Unit 7 - The Geochemically Scarce Metals Flashcards
Which of the geochemically scarce metals form minerals that can be found in common rocks?
Only copper, zinc, and chromium—the most common of the geochemically scarce metals—are able to form minerals in common rocks.
Describe the process of atomic substitution, and explain how a metal forms a mineral.
Atomic substitution, or solid solution, occurs when atoms of geochemically scarce metals take the place normally occupied by other atoms in a mineral. If the amount of those substituted atoms reaches saturation for that solid solution (about 0.1 per cent), a new mineral must form.
What factors may change the minimum concentration above background of geochemically scarce minerals to make them economical to mine?
The minimum concentration factor can change with demand and, by extension, price.
What are the modern industrial uses of chromium?
Chromium has a number of important uses. Chromium plating on steel and chromium alloys, such as stainless steel, keep the metal corrosion-resistant. Chromium-steel alloys are used in aircraft engines, military vehicles, weapons, and chemical industry, so the metal is considered strategic to national defence and industry. Added to steel for machine tools, it increases hardness and resistance to wear. Chromium is also used for making pigments and for tanning animal hides.
Differentiate between podiform and stratiform chromium deposits. Which type of deposit contains the world’s largest chromium resources? Where is the largest deposit located?
Chromite, the major ore mineral of chromium, is found in ultramafic rock. Podiform deposits (which have an elongated, lenticular shape) are found in highly faulted and deformed portions of tectonically active zones. The chromite appears as rounded granules enclosed in dunites and serpentinites, which are thought to be displaced fragments of the upper crust mantle. Stratiform deposits are layers of pure chromite which occur in large, mafic intrusions. Current research indicates that the chromite “grew” at the bottom of a cooling magma chamber during fractional crystallization when the magma periodically became contaminated with the melting country rock. Stratiform deposits contain the most abundant chromium resources, and the largest such deposit is in the Bushveld Igneous complex of South Africa.
Briefly discuss the uncertainty about present and future world chromium supplies.
South Africa accounts for seventy per cent of the world’s chromium supplies. While the demand is high, the supply could be affected by civil, political unrest.
What valuable properties does vanadium give to steel? What are the primary industries in which vanadium is used?
Vanadium makes steel hard enough to use in armour plating. The American automobile industry relies on vanadium-steel alloy because it increases steel’s strength, high temperature abrasion resistance, ductility, and ease of welding. Because the weight of an automobile body need not be as great with vanadium steel as with carbon steel, using vanadium-steel alloy increases the efficiency of vehicles. Vanadium steel is also used widely in gas and oil transmission pipelines.
Although vanadium is one of the most common geochemically scarce elements, vanadium ore deposits are rare. Why?
Vanadium-rich magnetites are the most important ore deposits. Vanadium, however, is easily able to substitute for ferric iron, and thus enters into solid solution with the abundant iron minerals. This substitution is so common that localized deposits with concentrations of more than about two per cent vanadium are rare.
Explain why vanadium deposits are found in both igneous and sedimentary geological environments.
Occurring in igneous rocks, vanadium ore is similar to chromium ore. It appears to have formed in cooling magma chambers by fractional crystallization. The difference between chromium ore and vanadium ore is that vanadium ore is found near the top of the magma chamber.
In the sedimentary situation, vanadium weathers out of the igneous rock and is oxidized to a more soluble state. Transported long distances in solution by surface or groundwater, it eventually precipitates by evaporation or in contact with organic matter, which reduces it to a less soluble state. This process is similar to those undergone by uranium and copper; in fact, vanadium is often found in concentration with uranium. Vanadium is also found associated with coal, crude oil, and the bitumen of tar sands.
How could Alberta become an important world producer of vanadium?
If the large scale mining of the Athabasca Oil Sands (in Alberta) continues, it has been estimated that over two million tonnes of vanadium could be recovered.
Describe the process of nickel plating. List the characteristics of nickel that make it valuable as an alloying agent for iron.
In the electroplating of nickel, a metal plate of nickel is attached to the anode of a battery, submerged in a solution, and an electric current is passed through the battery. The nickel from the nickel plate dissolves and migrates to the object connected to the cathode, where it is deposited as a thin layer. Nickel resists corrosion, can be polished to a high lustre, and adds strength.
Describe the formation of the nickel ore, pentlandite, which involves liquid immiscibility in the process of magmatic segregation.
Crystallization of a mineral compound from solution is a function of both temperature and concentration. When a mineral compound reaches its saturation point in a cooling magma, it begins to crystallize. If the temperature of the melt is higher than the compound’s melting point, however, the compound must remain a concentrated liquid droplet within the melt. The elements will not combine within the compound droplet: they are immiscible (cannot be mixed). This often happens within the iron sulphide mineral, pyrrhotite. The droplets gather atoms of other elements (including nickel) from the melt, and form an iron-nickel-copper-sulphide liquid. Being denser than the silicate magma, the droplets settle to the bottom and form a sulphide-rich zone at the base of the chamber. When this zone eventually crystallizes, pyrrhotite forms along with grains of nickel ore, called pentlandite. Ores with a nickel content of three or four per cent can be formed like this.
Describe the process responsible for forming garnierite in laterite ores of nickel.
When mafic and ultramafic igneous rocks undergo chemical weathering in a hot, humid climate, the silicate minerals decompose to form hydrous compounds and the iron oxidizes to the ferric state. The nickel present is released and either forms nickel silicate minerals or becomes part of the structure of other weathering minerals. The nickel silicate minerals are known as garnierite; they often have a nickel content of four to five per cent. Residual ores formed from weathering in this manner are called laterite ores.
Which three countries have the largest reserves of sulphide and laterite nickel ores?
Canada, Russia, and Australia (in that order) account for the largest reserves of sulphide ores of nickel. The largest reserves of laterite nickel ores are found in New Caledonia, the Philippines, and Cuba (in that order).
Describe the important properties and uses of molybdenum.
Most of the molybdenum mined today is used as an alloy with steel and cast iron. Used as a superalloy, molybdenum gives hardness, toughness, and corrosion and abrasion resistance, and adds strength at high temperatures. It is used throughout industry, especially in the production of cutting tools, in transportation, and in oil and gas production equipment. Nonmetallic uses include lubricants, catalysts, and pigments.
Describe the geological occurrence of nearly all known molybdenum ores.
Molybdenite, the chief metallic mineral of molybdenum, occurs in porphyry-type deposits (porphyry: very hard red, green, or purple igneous rock). Such deposits are associated with subduction zones at the edges of continental plate boundaries. In fact, a large percentage of molybdenum production is associated with porphyry copper mining.
Which countries have the largest reserves of molybdenum?
The USA and Chile contain the largest reserves of molybdenum, although significant reserves are also found in Canada, Russia, China, Mexico, and Peru.
What are the important qualities of cobalt alloys? Why is cobalt considered a strategic metal?
Cobalt alloys retain their strength at high temperatures and are resistant to corrosion by hot gases. Consequently, they are important in the production of jet engines, rocket nozzles, and gas turbines, and are thus considered to be strategic from a military standpoint.
With which three minerals is cobalt commonly associated?
Cobalt is often associated with copper and silver in stratiform copper sulphide ores. It also concentrates wherever nickel is concentrated.
Which countries contain the largest reserves of cobalt and dominate production of the metal?
The African countries of Congo and Zambia are dominant in cobalt reserves and production.
What are the important qualities of tungsten, and what are its uses?
Tungsten is the metal with the highest melting point and the highest tensile strength. Tungsten steels are used where toughness, durability, and impact resistance is important. The hardness of tungsten carbide is close to that of diamond, and is used in tools, drill bits, cutting edges, and armour-piercing projectiles. Because of its high melting point, tungsten is important in producing filaments for electric light bulbs, heating elements, and distributor points in automobiles.
