includes:* the Greater Caucasus Mountain Range and* the Lesser Caucasus Mountains....
, Russia
Russia
Russia or , officially known as both Russia and the Russian Federation , is a country in northern Eurasia. It is a federal semi-presidential republic, comprising 83 federal subjects...
and in the Bou-Skour mine, Bou Azzer
district, Morocco
Morocco
Morocco , officially the Kingdom of Morocco , is a country located in North Africa. It has a population of more than 32 million and an area of 710,850 km², and also primarily administers the disputed region of the Western Sahara...
.
Applications
Covellite was the first discovered natural superconductor. The framework of CuS3 /CuS2 allow for an electron excess that facilitate superconduction during particular states, with exceptionally low thermal loss. Material science is now aware of several of covellite's favorable properties and several researchers are intent on synthesizing covellite. Gramp et al. (2006) discovered that covellite can also be produced in the lab under anaerobicHypoxia (environmental)
Hypoxia, or oxygen depletion, is a phenomenon that occurs in aquatic environments as dissolved oxygen becomes reduced in concentration to a point where it becomes detrimental to aquatic organisms living in the system...
conditions by sulfate reducing bacteria at a variety of temperatures. However, further research remains, because although the abundance of covellite may be high, the growth of its crystal size is actually inhibited by physical constraints of the bacteria. It has been experimentally demonstrated that the presence of ammonium vanadates is important in the solid state transformation of other copper sulfides to covellite crystals.