Aluminium is one of the most common natural chemical elements. It is the third most abundant element in the Earth’s crust (about 8%) after oxygen and silicon. As D. I. Mendeleev wrote in his classical textbook ‘Basics of Chemistry,’ «aluminium is the most common element in nature; it is enough to indicate that it is contained in clay, in order to make it clear that aluminium is spread all over the Earth’s crust. That is why alum metal, is also called aluminIum, since it is found in clay.» Aluminium is a very chemical activity substance and therefore can be found only in a bound state, in the form of various minerals and rocks. About 250 various minerals contain aluminium; over 40% of them are represented by aluminium silicates. Aluminium silicates together with silicates form the Earth’s outer crust, underlying the continents. It’s not for no reason that scientists call it sial: by the symbols of silicon (Si) and aluminium (Al). As a result of destruction of initial aluminium silicates (mainly feldspars), multiple secondary rocks were formed, characterized by a higher aluminium content (bauxites, alunites, kaolins etc.), which contain aluminium in the form of hydrates (diaspore, boehmite, hydro-argillite) or hydrosilicates (such as kaolinite, various micas). In spite of the abundance of aluminium in the Earth’s crust, not every aluminium-containing rock can serve as a raw material for extracting alumina — the intermediate product, from which aluminium is extracted by means of reduction.
Aluminium oxide, or corundum is an igneous product which takes the first place in the table of essential minerals. This mineral has an elementary composition and is very rich in aluminium. Its hardness rate according to the Moss scale is 9, and it has a specific weight of 3.8 g/cm2. Due to the high stability of the crystal structure, corundum does not react with acids and alkali, which is why this mineral is not used as a raw material for aluminium production. But its crystals are known by literally everyone, which is not surprising, because these are ruby and sapphire used in jewellery. Rubies and sapphires adorn Monomakh’s Hat, and the orb and scepter of the Tsar Alexey Mikhailovich. Large sapphires also decorate the crowns of monarchs of the British Empire. Synthetic corundum produced by means of electrothermics is used in the abrasive industry. Cultured rubies also find a wide application in technology. For example, mechanical clocks are still valued by the amount of rubies used in them, which is often indicated on the back cover or on the dial. Ruby crystals serve as the major element of ruby lasers.
Hydrated forms of aluminium (diaspore, boehmite, hydro-argillite or gibbsite) are the rock-forming part of bauxites — the main raw material for aluminium production. As impurities, bauxites usually contain silicon oxides (quartz, opal, kaolinite), titanium oxides (rutile), iron oxides (in the form of carbonates, silicates, as well as hydrous and anhydrous oxides) and a few other compounds. By their appearance, bauxites are a rock similar to clay, having sometimes a stony and sometimes porous composition. The colour of bauxites is quite varied, but most often is of red tones. The specific weight varies from 1.2 g/cm2 to 3.5 g/cm2. The hardness rate of bauxites also varies in the same wide range (from 2 to 7 according to the Moss scale). The range of aluminium oxide content in industrially-used bauxites in the world is quite wide and can be from 35% to 60%.
The world production of alumina is mainly based on bauxites of tropical origin with a low silicon content, which are mined in Africa, South America, South-Eastern Asia, and Australia.
The first bauxite deposits in Russia were discovered in 1930–1931 in the Urals area. Low-silicon bauxites of the Northern Urals (SUBR) are the main raw material base for alumina production in Russia at present. In recent years, the Middle-Timan group of deposits in the Republic of Komi has been added to it. High-silicon bauxites of the North-Onega deposit in the Archangelsk region are not currently used for alumina production, though the corresponding technology for their processing has been created by the scientists of the Russian Aluminium and Magnesium Institute (RUSAL VAMI).
Besides the aluminium industry, bauxites also find application in production of refractory materials, electrocorundum, and cement.
Aluminium silicates
Aluminium silicates, the minerals formed in volcanic processes in the
high-temperature conditions characteristic for them, are some of the most widely found in the Earth’s crust. Except for
nepheline-containing rocks, aluminium silicates are currently of no practical interest for the aluminium industry, but they find wide application in other industries and fields.
Nepheline is the major rock-forming mineral of such alkaline formations as nepheline syenites, urtites etc. The hardness rate of nepheline according to the Moss scale is 5-6, and it has a specific weight of 2.6 g/cm2. In rock formations it can be found in the form of granular or solid aggregates. By appearance it is a colourless mineral, but it is often grayish-white or grey with a yellowish, brownish, reddish or greenish tone. The chemical composition of nepheline, due to the peculiarities of its formation, is versatile and depends on the geological situation, origin and composition of the rocks. When submerged into mineral acids, it loses transparency, becomes jelly-like and resembles a cloud, which is demonstrated by its name: ‘nephele’ means ‘cloud’ in Greek. In natural conditions, if affected by hot water solutions containing CO3, SO4 ions etc., nepheline is easily modified and substituted by cancrinites, zeolites etc. Depending on the content of nepheline itself and colour components — pyroxenes, feldspars — in the formation, they are distinguished into urtites, ijolites, malignites, uveites, syenites etc. The value of the nepheline raw material consists in its composite nature. Besides aluminium oxide, alkali composed of soda and potash are extracted from nepheline rocks, with the remains — belite mud — serving as the raw material for cement production.
Alumina production from nepheline raw materials in the world is implemented only in Russia. Our country does not only have significant resources available, but also has its own processing technology. The major deposits of nepheline ores in the territory of Russia are Kukisvumchorskiye, Uksporskoye, Rasvumchorskoye, Apatite Cirque (Kolsky peninsula, Murmansk region, apatite-nepheline ores), Mukhalskoey and Synnyrskoye (Buryatskaya autonomous republic; urtites, ijolites, melteigites, pseudo-leucite syenites), Bayanokolskoye (Tuvinskaya autonomous republic, feldspar urtites, uveite-urtites), Sakunsonskoye (Chita region, pseudo-leucite syenites).
Feldspars and plagioclase (orthoclase, microlin, albite, anorthite, labradorite spar) have found wide application in the construction, glass and ceramics industries. Labradorite spar is an isomorphic mixture of albite and anorthite. With its shining and radiant surface, the labradorite is used for expensive interior finishing in buildings. One of the types of orthoclase (the so-called moonstone) having a nacreous shine with tints and a delicate play of colours is used in jewellery.
Kyanite: pure kyanite is colourless, but the impurities of chrome and iron add a delicate colouration to it: from blue-greyish and blue-greenish to dark blue green and even yellow. Kyanite is formed by the re-crystallisation of sedimentary rocks rich in alumina; it can also be found in quartz veins. The hardness rate of Kyanite according to the Moss scale is 4.5-7, and it has a specific weight of 3.5-3.7 g/cm2. This valuable aluminous raw material is used for producing high-strength refractory and acidproof items, ceramics, and technical porcelain. In Russia, large deposits of kyanite have been discovered in the Urals, in the Republic of Karelia, and in the central part of Kolsky peninsula.
Kaolinite is aqueous aluminium silicate which is the major component of kaolins and clays. Clays are commonly found in alumina-containing rocks, formed as a result of weathering of feldspar rocks. As major impurities, clays contain quartz, micas, calcium and magnesium carbonates etc. Mono-mineral clay rocks of various white-colour tones, containing minerals from the kaolinite group (kaolinite, nacrite, dictite) are called kaolins. Kaolins play an important role in the structure of the Earth’s upper geosphere, forming the major part of sedimentary rocks. They are accumulated in glacier moraines, on the bottoms of lakes, in swamps and river mouths; they form the sludge on the bottom of oceans. They compose of massive deposits of the latest glaciations in Eurasia, North America, as well as Greenland.
There are over 20 various industries using clays as the main or auxiliary raw material (construction, casting, petroleum, paper, ceramics, porcelain and other industries). The efficient technology of using clays in alumina production has not been developed so far.
In Russia, there are numerous clay deposits; they are located in various parts of the country: in the central region, Karelia, and the Urals.
Mica is a transparent laminar alkaline or alkaline-earth aluminium silicate of complex composition, and it is colourless or yellow. Its hardness according to the Moss scale is 2.8-3.2, and it has a specific weight of 2.6-3.2 g/cm2. Laminar mica can rarely be found naturally. The ability of mica to split into very fine transparent leafs, retaining flexibility, elasticity and strength has been known from time immemorial. For example, before glass appeared, mica was used for ‘glassing’ the windows of the Terem Palace at the Kremlin and of rich boyar estates. At present, mica is widely used in electrical engineering, production of paints, varnishes, joint filler, liquid wallpaper and other types of building and finishing materials. The well-known nacreous car enamel contained in the ‘metallic’ paint owes its shine to tiny crystals of mica. Mica is non-toxic and is therefore used in the cosmetics industry as well for adding a nacreous shine to creams and ointments. As a result of baking crushed mica, a material called vermiculite is produced (‘blistered mica’) which is used for manufacturing heat-insulating items, sound-absorbing materials, plaster solutions, as well as filling agent for resins, plastics, paints, pesticides, and in agriculture — for improving soil structure.
Zeolites are a special group of natural framework secondary aluminium silicates, the hollow spaces which are filled with water molecules. In nature, zeolites are formed as a result of weathering feldspars, nephelines, as well as during volcanic eruptions, escaping to the hollow spaces in lava and various volcanic rocks. Dehydrated zeolite is able to absorb water, and it fills all its hollow spaces very quickly, displacing the air in them. It seems that a piece of mineral dropped in water is boiling. The name of the mineral can be explained by this: ‘zeolite’ means ‘boiling rock’ in Greek (Greek ‘zéo’ — boiling, and ‘lithos’ — rock). Zeolites have found wide application as molecular sieves in the chemical industry, cation-exchange materials in nuclear power engineering, and catalytic agents for reactions in the petrochemical industry. In Russia, Kamchatka, Buryatia, Sakhalin, and Primosky region are rich in zeolites.
Sodalites are framework alkaline aluminium silicates, which got their name from the Latin word ‘natrium.’ They are often formed during nepheline modifications. Their hardness according to the Moss scale is 5.5-6, and they have a specific weight of 2.3 g/cm2. In the Khibinskie mountains of Kolsky peninsula, the most well-known type of sodalite is hackmanite containing sulphur. This mineral gradually tarnishes in the air and acquires its colour again only if exposed to radiation. This property called photochromism allows using some synthetic types of this material in radio electronics and television. Natural sodalites are quite widely used as gemstones.
Alunite is the product of exposure of aluminium silicates to natural acid water. In mineralogical terms, alunite is alum, which can contain sodium instead of potassium or both sodium and potassium. Alunite ore, besides the alunite mineral, also contains quartz, kaolinite, alkaline aluminium silicates (sericite, pyrophillite etc.). In world practice, alunite ore is not currently used for alumina production. The specialists of the Russian Aluminium and Magnesium Institute (VAMI) developed the integrated processing of Zaklinskoye deposit alunites into alumina, sulfuric acid, and potassium sulphate and performed its industrial testing at Gjanjinsky alumina refinery (Azerbaijan). At present, this production facility is shutdown. In Russia, there is an alunite deposit in the Far East.
Precious, semi-precious stones and gemstones
A lot of natural aluminium silicates find their application in the jewellery industry. These are:
Beryl: depending on impurities, beryl crystals may have completely different colours. Green stones are called emeralds, blue — aquamarines, yellow and orange — heliodors, red — bixbyite, pink — vorobyevite. Colourless beryls can be found, but are very rare. The most well-known beryls are emeralds. ‘It is green, pure and as delicate as the spring grass, and when you look at it for a long time, your heart lights up’ is how poetically Kuprin described it in his story ‘Sulamif.’ The main supplier of emeralds in the ancient world was Africa. In Russia, the first emeralds were found in 1831, and this was by chance at the bank of the Tokovaya River in the Urals. The rich deposits discovered in this area were called the emerald mines. Now the collection of A.E. Fersman Mineralogy Museum keeps an emerald weighing 11,000 carats.
Topaz: The presence of magnesium, chrome, titanium, and vanadium ions as impurities determines the entire variety of colour tones, making it especially attractive for jewellers. A major deposit of blue topazes was discovered in our country in 1875 in the vicinities of Murzinka settlement in the Urals. The Urals mines are a genuine myriad of semiprecious stones. The largest domestic topazes of the pre-revolutionary period was a 32-kg blue-greenish stone. A large yellow topaz with two pearls adorns the ‘Kazan Hat’ — the tsar’s crown created by the masters of the Armoury in 1562 by the order of Ivan Grozny in commemoration of the conquest of Kazan City.
Garnet is the general name for the group, including up to 30 types of minerals. The most well-known garnet containing aluminium is almandine. The other representative of the garnet group, which contains aluminium is grossular. It got its name for a reason: By its colour and form it often resembles gooseberries (from Latin ‘grossularia’ — gooseberry). In 1968, an emerald-green grossular was found in Tanzania. This mineral used as a jewellery stone and gemstone has a very high value. Having a great hardness, garnets find application in production of sandpaper and grindstones.
Lazurite is a rather soft and fragile mineral which is easily treated and polished. The stones with goldish pyrite inclusions, which resemble a starry sky, have the highest value. The screw horns, beard, eyes, and hair at the back of the goats’ necks from the tombs of the ancient city of Ura (these memorials are dated back to the middle of the 3rd millennium B.C.) are made of Badakhshan lazurite, as are the 5-metre central columns on the iconostasis at St. Isaac’s Cathedral in St Petersburg. After the lazurite deposit was discovered near Lake Baikal in 1851, the Russian stone-cutting factories achieved high proficiency in making pieces of art from lazurite. The Peterhof Palace (near St Petersburg) contains a lazurite room: Panels, fireplaces, and various elements of finishing are made of this beautiful stone. Many outstanding painters used valuable lazurite paint. The walls and vaults of the Savior’s Transfiguration cathedral of Savior-Mirozhsky monastery in Pskov is completely covered with frescos painted on a bright lazurite background. Made in the mid-12th century, they have been preserved until today. Thanks to its unusual colour and mirror-polishing, lazurite is also widely used today. Beads, small vases, and decorations are made of it. Lazurite crust is used for making mosaics and high-quality ultramarine paint. Unlike many other paints, it does not grow dim in the sunlight, and is not sensitive to moisture and high temperatures.
Chrysoberyl and noble spinel are aluminates of beryl and magnesium, correspondingly, which do not contain silicon. Some aluminium atoms in Chrysoberyl are substituted with chrome atoms and this gem is very beautiful. It is called alexandrite. This stone appears emerald-green in the daylight, becomes red-violet in artificial light, and shines bright red if exposed to ultraviolet rays. In Noble spinel every molecule contains two atoms of aluminium, which usually forms tiny crystals of yellow, orange, red, violet, blue, light-blue, green, or black colour and it is not over 10 carats. The biggest pink spinel crystal (over 5 kg) was found at Pamir in 1986. In ancient times, certain types of spinel were often confused with rubies and Arizona ruby garnets. For example, the stones called ‘Ruby of the Black Prince’ and ‘Timur’s Ruby’ in the crowns of English kings are actually spinels. Its name has no definite interpretation. According to one theory, it originates from the Latin word ‘spinelia’ (‘spinelet’) which indicates the form of crystals. It is also possible that the term ‘spinel’ originates from the Greek word ‘spinos’ — ‘spark.’ As a matter of fact, faceted spinels reflect light at dusk.
Tanzanite is the youngest natural semi-precious stone containing aluminium; it was found in 1966 in Africa. It was named ’Mavenzi’ which is the name of the second highest peak of Kilimanjaro. The stone colour varies from blue to deep violet, but it always remains bright and rich. Little by little, tanzanite has stopped being perceived as an inexpensive substitute to blue sapphires and gained its own popularity. American jewellers call tanzanite ‘the semi-precious stone of the 20th century’ by rights. Like other related minerals, tanzanite has a significant shortcoming: It is very fragile. Decorations made of tanzanite cannot be worn every day.
