Lecture #8 Isomorphism and polymorphism Plan

Heterovalent isomorphism. In this type of isomorphism, atoms or ions of different valence chemical elements with equal or close ionic radii exchange places with each other. Even as a result of such an exchange, as is certainly known from general chemistry, the electrostatic balance between anions and cations in the compound must be maintained. Since the excess valence caused by the exchange of a small valence ion with a large valence ion is in the cation part of the compound, it is necessary for ions to exchange places with other ions in the anion part of this compound. As a result, the balance between anions and cations is maintained, and vice versa, as anions of unequal valency are isomorphically exchanged, the cation part of this compound must be replaced to the extent that electrostatic balance is maintained. For example, when Mg ²⁺ is replaced by Al ^{3+ in micas, three of the divalent Mg cations are exchanged with two of the A1 cations, that is,} the electrostatic balance in the compound is maintained only when Mg ₃ →Al _{2 .}
Also, isomorphous substitution is very common among complex anions with similar or equal radii, such as [SiO ₄ ] ^4- , [AlO ₄ ] ^5- , [PO ₄ ] ^3- and [SO ₄ ] ^{2- .} In particular, infinite mixtures of plagioclase group minerals albite Na[AlSiO ₃ O ₈ ] with [Al ^III O ₄ ] ^5- anorthite Ca[Al ₂ Si ₂ O ₈ ] are widespread in magmatic rocks. In this case, as a result of the isomorphous exchange of [SiO ₄ ] ^4- and [AlO ₄ ] ^5- complex anions, due to the fact that Al is trivalent and Si is quadrivalent, the excess negative valence in the anion part of the compound is due to the replacement of the Na cation with the divalent Ca cation. comes to equilibrium. Therefore, the valency of isomorphous exchangeable ions in the anion part and the cation part of the compound must always be equal.
In the examples given above, isomorphous substances can be interchanged with each other in infinite proportions. Such isomorphs are perfect isomorphs; and isomorphism is called perfect isomorphism. In addition, there are limited isomorphs. For example, BaSO ₄ and KMnO ₄ can be isomorphically exchanged only in certain proportions, that is, it can form a mixed crystal. Such an isomorphism is called a restricted - imperfect isomorphism . Calcite Ca[CO ₃ ] and magnesite Mg[CO ₃ ] are also finite isomorphous substances, only in a certain ratio they form a mixed crystal (dolomite − CaMg[CO ₃ ] _2. Each of these substances is the unit cell in the crystal system. due to the much larger difference between the size (6.412A° and 6.064 A°) and the radius of the cations (1.06A° and 0.78 A°), it cannot form an infinite mixture. In the case of isomorphous mixtures, the difference between the radii of the units of the exchange system is much greater, that is, 20-25%. In addition, a complex situation, isovalent and heterovalent isomorphisms can appear simultaneously in one mineral.
Elements that form an infinite isomorphism (mixture) in the composition of minerals