Aluminosilicates

Aluminosilicates

 

aluminum-silicon saltlike compounds, which in nature make up a group of widely distributed minerals. In the crystalline structure, aluminum-like silicon has a coordination number 4—that is, it is in the center of a tetrahedron surrounded by four oxygen atoms. Aluminum can replace silicon structurally as well as chemically. This means that alumina is similar (but not identical) to silica in its chemical nature. Aluminum can enter into the composition of silicates but still have, like magnesium and other typical base metals, a coordination number 6 (octahedron). In this case the corresponding compounds are aluminosilicates—for instance, mineral topaz, pyrophyllite, and others. Sometimes the silicic anhydride (Si0₄)^4- is replaced by (AlO₄)⁵” in the structural anionic complex of a silicate. This gives rise to an additional negative charge which is compensated by addition of cations, usually K, Na, or bivalent Ca and Ba with large ionic radii.

Silica-type minerals are always aluminosilicates, including aluminosilicates of potassium such as orthoclase and microcline (KAlSi₃O₈), aluminosilicates of sodium such as albite (NaAlSi₃O₈), aluminosilicates of calcium such as anorthite (CaAl₂Si₂O₈), and others. Possibility of replacement of the NaSi group by CaAl permits the formation of several compounds of the solid solution type with unlimited solubility, called plagioclases. Aluminosilicates also make up nepheline KNa₃[AlSiO₄] ₄, 4 leucite K[AlSi₂O_e], the scapolite group, zeolite, and others. Aluminosilicates are also found quite often among the silicates of sheet structure. To this group belong minerals of the mica family such as muscovite KAl₂[AlSI₃O₁₀](OH)2, the family of brittle micas such as margante CaAl₂[Al₂Si₂O₁₀](OH)₂, the family of chlorites such as amezite (Mg, Fe)₄Al₂[Al₂Si₂O₁₀](OH)₈, and others. Aluminosilicates are seldom found in other chemicostructural types of silicates. Rare examples of these are hornblende (belt structure), augite (chain structure), and cordierite (ring structure). On disintegration on the earth’s surface aluminosilicates form clay minerals or less often hydrated mica bauxites. The term aliumosilikat was introduced into mineralogy by academician V. I. Vernadskii. He

Table 1. Basic properties of the most widely used aluminosilicate refractory products in the USSR
	Semiacid products		Fireclay products		High alumina-content products
	Class A	Class B	Class A	Class B	VGO-62	VGU-62	VGO-72
1 MN/m2 ≈ 10 kgf/cm2
Lowest fire resistance ( C) ..................	1710	1670	1730	1670	1800	1800	1800
Highest apparent porosity (%)...............	27	30	30	30	24	17	24
Lowest limit of durability under pressure (meganewtons per square meter)1 ..........	10	15	12.5	12.5	25	60	30
Slag resistance ............................	moderate	moderate	good	good	good	excellent	excellent
Thermal resistance .........................	good	moderate	good	good	good	moderate	good

first showed the similar role played by Al and Si in geochemical processes and in the composition of natural compounds. This became a foundation for his aluminum acid theory of silicate structure.

REFERENCES

Vernadskii, V. I., and S. M. Kurbátov. Zemnye silikaty, aliumosilikaty i ikh analogi, 4th ed. Moscow-Leningrad, 1937.
Povarennykh, A. S. Kristallokhimicheskaia klassifikatsiia mineral’nykh vidov. Kiev, 1966.

G. P. BARSANOV

Artificial aluminosilicates are produced by synthesis. The most important are artificial aluminosilicates of the natural mineral zeolite type, the so-called molecular sieves and permutites. Methods of synthesis of aluminosilicates are actually imitations of natural geochemical processes, taking place in overheated water solutions under pressure. Molecular sieves are created in autoclaves at 60° −450°C. A solution of sodium aluminate Na[Al(OH)₄] and a water suspension of silicic acid nSiO₂ x mH₂0 with some alkali addition are used as raw materials. Obtained from this mixture, aluminosilicate gel is washed and dried at a temperature of about 100°C. Molecular sieves can also be formed through recrystallization of some minerals in concentrated salt solutions. Permutites can be produced by sintering kaolin Al₄[Si₄O₁₀](OH)₈ or feldspar KAlSi₃O₈ with quartz α-SiO ₂ and soda Na₂Co₃ at 1,000°C and by other methods. Artificial aluminosilicates are used in chemical and other industries. Especially widely used are molecular sieves. They are used for drying, cleaning, and gas separation in chromatographical analysis of gases and liquids. Permutites are mainly used to decrease hardness of water.

V. S. PSALIDAS