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Aluminium silicates

The silicate Al2SiO5, i.e. Al2O3.SiO2, occurs in nature as three distinct minerals: cyanite, which crystallises in the triclinic system (a:b:c = 0.899:1:0.709, α = 90°5.5', β = 101°2', γ = 105°44.5'), andalusite, which occurs in rhombic crystals (a:b:c = 0.986:1:0.703), and sillimanite, which is also rhombic (a:b:c = 0.9696:1:0.7046). The only one of these forms of the silicate stable above 1300° is sillimanite.

equilibrium alumina-silica
Equilibrium diagram for the system alumina - silica.
Thermal investigation of the system silica-alumina shows the existence of only one aluminium silicate, viz. Al2O3.SiO2, which separates from a fused mixture in crystals of sillimanite, melting at 1816° (fig.).

Aluminium enters into the composition of numerous complex silicates, many of which are important rock-forming minerals. Only a very brief description of the more important of these can be given here.

The felspars are the most important of all rock-forming minerals. The chief felspars are orthoclase or potash felspar (monoclinic; a:b:c = 0.658:1:0.555; β = 63°57') albite or soda felspar (triclinic; a:b:c = 0.6335:1:0.5577; α = 94°.3', β = 116°29', γ = 88°9'), anorthite or lime felspar (triclinic; a:b: c = 0.6347:1:0.5501; α = 93°13', β = 115°55', γ = 91°12'), and the plagioclases, of which the minerals oligoclase, andesite, labradorite, and bytownite are examples. The chemical formulae are KAlSi3O8 for orthoclase, NaAlSi3O8 for albite, and CaAl2Si2O8 for anorthite. As originally supposed by Tschermak in 1864, the plagioclases are isomorphous mixtures of albite and anorthite, Anorthite melts at 1550°.

Orthoclase is mined in large quantities for use in the manufacture of porcelain. Numerous attempts have been made to utilise this mineral as a commercial source of potash. The Ceylon "moonstone," which is used as a gem, consists of orthoclase having a pale blue, pearly opalescence.

The micas are minerals of very common occurrence as constituents of rocks. Chemically, they are silicates of aluminium and either an alkali or iron and magnesium, and in addition they contain fluorine and water of constitution. They are monoclinic, but pseudohexagonal, and possess a highly perfect basal cleavage, which gives white mica or muscovite its commercial importance. Other micas are biotite or dark mica and lepidolite or lithia mica.

The garnets are a well-defined series of minerals of the general formula R3''R2'''(SiO4)3, where R'' is Ca, Mg, Fe, or Mn, and R''' is Al, Fe, or Cr. The following varieties are distinguished: grossular (Ca,Al), pyrope (Mg,Al), spessartite (Mn,Al), almandine (Fe,Al), uvarovite (Ca,Cr), and andradite (Ca,Fe). The garnets are cubic minerals, distinguished by their dodecahedral or icositetrahedral form, greasy lustre, strong refraction, and ready fusibility. Pyrope constitutes the common red garnet used in jewellery.

The zeolites are hydrated silicates of aluminium and the alkali metals (and generally of calcium as well). The following minerals are among those classed as zeolites: - Stilbite – CaAl2Si6O16.6H2O; Analcite – Na2Al2Si4O12.2H2O; Heulandite – H4CaAl2Si6O18.3H2O; Natrolite – Na2Al2Si3O10.2H2O; Chabazite – CaAl2Si4O12.6H2O; Scolecite – CaAl2Si3O10.3H2O;

The water is very loosely held, and they intumesce before the blowpipe. With loss of water the zeolites lose their transparency, but many of them, after dehydration, can re-absorb the amount of water they originally contained, thereby regaining their transparency and original optical properties. Further, dehydrated zeolites absorb ammonia, hydrogen sulphide, alcohol, and many other vapours. The nature of these interesting minerals has therefore been the subject of numerous researches.

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