Chromium (Cr, 24) and manganese (Mn, 25) are "transition metals" inhabiting the middle of the periodic table. They illustrate very well the properties of these elements, which are similar in some ways, but in others having fascinating idiosyncrasies. Nearly two-thirds of the elements of the periodic table are transition metals, in which the d and f atomic orbitals are being filled. These include the 14 "rare earths" from cerium (Ce, 58) to lutetium (Lu, 71) and the 14 "actinides" from thorium (Th, 90) to lawrencium (Lr, 103). Those at the far right: Cu, Ag, Au and Zn, Cd, Hg--are atypical, more "normal" elements in which the members of a column resemble one another. In them, the d shell is filled with its complement of 10 electrons, so d-electrons only play a role when they are promoted to higher orbitals, and do not dominate the chemistry. Chromium and manganese each have a half-filled d-shell, which is responsible for their unique properties. To the right are the triads (Fe, Co, Ni), (Ru, Rh, Pd), and (Os, Ir, Pt), all peculiar metals. Therefore, we are talking mainly about the 14 elements in the third through fifth periods to the left of these, omitting technetium (Tc, 43), which does not occur in nature. If it did, it would be like the rest. It was once hoped to be masurium, discovered along with rhenium in 1924, but masurium turned out to be an illusion.
Actually, Tc does occur in very small amounts as a fission fragment from the spontaneous fission of uranium, and some modern measurements with X-ray spectroscopy, the same method used by Tacke, have indeed detected it. However, Tacke did not isolate element 43 nor report any of its properties, only an amount far in excess of what could possibly be present. Whether credit for discovery should rest on such slim evidence is doubtful. Technetium was made by Perrier and Segrè in 1937 by the Mo98(n,γ)Mo99 reaction (or similar) followed by beta-decay to Tc99, the longest-lived isotope of technetium (2.1 x 105 y). It can now be produced in kilogram amounts by the nuclear industry.
Professor Fred Allison, a physics professor at the Alabama Polytechnic Institute (now Auburn University) announced the discovery of Alabamium (Ab, 85) and Virginium (Vi, 87), as well as the hydrogen isotope H3, in 1931. He used the Allison Effect, connected with the Faraday Effect, for isotopic and chemical analysis. This proved to be an example of pathological science, and Allison was debunked by Irving Langmuitr, after he had taken in many eminent chemists. It is now estimated that in the outer mile of the continental crust there is in all only 68.6 mg of element 85, and 24.5 g of element 87 (for comparison, there are some 4000 tons of Po210). Both elements have only short-lived radioactive isotopes, and Professor Allison was probably not looking in the right place anyway. Virginium had previously been claimed by Russian scientists, who proposed the names Russium and Moldavium. They are now called Astatine (At, 85) and Francium (Fr, 87), discovered in 1940 by Corson, McKenzie and Segrè, and by Mlle. Perey in 1939, respectively. Promethium (Pm, 61) was discovered in Italy and named Florentium in 1924, then by B. Smith Hopkins at the University of Illinois in 1926 and named Illinium, but was actually discovered at Oak Ridge some years later. It does not occur in nature, since its longest-lived isotope has a half-life of 25 years.
The chemical elements show very well that they have no intrinsic qualities: they are just nuclei of differing charges with enough electrons to make them electrically neutral, and that the electrostatic forces between them are responsible for all the wonderful variety of materials. The qualities are given entirely by the structure, none by "essence." For example, it is energetically favorable (lowest free energy) for the outer electrons to rub off when the atoms are brought together and form a metallic bond in which they are delocalized and can more or less roam freely among the atom cores. This is responsible for the metallic lustre and the simple crystal structure: body-centered cubic, face-centered cubic or hexagonal. To this degree, all the elements look alike. We will be very interested in structure, for the understanding it can give us of the observed properties.
Both chromium and manganese are essential in the iron and steel industry, and about 90% of each is used in this application. The small resources of chromium and manganese ores in the United States are exhausted, so dependence on foreign sources for new metal is total. A small secondary (recovered from scrap) chromium production now exists, that provides about 20% of the national needs. Both metals are on the list of strategic and critical materials, with emergency stores of 4 x 106 tons of Mn and 1 x 106 tons of Cr. as of 1989. World production of chromite is a little over 107 tons, and world reserves are estimated at 7.6 x 109 tons. United States consumption of chromium is 0.54 x 106 tons, of which secondary chromium supplies 0.11 x 106 tons. The strategic reserves amount to about a 2-year supply, which is considerably better than the 30-day supply of crude oil on reserve. At 1989 prices, chromium was $930 per ton, manganese $215 per ton.
Chromium and manganese form many ionic complexes, often colorful ones, responsible for the colors of gems and pigments. The study of their structure is called "ligand-field theory," an interesting branch of inorganic chemistry. A few of their compounds are often used in the laboratory, like manganese dioxide, potassium permanganate and potassium dichromate. We shall take a look at the chemistry of chromium and manganese to make these things clear.
Chromium was discovered and named by Nicolas-Louis Vauquelin, professor of chemistry at the École des Mines in Paris in 1797, whose work was confirmed by H. M. Klaproth soon after. The name is from Greek to xrwma, "the skin," by extension the "color of the skin" and finally to "color" in general. Xrwmata was used figuratively for "ornaments," perhaps brightly colored. Vauquelin was impressed by the vivid colors of chromium compounds, such a joy after the endless colorlessness of sodium and potassium compounds. Chromium was soon used in pigments. Lead chromate, PbCrO4, found as the mineral crocoite is known as chrome yellow. Basic lead chromate, PbCrO4·PbO, is chrome red. Chromic oxide, Cr2O3, is chrome green.
Manganese was first identified by the ingenious Swedish apothecary K. W. Scheele in 1774, but Gahn seems to have first isolated the metal later the same year. Bergmann also contributed to the knowledge of manganese. Chromium and manganese were merely chemical curiosities, without uses, except for the chromium pigments and manganese dioxide. As early as 1740, Pott had determined that pyrolusite contained no iron, and made some manganese compounds. The crystal structures of manganese metal are surprisingly complicated. There are two, α- and β-manganese. Look them up on the NRL Crystal Structures website; a link is given in the References.
The name "manganese" has an interesting history. In ancient times, two black minerals from Magnesia in Asia Minor were called Magnes, but were thought to differ in sex. The male Magnes attracted iron, and was, of course, magnetite, and gave us the term Magnet. The female Magnes did not attract iron, but was used to decolorize glass. The female Magnes was later called Magnesia, but we now know it as pyrolusite or manganese dioxide. In the 16th century, it was called Manganesum by glassmakers, while the alchemist Camillus Leonardus called it Alabandicus (from a location where it was found), and it was also known as brownstone (Braunstein). Mercati called it Manganesa, and finally the metal isolated from it became known as Manganese (German: Mangan). The name Magnesia was then free to be applied to white magnesium oxide, which provided the name Magnesium for the metal. Magnet, manganese, magnesia and magnesium all have the same etymological source.
The most remarkable physical properties of chromium are its hardness and its passivation by a thin transparent surface layer of oxide. Ordinary chromium metal has a hardness of Mohs 9, equivalent to corundum, inferior only to diamond. It may be the hardest metal, absolutely unscratchable by any other. It cannot be scratched by a file, which has a hardness of about 6.5. Very pure chromium, and the porous chromium of decorative chrome plating, may not be quite as hard, but they are still up there. This means also that chromium will take a superb polish, and the passivation means that this surface will not corrode. A few other metals, such as aluminium and nickel, can also be passivated, and non-metals like silicon as well, but they are not nearly as hard. The "stainless" in stainless steel results from this same property, when surface chromium atoms form a passivated surface that resists chemical attack.
Manganese, on the other hand, has a hardness of 5, about the same as a knife blade, so it is hard, but not exceptionally so. Most transition metals are characteristically this hard. Like iron, it rusts, and perhaps even more readily. Along with its hardness it is brittle, a property shared with chromium, which means that neither pure metal is suitable as a material of construction. Both metals are silvery on a fresh surface, which we noted above was a common characteristic of metals with free conduction electrons. The electrical resistivities of chromium and manganese are given in Lange as 2.6 and 5 μΩ-cm; another source gives 13.1 and 5, and the Sargent-Welch periodic table 12.9 and a rather unbelievable 144. For comparison, copper's is 1.68.
The melting points of chromium and manganese are 1857°C and 1244°C, and the boiling points are 2642°C and 2040°C. High melting and boiling points are typical of transition metals, tungsten having the highest of all, with tantalum and rhenium not far behind. The densities are 7.19 and 7.43 g/cc, about the same as those of iron and other familiar metals in the third period.
The electron configuration of chromium is Ar3d54s. There are six electrons outside an argon kernel, and the energies of the 3d and 4s orbitals are not very different. Chromium is found with oxidation numbers of 0, +2, +3 and +6. This certainly does not mean that bare ions of the corresponding charge exist, but that the charged chromium kernel produces an attractive field for negative ions, such as O--, or for the negative ends of polar neutral groups, such as H2O. Structures may be different in aqueous solution, indeed in acid or basic solutions, and in crystals. Bonds may be ionic or covalent, or of any intermediate character. Whatever gives the minimum free energy is what happens. Oxidation number zero is, of course, metallic chromium. Cr++ is called chromous, usually quickly oxidized to Cr+++, or chromic. In acid solution, these ions coordinate 6 water molecules around them in an octahedral arrangement, attracting the lone pairs. The chromous ion is blue, the chromic ion violet. In basic solution, four hydroxyl ions are bound tetrahedrally, to give Cr(OH)4-- [Cr(OH)2 + 2OH-], or Cr(OH)4- [CrO2-, chromite ion, which is green]. The +6 state coordinates four O-- ions to form CrO4-- [chromate] in basic solution, or Cr2O7-- [dichromate] in acidic solution. The dichromate ion is two tetrahedral chromate ions joined at one vertex. There is no change of oxidation number between chromate and dichromate, but the color changes from yellow to orange. The situation is a little messier than this in detail, since H2O and OH- ions can be mixed, but this gives the general appearance and behavior of the soluble compounds. Zinc metal can be used as a reducing agent, and hydrogen peroxide as either an oxidizing or a reducing agent, in working with chromium ions. Soluble chromium compounds are poisonous.
When dichromate and concentrated sulphuric acid are mixed, the reaction gives a sulphate and chromic acid, which is dehydrated to red chromium trioxide, a powerful oxidizing agent. This is the familiar cleaning solution for laboratory use. To make it, mix 800 cc of technical concentrated sulphuric acid with a solution of 92 g of technical Na2Cr2O7·2H2O in 458 cc of tap water (or corresponding proportions). Leave the precipitate alone; it is part of the solution. It should be used warm or hot, and only after preliminary cleaning with detergents has left only the slightest residue. It is expensive and hazardous, and not recommended for home use. When the solution turns greenish, it is exhausted and should be discarded, followed down the drain by ample water to dilute it, since it is strongly acid, but not hazardous in a dilute condition. It will help keep the drains clear.
Manganese has one more electron, and shows even more oxidation states: 0, +2, +3, +4, +6 and +7. The +2 manganous ion is rather stable and not easily oxidized in neutral or acid solutions. The manganous ion is colorless in aqueous solution, but some manganous salts are pink. In a basic solution, white Mn(OH)2 is formed, but it rapidly oxidizes to a +3 oxide. The +3 manganic ion quickly disproportionates, some reducing to +2 while an equal amount oxidizes to +4. The +3 state can be preserved only by complexing with CN- or C2O4-- or a similar group, or by forming an insoluble salt. The oxalate is a chelating ligand that binds at two points, a favored structure. Ligand is just a name for any group of atoms that is bound to the central ion. The principal compound of the +4 state is MnO2. It is an oxidizing agent when the manganese is reduced to Mn++++, and a reducing agent when the manganese is oxidized to +6. Heating the dioxide with a basic substance in air makes dark green manganate ion, MnO4--, with +6 manganese. When acid is added, the manganate disproportionates to MnO2 and +7 manganese in the deep violet permanganate ion, MnO4-. Thus, K2MnO4 is potassium manganate, while KMnO4 is potassium permanganate, the familiar soluble violet salt. Permanganate is the strongest common oxidizing agent, because it is readily reduced. The disappearance of the violet color gives the end point in titrations.
Manganese dioxide is used as a depolarizer in dry cells. By "polarization" is meant the appearance of a layer of nonconducting adsorbed molecular hydrogen. Manganese dioxide removes this by oxidizing the hydrogen to water, perhaps by the reaction MnO2 + H2 → MnO + H2O. It can also be used to oxidize the chlorine in HCl to Cl2 in the laboratory production of chlorine. It is also a catalyst in the thermal decomposition of potassium chlorate, 2KClO3 → 2KCl + 3O2, the usual laboratory source of oxygen. I have not heard of a theory of why this takes place, or if the manganese dioxide must be prepared a certain way so that it has a large surface area.
Trivalent transition metal ions can replace trivalent aluminum ions in corundum, Al2O3. Electron transitions in these ions, unlike in the colorless aluminum which does not have such electrons available, can absorb strongly in the visible spectrum and give bright colors. Cr+++ absorbs the blue and green, leaving only a deep-red color, making a ruby. Ti+++, on the other hand, absorbs the green and red, making the blue sapphire. In beryl, Be3Al2(SiO3)6, Cr+++ absorbs blue and red, creating the green emerald. The crystal environment of the ion is different in beryl than in corundum. Many other clear crystals are colored by transition-metal ions. The amounts required are so small that in many cases the responsible ions have not been identified.
Ruby absorbs light over a broad spectrum. In an ordinary crystal, the energy is mostly dissipated by phonon interactions, and mainly the result of the absorption is seen. When excited by ultraviolet, however, the red emission can often be seen, especially in artificial ruby intended for lasers. In a laser, the ends of the crystal may be carefully polished parallel to make a resonant cavity for red light (or external mirrors can be used). When the ruby is "pumped" with white light, such as that from a xenon flash lamp, it may emit stimulated coherent radiation in the range 693-705 nm, a very deep red. This radiation comes from the Cr+++ ions.
Mineralogy and Metallurgy of Chromium and Manganese
Chromium and manganese are both widely distributed in the earth's crust. The abundance of manganese is 0.096%, and that of chromium is 0.037%. Chromium has not been detected in sea water, but manganese is present to about 1-10 ppb. Their compounds are very insoluble, so this scarcity is not surprising. Chromium occurs almost entirely as ferrous chromite, Fe(CrO2)2, which is called chromite, a sample of which is shown in the photograph (see References for the source of this copyrighted image). This substance crystallizes early out of a basic magma, and so is found associtated with peridotites, serpentine, olivine and other ultrabasis rocks. It is heavy and can be concentrated by gravity methods. The largest producers of chromite are Kazakhstan and South Africa, responsible for about 75%. India, Zimbabwe, Turkey and New Caledonia (more on New Caledonia below) are other sources. It is a hard, black crystalline substance that leaves a brown streak on unglazed porcelain.
Some chromite is ground, mixed with a little lime and a binder, formed into bricks and baked to form a refractory furnace lining. Chromite does not react with either silica or magnesite refractories, and so is called a neutral refractory, of which it is practically the only example, and so is indispensable.
Chromite was first mined in the United States in 1828, in Pennsylvania and Maryland. In World War II, there was production from California and Oregon. Chromite is not currently mined in the United States, but about 20% of current consumption is met by secondary chromium, that is, chromium recovered from scrap stainless steel. 18-8 stainless steel can be directly charged into electric furnaces that make most stainless steel. The nickel in scrap steel is more valuable than the chromium.
To form chromates, the chromite is first roasted with sodium carbonate in the presence of air. The result is ferric oxide, carbon dioxide and sodium chromate. The chromium is oxidized from +3 to +6 in this reaction. When the chromate is dissolved in dilute sulphuric acid, sodium sulphate and sodium dichromate result. The sodium sulphate is less soluble than the dichromate, so it is crystallized out, and the dichromate recovered. When sodium dichromate is heated with sulphur, sodium sulphate and green chromic oxide results, which is insoluble and remains when the sulphate is washed out. Chromic oxide and aluminium powder are mixed and ignited (Goldschmidt or Thermite process) to produce molten chromium metal of 99.5% purity. If dry sodium chromate is treated with concentrated sulphuric acid, sodium sulphate and chromic acid, H2CrO4 are produced. The chromic acid is immediately dehydrated by the sulphuric acid, and red CrO3 precipitates. Chromium trioxide is used in chromium electroplating, where it is known as "chromic acid."
To make ferrochrome, the alloy used in making alloy steels, chromite can be smelted with carbon in a blast furnace. Ferrochrome should contain about 60% Cr and as little carbon as possible. It is possible to decarbonize ferrochrome in a reverberatory furnace under a basic slag of lime and feldspar. The lower the carbon, the higher the value of the ferrochrome. Although the U.S. once produced most of its ferrochrome, it is now mostly imported. The price of chromite is now about $75 per metric ton, ferrochrome $1000 per metric ton, and metallic chromium $7500 per metric ton, about $3.30 per pound. Nickel, by comparison, is 5 to 8 times as expensive.
Manganese, by contrast to chromium, occurs widely disseminated in igneous rock, and so in soils and sedimentary rocks. The only economic ores are those that have been concentrated by supergene enrichment, dissolved by descending waters and deposited in favorable locations. The most common mineral is pyrolusite, which is mainly MnO2. In its crystalline form, which is very rare, it is hard and black. A crystalline specimen is shown at the left. (See References for the source of this copyrighted image). Usually pyrolusite is found in soft masses of colloidal particles, hardness 1-2, and staining the hands like charcoal. An impure form containing mixed oxides is called wad. The name pyrolusite comes from the Greek for "fire" and "dissolve" since if added to molten glass, it oxidizes iron from ferrous to ferric, "washing out" the green color and making clear glass. It often contains iron impurities, and so may be slightly magnetic, which led to an early confusion with magnetite, and the "magnetic" name of manganese ("magnanese" was awkward). The shales of South Dakota contain some manganese, about 1%, and means of beneficiating this to 18% by leaching have been devised. The product can be used to extend more concentrated ore. An ore of about 40% metallic manganese is required for present processes.
Manganese is also found in several rare minerals, such as pink rhodochrosite, MnCO3, and rhodonite MnSiO3, and black manganite, MnO(OH), and alabandite, MnS. Rhodochrosite crystals resemble calcite, and those found in Lake County, Colorado, are a beautiful clear pink. They are the "state mineral" and are sometimes cut as gems. An attractive specimen of rhodochrosite is shown at the right (© Amethyst Galleries). These minerals can sometimes be ores of manganese where they occur as a by-product, as rhodochrosite was at Butte, Montana, but are seldom found in sufficient amounts to be worth mining.
In the semi-arid country of southeastern Utah, red sandstone cliffs are often seen to be covered by a black film. In some places, petroglyphs have been created by the early inhabitants (and some by recent more barbarous characters), scratched through the black surface. This film is desert varnish, and is made black by its content of manganese dioxide. Infrequent moisture is first absorbed by the rock, then is drawn to the surface as the moisture evaporates, leaving dissolved substances behind, among which is the manganese, as well as iron from the cement of the sandstone. Manganese and iron oxides form a tenacious layer on the surface of the rock, which is the desert varnish. As the rock weathers, new varnish appears to recruit the old. This shows again how common manganese is in the environment, though in very small concentration.
Manganese nodules, or ferromanganese concretions, in the deep ocean can contain up to 36% Mn, but 30% is usual in rich nodules. These nodules are found in both Atlantic and Pacific, but principally in the Pacific. One locality is the Manihiki Plateau east of Samoa, from 5°-15°N, 120°-160°W, at depths of 12,000-18,000 feet. The nodules are concretions of 1" to 9" diameter, lying in diatomaceous ooze. The concretions begin on a shark's tooth or whale's earbone, which are very insoluble, or other nucleus, and grow about 1 mm in a million years. They grow from their bottom surfaces, and are kept moving by benthic organisms. There can be 100,000 tons of nodules per square mile. They also contain copper, nickel, cobalt and tungsten. Mining, apparently mainly for copper and nickel, was to begin in 1977-1980, but the four American companies then involved have abandoned their plans. There are complex legal and environmental obstacles to be overcome in nodule mining in the deep sea. The nodules were discovered on the pioneering oceanographic cruise of HMS Challenger in 1873-1876.
Russia and South Africa produce about 85% of the world's pyrolusite. Large new sources have been found at Moanda, Gabon; in the Kalahari Desert of north Cape Province, South Africa; and at Groote Eylandt, Northern Territories, Australia. Brazil, New Caledonia and even Fiji also have manganese ores. In the United States, some pyrolusite was found in South Carolina, but the reserves there have now dwindled to nothing. World War II caused the small domestic reserves of richer manganese ores to be intensively exploited, so that they no longer are an economic source. The United States currently mines no manganese.
Pyrolusite cannot be beneficiated to any great degree because of its powdery nature, so is used as it comes. Pyrolusite can be reduced with carbon in a blast furnace. Since the ore also contains a large amount of iron, the result is a white cast iron called spiegeleisen, or "mirror-iron" that can apparently be polished to make a mirror. This alloy contains 15-20% manganese and 5% carbon, and is inexpensive because it uses available raw materials and does not involve a chemical separation. This was the alloy first used by Mushet in 1856 to make the Bessemer process successful. A higher-quality product is ferromanganese, containing 75 to 81% manganese and 5% carbon. The pyrolusite must be purer, and a more limestone is used as a flux. Nevertheless, the manganese loss in the slag and vapor is as much as 15%. Ferromanganese can be decarbonized by melting with manganese dioxide in an electric furnace; the MnO2 burns off the carbon as CO2, and the carbon can be reduced to less than 1%. Carbon-free manganese metal can be produced from pure MnO2 by the Goldschmidt process for use in softer high-manganese steel.
Chromium and manganese are both obtained from single ores, chromite and pyrolusite, that are widely distributed enough that monopolies cannot easily be formed. The United States, Europe and Japan have now exhausted the small resources they once had of both chromite and pyrolusite. The world supply is not in immediate danger, but the rate of exploitation of inexpensive chromium and manganese is high, and will not last a millennium, a very short time in geological terms. The U.S. strategic reserve of chromium has been sold off after the dissolution of the Soviet Union in 1991.
Manganite, as we have mentioned, is the mineral containing MnO(OH). The same name has been given by solid-state physicists to a series of crystalline compounds like La1-xCaxMnO3, where 0 ≤ x ≤ 1. The lanthanum can be replaced by any trivalent rare earth, and the calcium by any alkali metal. For x = 0, we have LaMnO3, in which the manganese is +3, and for x = 1, we have CaMnO3, in which the manganese is +4. The electronic configuration of +4 Mn is Ar3d5, with exactly a half-filled d shell, while with +3 Mn there is an extra sixth electron. By controlling the relative amounts of La and Ca, we can control the density of the sixth electron, and so the electronic properties of the material. The manganese will be a mixture of +3 and +4 ions, which can arrange themselves randomly or in an ordered structure.
The manganites have a perovskite structure, of which a unit cell is shown at the right. Perovskite is the rare mineral composed of CaTiO3. It is hard (5.5), heavy (4.0 g/cc), varying from black with a metallic lustre to yellow. It was named after the Russian mineralogist L. A. Perovski (1792-1856) when first described in 1839. It has been found at Magnet Cove, Arkansas, among other places. The violet circle represents the Ti in perovskite, or the Mn in manganites. It is surrounded octahedrally by six oxygen ions, represented by blue circles. The charge is neutralized by eight cations at the corners of a cube. Since the unit cell has a half-interest in each oxygen represented, and an eighth-interest in each cation, while the manganese is fully owned, the formula is CaMnO3, if the cations are Ca++ and the Mn is +4.
When crystals of perovskite were measured optically, the symmetry was found to be cubic, or isometric. However, X-ray measurements of the crystal structure showed that the axes were slighly different in length from each other, so that the symmetry was actually orthorhombic. This small difference, lowering the symmetry from cubic to rhombic, has an important theoretical explanation. It is an example of the Jahn-Teller effect, first observed in symmetric molecules. Whenever the ground state of a symmetric molecule, or a symmetric crystal, is degenerate, a lower energy can be obtained through a slight distortion that decreases the symmetry. The decrease in energy is EJT, the Jahn-Teller stabilization energy. The sixth electron in a manganite, if localized on a particular ion, can stabilize the crystal by the Jahn-Teller effect. This is often called an electron-phonon interaction, though the phonons appear to be zero-frequency.
Another thing that the sixth electron could do is to become delocalized and wander about the crystal freely. This also stabilizes the crystal, since the kinetic energy of an electron free to wander is lower than if it is localized in a small volume. This can be understood by the uncertainty principle, ΔpΔx ≤ h/4π. If an electron is confined, then Δx is small, so Δp must be large, and vice-versa. The 1s electrons close to the nucleus of, say, a manganese ion with nuclear charge +25, are confined to a very small volume, but are also necessarily moving at a very high speed. This effect is partly responsible for the popularity of metallic lattices.
A third factor in the crystal structure of manganites is electronic magnetism. The three electrons in +4 Mn occupy the three degenerate orbits belonging to a triply degenerate T representation of the cubic group made from the d orbitals. The remaining two orbitals belong to a doubly-degenerate E representation and are at a higher energy (higher by the ligand-field splitting). The spins are parallel, since this allows the electrons to be as far apart as possible. Purely magnetic interactions tend to align spins parallel, but this is not a large effect, and most of the stabilization energy is an "exchange" effect arising from electrostatic repulsion. The three electrons form a core moment on each Mn ion.
All of these factors compete with each other, and with the disordering effect of thermal agitation, to determine the electronic state of a manganite, and the battle is indecisive, since all are of comparable strength. The effect of thermal agitation appears in the term -TS in the free energy. At higher temperatures T, states of greater entropy are favored over more ordered ones. Three principal kinds of phases can be recognized, and a schematic phase diagram is shown at the left. The abscissa is the relative strength of the Jahn-Teller stabilization with respect to delocalization. If the Jahn-Teller stabilization is small, then the electrons form a degenerate Fermi gas the usual way, and the crystal is a conductor. The exchange interactions between the core moments, mediated by the free electrons, align the core moments ferromagnetically, so the crystal is ferromagnetic. This phase is denoted FMM. If the Jahn-Teller stabilization predominates, then the sixth electron is localized, and the crystal becomes an insulator. The exchange interaction is such that a crystal with an intermediate value of x, say x = 0.5, becomes antiferromagnetic, with the core spins alternating in direction. This phase is called COI, for "charge-ordered insulator," and the electrons may well be described as an "electron solid."
As the temperature is raised, a disordered system where some electrons are localized and others are not, and the core spins are not locked parallel or antiparallel, is formed. This structure is paramagnetic, and is represented on the phase diagram by PM. The phase transitions, since they represent abrupt changes in entropy, are the usual first-order phase changes with latent heats. Since the system is quite complex, there may also be additional phases of the same general character, showing second-order phase transitions. In the PM structure, resisitivity is high, and the electron state may be described as a "liquid."
In manganites, the indecisive battle of the states may result in the simultaneous presence of different phases in the same sample. This can occur in polycrystalline samples, or where there is incomplete thermal equilibrium. A sample of Bi0.24Ca0.76MnO3 investigated by scanning tunneling microscopy (STM) showed a phase boundary between FMM and COI, or at least between surface phases of these types, the first disordered and conducting, the second ordered and insulating. The V-I characteristics of the individual regions were also measured, and had the expected forms. In the ordered state, the +4 and +3 Mn alternated in a unit cell twice as large as the basic one. Another imaging of a 5μm square region by magnetic force microscopy (MFM) clearly showed patches of FMM and COI. Such "mesoscopic texture" is exiting to those who study phase changes.
Another curious effect arises in the magnetoresistance of the crystals. Transverse magnetoresistance is generally seen as an increase in resistivity in the presence of a perpendicular magnetic field. For a single type of carrier and constant relaxation time, the Hall field can cancel any effect of the magnetic field on resistance with a drift velocity, but this is not possible in more complex cases, and the resistance increases with the magnetic field. The effect is small, but can be used to study the Fermi surface. In the case of manganites, a magnetic effect called Colossal Magnetoresistance, or CMR, appears. It is called "colossal" because in some cases the difference can be a factor of 107, since one phase is insulating. In the transition from FMM to PM, the resistivity generally spikes as the electrons become localized, decreasing at higher temperatures as the "electron liquid" beomes less viscous. This behavior is shown in the diagram by the curve marked "0 T." A very strong magnetic field will keep the core spins aligned, which facilitates the movement of the electrons, and so the resistivity does not rise as much, as shown by the curved marked "4 T," for a field of 4 tesla. Producing a field this strong takes special measures; it cannot be done with mere iron.
The Bessemer steel-making process burns the carbon out of molten pig iron by blowing air through it. In a few minutes and with a shower of sparks, tons of low-carbon steel are produced. The alternative was hours of tedious "puddling" to make wrought iron. Bessemer steel brought the age of low-cost steel to engineering in 1855. It was difficult to judge the end-point accurately, so excess oxgen was dissolved in the steel, which had a very deleterious effect. The Bessemer steel was hard and unworkable, causing Henry Bessemer to despair. Robert Mushet solved the problem in 1856 by throwing a little spiegeleisen in the blown iron. The manganese removed the oxygen by forming harmless MnO2, and the carbon in the spiegeleisen brought it up to the desired concentration of something less than 1%. Manganese also reduced the deleterious effects of sulphur and phosphorus by preventing them from concentrating at the grain boundaries. Sulphur makes steel crumble when it is rolled red-hot, whild phosphorus makes it brittle when cold. The addition of less than 2% manganese made Bessemer steel widely acceptable. No matter how steel is now made, a little manganese is an essential addition for deoxidation and the other beneficial effects. About 16 pounds of manganese metal are required for each 2000 pounds of steel. In the 1980's, new methods of making pig iron reduced the manganese requirement by about 20%.
Steel is not considered an "alloy" steel unless it contains 2% or more of manganese, more than 0.1% molybdenum or vanadium, 0.3% tungsten or cobalt, or 0.5% chromium or nickel. This statement gives an idea of the metals that are usually added to iron to give desirable properties, such as strength, toughness, ease of heat treatment, corrosion resistance, machinability and so forth. They are all transition metals of the kind we are discussing, or else members of the Fe-Co-Ni triad. Carbon, up to a few percent at most, is essential for giving steel its desirable properties, through the formation of iron carbide, Fe3C, cementite, hard and white, which affects the microstructure of the metal. Low-carbon steel, with less than 0.83% carbon, is a mixture of ferrite or α-iron, iron with a little dissolved carbon, and pearlite, the eutectoid mixture of ferrite and cementite. Ferrite is magnetic and hard, with a body-centered cubic crystal structure. When steel is heated to red heat, about 910°C for pure iron, the ferrite changes to soft, face-centered cubic austenite or γ-iron, which can easily be worked, and is non-magnetic. Iron ceases to be magnetic above 769°C, the Curie Point. The heat treatment that is possible because of these phase changes is one of the principal advantages of steel.
The addition of chromium to steel hardens it and impedes the transition to austenite, raising the temperature of the phase change. Chromium and ferrite are both body-centered cubic, while austenite is face-centered. Nickel (and manganese) encourage austenite, and lower the temperature of the phase change. In fact, enough nickel can be added to make austenite stable at room temperature, and the steel remains nonmagnetic. Nickel and chromium largely remain in the ferrite, but chromium forms a little carbide, which explains its hardening effect. The effect on austenite can be minimized by adding both chromium and nickel together, since this bad effect tends to cancel, while the good effects of the additions tend to add. Chromium is added in the form of ferrochromium and nickel as ferronickel.
The first alloy steel was Sir Robert Hadfield's high-manganese steel, created in 1882. This austenitic steel contains 12.5% manganese and 1.2% carbon, and is used quenched from 1050°C so the carbides remain in solution. It is hard and has a tensile strength of 123,000 psi. It work-hardens to resist abrasion to a remarkable degree, so that it is used in crushers and dredgers, and in railway track fittings that must resist great wear. Alloys with less magnesium, around 1.5%, and lower carbon, are inexpensive substitutes for nickel-chromium steels.
More than about 10% chromium renders iron or steel corrosion-proof, from the passivation of the chromium at the surface. A "stainless iron" with 13% chromium and less than 0.05% carbon is used for sinks and food containers. A higher-carbon alloy of 0.3% carbon is a stainless steel, and is used in stainless-steel knives. This alloy is martensitic with small particles of carbide. Most stainless alloys with chromium only are ferritic in structure and magnetic. The most commonly used stainless steel is Type 302 or "18-8 stainless steel." Its analysis is 17%-19% Cr, 8%-10% Ni, with C > 0.08%, Mn ≤2% and Si ≤ 1%. It is austenitic and nonmagnetic. It is subject to work-hardening, so is not suitable for deep-drawing or machining, but is easily worked cold and welded. Extra-low carbon stainless steels (Types 304, 304L) avoid precipitation of carbides near welds. The strongly-worked metal may become magnetic. An alloy with 12.5% each Cr and Ni is suitable for deep-drawing. An 18-8 steel with 1.5% Mn and a 0.25% S (Type 303) is free-cutting. Type 302 has excellent oxidation resistance up to 870°C, and resists corrosion by all common chemicals except hydrochloric acid and sauerkraut brine. In particular, it is not attacked by nitric or sulphuric acids. It gives a deep green solution when dissolved in hydrochloric acid. Type 301 is a 17-7 stainless, cheaper because less nickel and chromium are used, but is not as corrosion-resistant as Type 302.
Small amounts of manganese added to aluminium harden it without affecting the corrosion-resisting properties. Duralumin contains 4% Cu, and 0.5-1.0% each of manganese and magnesium. Another alloy has 1.0-1.5% manganese and 0.2% copper. Brass with up to 3.5% manganese becomes "manganese bronze" with superior strength characteristics, equivalent to mild steel, while the brass can still be cast the usual way. Manganese is very commonly used to harden magnesium, in nearly all of the Dowmetal alloys. Dowmetal F is 4% Al, 0.3% Mn and 95.7% Mg. Nichrome IV or Chromel A is 80% Ni, 20% Cr (resistivity 103 μΩ-cm). There are many alloys called "nichrome" of varying constitutions. One is Ni 60%, Fe 24%, Cr 16%, C 0.1%. Resistance alloys may be designed for small temperature coefficient of resistance, for stability at high temperatures, or for maximum resistivity.
The importance of hard steels for making cutting tools for lathes and milling machines, for punches, shears and dies cannot be overestimated. The steel must not temper (soften) when hot, must resist oxidation, and should be easy to manufacture. Fast cutting means hot tools, and was the origin of the term "high speed steel." Robert Mushet was the first to make a tool steel when he made a manganese steel in 1861 with the accidental addition of 6% tungsten. The manganese was replaced by chromium around 1881, and more tungsten was added. In 1900, the first modern high-speed steel with 18% W and 4% Cr was exhibited cutting at red heat. Vanadium was introduced in 1906, and then molybdenum as a substitute for tungsten. W, Mo, Nb, and V all form very hard carbides, which are embedded in a tough matrix. V4C3 is a particularly abrasion-resistant carbide. High speed steel is 75% Fe, 18% W, 6% Cr, 0.3% V, 0.7% C, and can be forged and heat treated. The matrix is Fe-Cr. Stellite 3 is 55% Co, 20%-23% Cr, 15-20 W, 3%-5% Fe, 1.5%-4% C. Stellite must be cast, and ground to shape. Molybdenum steel has been specially popular in the United States, because of a lack of domestic sources of the other alloying metals. The famous mine at Climax, Colorado has now been closed, however. The Vickers Pyramid Hardness Number (VPN) of high-speed steels runs from 800-1000.
Nouvelle Calédonie is a French overseas territory in the southwestern Pacific, a slender island 250 miles long and 30 miles wide oriented NW-SE, about 1000 miles east of Australia and the same distance northwest of New Zealand, at latitude 21°S and longitude 164°E, just north of the Tropic of Capricorn. The Loyalty Islands are scattered along the east coast, with Fiji another thousand miles to the northeast, on the way to Samoa. New Hebrides, now Vanuatu, lies to the north. The main island, La Grande Terre, is one long mountain range, La Chaîne Central, from Mt. Panié, 5413', in the north to Mt. Humboldt, 5360', in the south. There are coastal plains in the west, but in the east the mountains come down to the sea. The three main Loyalty Islands are former coral atolls that were elevated in the Tertiary and are now flat round plains ringed by cliffs. One island, Ouvéa, the northernmost, largely retains the atoll configuration. Lifou and Maré are the other two. A number of smaller islands are also in the territory. Of these, Nou and Pines were penal colonies from 1864 to 1897. The mixed population of La Grande Terre is less than 200,000, of which about 50,000 inhabit the capital, Nouméa, at its southern tip. An aerial view of Nouméa is shown in the heading (source: www.sponline.com; see References).
The islands were inhabited by Melanesians at least as early as 2500 BCE. Polynesians arrived in the 12th to 17th centuries, apparently from the northeast, and settled the Loyalties. The island was discovered and named by Capt. James Cook in 1774. Napoléon III annexed the territory, a land of cannibals, in 1853. Nickel was discovered in 1863. Prisoners from the Commune de Paris were sent there in 1872. Mining began in the late 19th century, and southeast Asians came to work the mines, mainly as indentured servants. It was on the sidelines in World War II, but a large U.S. base was established in 1942. The population is 43% Melanesian, called Kanaks, and 37% of European origin. The remainder are descendants of the indentured laborers. Indentured servitude was abolished in 1951, and many indentured miners returned to their native lands. Nouvelle Calédonie became an overseas territory in 1946. The various ethnic groups have not integrated, and struggle for supremacy. The Kanaks want independence, the Europeans to remain associated with France. A referendum in 1987 (boycotted by Kanaks) decided for remaining in the French commonwealth, although the surrounding island groups, such as Vanuatu (New Hebrides) and Tonga (Friendly Islands), are now largely independent nations. There is still agitation for independence, of course, and rumblings continue. The direct dialing code is 687, and its time zone is +11, one hour later than eastern Australia. Most of the rugged 6530 square miles are unsuitable for agriculture, but some of the typical agricultural crops of the region--coconuts and yams--are produced, chiefly along the west coast.
The island lies southwest of the New Hebrides trench, in a region where the Indian plate is subducting beneath the Pacific plate, scraping off the mountains of New Caledonia and creating many volcanic islands and atolls. The igneous rocks have provided a wealth of minerals: Ni, Cr, Fe, Co, Mn, Ag, Au, Pb and Cu are all mined. The island is the principal source of nickel, which is arsenic-free, after Canada. The lateritic nickel mines were opened in 1875, and there was a Nickel Boom, 1968-71, after Canadian labor problems caused a nickel shortage. Chromium and manganese are also produced here, though not often shown as a source in references. I do not know the status of the mines and the reserves, and the website scarcely mentions mining. Petroleum, coal and coke are imported to fuel the mineral industries. I was only dimly aware of New Caledonia before writing this article, and had no idea that it was a mineral powerhouse.
An excellent source for crystal structures is NRL Crystal Structures. Look under "Prototype Index" for the formula of the crystal. Download the pictures if you wish; Microsoft Photo Editor can read and display them. In particular, look for the α and β manganese structures, which are very complex. Once you discover this site, you will probably want to browse it.
L. E. Orgel, An Introduction to Transition-Metal Chemistry: Ligand-Field Theory 2nd ed. (London: Methuen, 1966). This is not the clearest possible explanation of transition-metal complexes, because it is rather non-mathematical, though is very well done and at least shows how complex the field can be.
N. Mathur and P. Littlewood, Mesoscopic Texture in Manganites, Physics Today, January 2003, pp. 25-30.
R. A. Higgins, Engineering Metallurgy, 3rd ed. (London: The English Universities Press, 1971). An excellent introduction to physical metallurgy, concentrating on iron and steel.
M. J. Sienko and R. A. Plane, Chemical Principles and Properties 2nd ed. (New York: McGraw-Hill, 1974). pp. 504-511.
J. L. Bray, Non-Ferrous Production Metallurgy, 2nd ed. (New York: John Wiley & Sons, 1947). Chapters 10 and 16.
Pictures of chromite, pyrolusite and rhodochrosite kindly furnished by Amethyst Galleries, Inc.. This is an excellent website and specimens can be purchased online. This is probably the best mineral website, and the company should be supported for making it available.
Current information on the minerals industries, including prices, production and consumption, is available from USGS.
C. P. Idyll, Abyss (New York: Crowell, 1976), pp. 343-347 (manganese nodules).
E. Seibold and W. H. Berger, The Sea Floor, 3rd ed. (Berlin: Springer, 1996). pp. 289-294. A more technical account of manganese nodules.
The very interesting New Caledonia website is at New Caledonia, from which the image was sampled to make the location seem more real. There is, unfortunately, nothing on mining on the site. It would certainly be an exotic place for a winter vacation. Today (27 January 2003) it was partly cloudy and 75°F at 1.30 pm.
For the Allison Effect, see Allison.
Composed by J. B. Calvert
Created 24 January 2003
Last revised 29 May 2004