Mineral Info, Malachite

 

Mineral Information

 

PHOTO OF THE MINERAL MALACHITE, (Cu2(CO3)(OH)2, Copper Carbonate Hydroxide)
A COPPER BEARING MINERAL WITH AS MUCH AS 50% COPPER, WHICH IS WHERE IT GETS ITS GREEN COLOR. The distinctive bright-green hydrous CARBONATE MINERAL malachite is a common but minor ore of copper. It is usually found in copper deposits associated with LIMESTONE, occurring with AZURITE as the weathering product of other copper ore minerals. Hardness is 3 1/2 to 4, streak is pale green, specific gravity is 3.9 to 4.1, and luster is adamantine to silky. Malachite forms needlelike prismatic crystals (monoclinic system) that are rarely distinct; it is usually found in granular, earthy, or fibrous masses and rounded, banded crusts. Malachite is used as a decorative stone when cut and polished, a semiprecious gem, and a green pigment.

The chemical element copper is a reddish metal at the head of group IB in the periodic table. Its symbol is Cu; atomic number, 29; and atomic weight, 63.546. Copper follows the first transitional series of elements. Copper was the first metal used by humans and is second only to iron in its utility through the ages. The name is derived from the Latin cuprum, copper, from the earlier Latin Cyprium, Cyprian metal. The discovery of the metal dates from prehistoric times, and it is estimated that copper was first used about 5000 BC or even earlier.
Natural Occurrence and Extraction In Roman times much of the copper was obtained from the island of Cyprus, as the name implies. Copper today is mined in many parts of the world, the largest producers at present being Chile, Peru, Poland, the United States, Zaire, and Zambia. More than 160 minerals containing copper are known. Copper constitutes 70 parts per million of the Earth's crust and is present to the extent of 0.020-0.001 parts per million in seawater. Copper in its native state--such as that found in the Lake Superior region of North America--is often so pure that it requires only melting with a flux to produce "lake copper," which for many years was the world standard for pure copper. About 80% of all copper mined today, however, is derived from low-grade ores containing 2% or less of the element. Half of the world's copper deposits are in the form of chalcopyrite ore. All important copper-bearing ores fall into two main classes: oxidized ores and sulfide ores.

Sulfide ores are more important commercially. Ores are removed either by open-pit or by underground mining. Ores containing as little as 0.4% copper can be mined profitably in open-pit mining, but underground mining is profitable only if an ore contains 0.7%-6% copper. The oxidized ores, such as cuprite and tenorite, can be reduced directly to metallic copper by heating with carbon in a furnace, but the sulfide ores, such as chalcopyrite and chalcocite, require a more complex treatment in which low-grade ores have to be enriched before smelting begins. This involves the ore-flotation process, in which the ore is crushed and powdered before it is agitated with water containing a foaming agent and an agent to make the copper-bearing particles water-repellent. These particles accumulate in the froth on the surface of the flotation tank, and this froth is skimmed off and heated to about 800 deg C to remove some of the water as well as antimony, arsenic, and sulfur, which are also present.

The residue is then mixed with silica and melted in a furnace at 1,400 deg-1,500 deg C. This produces two liquid layers: a lower layer of copper matte (cuprous sulfide mixed with iron sulfide and oxides), and an upper layer of silicate slag, which is drawn off. Silica or siliceous copper ore is added to the liquid matte in a converter, and air under pressure is blown through the liquid. Upon removal of the iron slag, the copper(I) sulfide that remains is reduced to copper by heating in a controlled amount of air. The remaining molten copper, which is 98%-99% pure, is either cast into blocks of blister copper or into anodes. The final stage of purification is mainly by electrolytic refining, which yields copper of 99.95%-99.97% purity. The impure copper is made the anode of an electrolytic cell that contains pure strips of copper as the cathode and an electrolyte of aqueous copper(II) sulfate. During electrolysis, copper is transferred from the anode to the cathode. An anode sludge containing silver and gold is produced during this process, and this increases its economic feasibility.
Physical and Chemical Properties Eleven isotopes of copper are known, two of which are not radioactive and occur with a natural abundance of 69.09% and 30.91%, respectively. Copper melts at 1,083.4 deg plus or minus 0.2 deg C (in a vacuum), boils at 2,567 deg C, and has a density of 8.96 at 20 deg C. The element has a hardness of 3, takes on a bright metallic luster, has a cubic crystal structure, and is malleable, ductile, and a good conductor of heat and electricity, second only to silver in electrical conductivity. The outstanding feature of copper and the other metals of Group IB (gold and silver) is their resistance to chemical attack. Copper is slowly attacked by moist air, and its surface gradually becomes covered with the characteristic green patina that consists of basic sulfate. At about 300 deg C copper is attacked by air or oxygen, and a black coating of copper(II) oxide forms at the surface; at a temperature of 1,000 deg C copper(I) oxide is formed instead. The metal is attacked by sulfur vapor, with the formation of copper(I) sulfide; and by the halogens, which form copper(II) halides, except iodine, which forms copper(I) iodide. Copper is not attacked by water or steam, and dilute nonoxidizing acids, such as dilute hydrochloric and dilute sulfuric acids, have no effect in the absence of an oxidizing agent. The metal is attacked by boiling concentrated hydrochloric acid with the evolution of hydrogen, by hot concentrated sulfuric acid, and by dilute or concentrated nitric acid.
Alloys of Copper Copper mixes well with many elements, and more than 1,000 different alloys have been formed, several of which are technologically significant. The presence of the other element or elements can modify the hot or cold machining properties, tensile strength, corrosion fatigue, and wear resistance of the copper; it is also possible to create alloys of pleasing colors.

The best-known alloy of copper is BRASS, which consists of copper containing between 5% and 40% zinc. It possesses a high tensile strength, hardness, and wear-resistance. The addition of 0.5%-3% lead to a brass alloy (leaded brass) improves the machinability of brass, and brass containing 30%-40% zinc and 1% tin (tin brass) has a high corrosion-resistance. Another useful alloy of copper is nickel silver, which consists of copper (55%-65%), nickel (10%-18%), and zinc (17%-27%). It is used as a base for silver-plating items such as costume jewelry and tableware. Phosphor bronze is formed by the addition of up to 0.35% of phosphorus to copper-tin alloys containing up to 10% tin. This alloy has great resiliency, fatigue endurance, hardness, and corrosion resistance; these properties make it suitable for use in springs and diaphragms. Silicon bronze, consisting of 1%-3% silicon, 95%-96% copper, and small amounts of other metals--for example lead, tin, zinc, manganese, iron, or nickel--is as strong as mild steel and has a high resistance to corrosion. It is used in the production of equipment for chemical plants in which corrosive liquids are handled. Aluminum-copper bronzes contain aluminum (5%-12%) and sometimes zinc and silicon; they are also corrosion resistant, and have good strength, hardness, and wear resistance. They are used for carrying corrosive liquids such as hot brine in salt refineries. Beryllium-copper alloys, containing 2% beryllium, have a high corrosion resistance and high tensile strength, with considerable fatigue and wear resistance. They find wide application where high strength is required and for making non-spark-forming tools.

Copper is a trace element essential to the healthy life of many plants and animals, in which it usually occurs as part of the oxidizing enzymes such as ascorbic acid oxidase, tyrosinase, lactase, and monoamine oxidase. These enzymes, which are high-molecular-weight proteins containing 0.05%-0.35% of Cu, play an important part in living oxidation and reduction reactions, in which the copper undergoes cyclic changes between Cu(I) and Cu(II) oxidation states. The metal is tightly bound to ligand sites, containing oxygen, sulfur, or nitrogen atoms on the protein. The normal diet of humans includes between 2 and 5 mg of copper per day, exceeding the body maintenance requirements of about 2 mg per day. The hereditary deficiency of the protein ceruloplasmin, known as Wilson's disease, is associated with a pathological increase in the copper content of almost all tissues, particularly the brain and liver. Albino mammals lack the normal form of the copper-containing enzyme tyrosinase, which participates in the synthesis of the pigment melanin. Copper can be toxic in large quantities, especially to lower organisms such as bacilli, fungi, and algae.

Applications of Copper and Its Compounds The electrical industry is a major consumer of copper. The metal is used for the windings of generators and for conveying electrical power. Its resistance to chemical attack and its high thermal conductivity make copper a useful metal for condensers in chemical plants and for car radiators. Copper tubing is widely employed in plumbing, and finely divided copper is used as an industrial catalyst in the oxidation of methanol to formaldehyde. Copper compounds, such as Fehling solution, are used in analytical tests for sugars. Copper(II) sulfate has many industrial applications, including the preparation of Bordeaux mixture (a fungicide) and the manufacture of other copper compounds. It is also used in electroplating solutions, in textile dyeing, and as a timber preservative. Copper wire for electric and electronic uses was the most important area of refined copper sales. Brass products, industrial machinery, automobiles and other forms of transport, and consumer products were all large copper consumers. In the fields where copper traditionally has been strong, only housing failed to revive. Solvent extraction, a new copper refining technique, is proving highly successful. In a process resembling that recently developed for treating gold ores, low-grade copper ores--once considered valueless--are treated with a weak acid solution, and after being combined with other chemicals, the leachate is subjected to electrolysis. Copper derived from leachates is called "electrowon," and costs for electrowon copper are far below those for copper derived from smelting.

 

THE ORE AT LEFT IS THE MINERAL REALGAR, COMPOSED OF THE ELEMENTS OF ARSENIC AND SULFUR. IT IS A MAJOR SOURCE OF ARSENIC

Arsenic is a metallic chemical element in Group VA of the periodic table. Its symbol is As, its atomic number 33, and its atomic weight 74.9216. The Earth's crust contains only about 5.5 parts arsenic per million. Arsenic occurs in numerous minerals, in particular realgar, orpiment, and arsenopyrite. Arsenic and some arsenic compounds have been known for a long time. Aristotle thought that arsenic was a kind of sulfur. In about 1250, Albertus Magnus became the first to describe a method of manufacturing arsenic. Since then the method has scarcely changed: the mineral arsenopyrite is heated and decomposes with the liberation of arsenic gas. The gas can be condensed on a cold surface. Arsenic exists in three allotropic modifications: the yellow (alpha); the black (beta); and the metallic, or gray (gamma). Normally arsenic is found in its metallic form, which is the most stable and at normal pressure does not melt but sublimes at about 615 degrees C. It forms alloys with other metals. The alpha and beta modifications have no metallic properties. Arsenic is fairly reactive. Above 400 degrees C it burns with a bluish flame, forming arsenic trioxide. This compound is known as white arsenic and is used as a rat poison. Arsenic was used in Aristotle's time to harden copper. Orpiment and realgar have long been used as depilatories in the leather industry. When orpiment is rubbed on silver, it gives the surface a golden color. Orpiment thus appears to have one of the properties attributed to the philosophers' stone, and it was therefore an important material for alchemists. The toxic quality of arsenic also has been known since ancient times. In the human body it accumulates in the nails and the hair, where it can be detected--even in the bodies of persons long dead--by the atomic absorption method. The acute symptoms are diarrhea and cramps. In cases of chronic poisoning, anemia and paralysis may appear. If there is prolonged contact with the skin, malignant skin tumors can develop. BAL (British Anti-Lewisite) was developed as an antidote against the arsenic-containing war gas Lewisite, but it also proved useful in treating common arsenic poisoning. In medicine, 4-aminobenzene arsenic and 4-hydroxybenzene arsenic compounds are used in certain infections. The best known is Salvarsan, an antisyphilis drug. Commercially, arsenic is added to lead to harden it and is used in the production of herbicides and pesticides.

THE PHOTO AT LEFT IS SILVER. SILVER IS USED IN PHOTOGRAPHY (PRIMARY USE), COINAGE, MIRRORS, ELECTRONIC CIRCUITS, SWITCHES, SOLDERS, BATTERIES, BEARINGS, AND SILVER IODIDE IS USED TO PRODUCE RAIN BY SEEDING THE CLOUDS. THE ELEMENTS OF SILVER, GOLD, AND COPPER ARE OFTEN FOUND IN NATURE TOGETHER OCCURING IN THE SAME DEPOSIT.

Silver is a heavy metallic element with a brilliant white luster. The chemical symbol for silver, Ag, derives from its Latin name, argentum, meaning "white and shining"--an apt description. The metal has been used as currency since ancient times, both in the mass (bullion) and in the form of coins. Silver shares Group IB of the periodic table with gold and copper. Silver, gold, platinum, and mercury are together categorized as noble metals because they do not oxidize readily when heated, nor do they dissolve in most of the inorganic acids. Because of silver's value it is also categorized as a precious metal, as are gold, platinum, iridium, and palladium. Besides being a monetary metal, silver has also long been used for making jewelry, solid silver or silver-plated objects such as eating utensils, and other tokens of wealth because of its beauty.

Only about 6% of the world production of silver goes to coinage, and only about 10% to jewelry, silver plate, and sterling ware. By contrast, more than 40% of all silver produced is used in photography because of light-sensitive properties of silver compounds. Thus photographic film is composed of an acetate upon which a thin gelatin layer, the emulsion, is spread. The emulsion contains a silver salt, a halide, as well as organic and inorganic sensitizing compounds. Silver iodide is used for fast films, silver bromide for slower films and fast-printing papers, and silver chloride for most printing papers. When light falls upon them the silver compounds are photooxidized. That is, electrons are released from the halide ions and combine with the silver ions, forming elemental silver atoms suspended in the gelatin emulsion. Placing the film in a developer continues the oxidation process in the salt crystals already affected by light but has no effect on unaffected crystals. Thereafter the film is in a sodium thiosulfate bath to halt the oxidation process, leaving a negative image from which a positive print can be made. Color films also employ dyes. The rest of the silver produced goes into various industrial and medical applications and into the silvering of mirrors. Silver's exceedingly high electrical conductance and resistance to oxidation make it valuable in critical electrical contacts, switches, printed circuits, solders, long-lasting batteries, and many forms of electrical and electronic equipment. It is used in bearing alloys for airplanes and diesel engines and in some automobiles as well, and, alloyed with copper, it is used for welding. In colloidal form it serves as a catalyst in the manufacture of certain alcohols. Alloyed with cesium, it is employed in photocells. Finally, in the form of silver iodide, it is used to seed clouds for WEATHER MODIFICATION purposes.

Occurrence of Silver is a relatively rare element, ranking about 65th in cosmic abundance and about 68th in abundance in the Earth's crust. It is found in small quantities in many locations on Earth, sometimes in relatively pure native form but more often in silver sulfide--Ag(2)S--ores (see ARGENTITE), in other ores such as cerargyrite, or horn silver (silver chloride, AgCl), and as a minor constituent in ores of copper, lead, and zinc. It is also found in minute quantities in seawater. Silver is sufficiently widespread as a native metal for the Egyptians to have used it without complex refining processes before 3500 BC. Historically, however, the most important mines have been those that were discovered by the Spaniards in Central and South America during the 16th century AD, as well as the U.S. COMSTOCK LODE discovered in Nevada in 1859 and the deposits in the Coeur d'Alene area of Idaho.

Next to gold, it is the most malleable and ductile metal known. It is harder than gold but softer than copper. This softness limits its use, even for coinage, unless it is alloyed with about 10% copper. When alloyed with 7.5% copper, it is known as sterling silver. As a pure element it can absorb oxygen in the amount of 20 times its own volume at its melting point of 960.8 deg C (1,761.4 deg F). The boiling point of the metal is 2,210 deg C (4,010 deg F). Silver's atomic number is 47, and its atomic weight is 107.868. Naturally occurring silver consists of two stable isotopes: silver-107 (51.82%) and silver-109 (48.18%). Crystals of silver have a face-centered cubic lattice structure

The CYANIDE PROCESS is generally the extractive method of choice. It can also be used to reclaim silver from zinc ores after the volatile zinc has been distilled from them. In this process a dilute sodium cyanide solution is used to dissolve the silver in the presence of air. This yields a silver cyanide complex that, after removal from the unreacted ore, is treated with zinc dust to recover elemental silver. The cyanide process, which appeared about 1859, was responsible for the recovery of the silver of the Comstock Lode. By 1910 the so-called froth process of concentrating silver ores became common, and today it accounts for about 75% of all silver recovered. The ore is ground to a powder, placed in large vats containing water suspensions of frothing agents, and thoroughly agitated by jets of air. Depending on the agent used, either the silver-bearing ore or the useless residues adhere to the bubbles of the froth and are skimmed off. The final refining is electrolytic. Silver is also recovered from used photographic film by burning the film and chemically treating the ashes as if they were copper ore. Unfortunately, less than 20% of all silver used for film is recovered. The remainder is irrecoverable by any practical method. Silver as a Monetary Metal By at least the 8th century BC it is likely that all countries of the ancient Middle East were using both silver and gold as currency. This practice is called bimetallism when a fixed ratio is set between the values of the two forms of coins. In the United States, bimetallism was officially adopted in 1792. The gold discoveries of the mid-19th century thereafter led to lower gold and higher silver prices, silver eventually being sold in the market for higher values than it held at the mint. As a result, a prohibition was placed on silver coinage in the United States in 1873. Later discoveries of silver in the western states changed the picture again. In the late 19th century the metal became involved in a contest between tight money adherents and the FREE SILVER advocates--farmers, debtors, and silver-mine owners who advocated a return to bimetallism, thereby increasing the supply of money in circulation. In 1900, Congress officially adopted a single gold-based monetary standard. In the 1960s the Treasury Department ceased issuing silver certificates, the paper money secured by silver. By 1970, when the silver content of the half dollar was reduced to nothing, the silver content of all regular coinage had been eliminated entirely.

 
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