Lecture #8 Isomorphism and polymorphism Plan

Table 1.5

El. positive elements	El. positive elements	El. no. elements	El. positive elements
KH 3 O(1)*	La Ce(2)	Cl-Br(I)	Na-Ca(5)
Ag-Au(I)	Ce-Y(3)	OH-F(6)	Ca-Ce(l)
Sr-Ca(l)	Al-Fe ^3+(5)	S-Se(2)	Mg-Al(2)
Ba-Sr(I)	Fe ^3+-Mn ^3+(1)	Te-S(l)**	Fe ^2+-A1(2)
Ca-Mn(2)	Sb'''-As'''(1)	CO 3 -SO 4 (l)	Fe ^2+-F ^3+(3)
Mn-Mg(1)	Sn-Ge(l)	AsO 4 -PO 4 (2)	Mn ^2+-Fe ^3+(2)
Mn-Fe(20)	V ^v-As ^v(l)	Na 3 [PO 4 ]- H 2 O(1)	Si-Al(6)
Mg-Fe(22)	Ta-Nb(5)		Ti-Nb(l)
Zn-Cu(I)	Os-Ir(l)
Co-Ni(4)

*In parentheses are the number of pairs of elements identified in the mineral world
**Atoms are replaced layer by layer, not atom by atom.
Physical properties of those minerals as a result of complex isomorphous exchange of such elements. specific gravity; hardness and other properties and system dimensions will vary.
In addition to the formal mixed crystals described above, there are also complementary mixed crystals, in which the intervening substance is not located in the place of the system units, but in the spaces between them. In this respect, the parent crystal system is similar to the solvent, and the intervening substance is similar to the solute.
Metals mixed with some metal alloys occupy such voids in crystal systems. Blue table salt (halite − NaCl), which contains sodium metal compounds, is an example of these complementary mixed crystals.
Polymorphism . Depending on the conditions of crystallization (pressure, temperature, composition of solution or alloy), a certain chemical element or chemical compound can form different crystals that are completely different in terms of shape, structure and physical properties. Substances with the same chemical composition and different crystal systems are called polymorphic modifications. Polymorphic modifications are distinguished from each other, first of all, by the difference in the crystal system. This is manifested in the ability of different system units to form different crystal systems according to certain conditions, that is, in different locations in the system. Some chemical elements or compounds can have several - two, three or even more polymorphic modifications. Sometimes in different crystal systems, atoms and ions differ from each other according to their position and arrangement. For example, it crystallizes in two or more forms, such a property is called polymorphism.
Existing crystal systems can be transformed into various polymorphic modifications by changing the conditions of their formation. In general, modifications of chemicals that occur under the influence of high pressure and high temperature have a higher degree of symmetry than other modifications that occur in low temperature and pressure environments. For example, quartz (SiO ₂ ) crystallizes in hexagonal syngonia in a high-temperature environment, and in a trigonal syngonia in a low-temperature environment; C - carbon is also diamond - cubic, graphite is hexagonal, calcium carbonate CaCO ₃ is also calcite-trigonal, aragonite-rhombic crystallized in such different environments. But if a certain substance, for example, diamond and graphite, needs a very high temperature and pressure to change from one modification to another, a very small pressure and temperature is enough for another substance to change. For example, all five existing modifications of ammonium nitrate salt occur in the temperature range of 17-80°. There are also substances for which the external environment becomes more important than temperature and pressure for the formation of any of their modifications. For example, under the same thermodynamic conditions, Fe ²⁺ crystallizes in the form of pyrite in cubic syngonia in an acidic environment, and in the form of marcasite in rhombic syngonia in an alkaline environment. All such polymorphic modifications are stable under certain temperature and pressure conditions and can meet together. TiO ₂ , which crystallizes in the form of rutile, anatase, and brookite, and ZnS, which is found in the form of sphalerite and wurtzite, are also included.
In the process of polymorphic changes shown above, not only the crystal system of the substance changes, but also the type of chemical bond between the system units in some cases. Polymorphic substitutions are so numerous and varied that polymorphism can be divided into three types as follows.
In the process of polymorphic substitutions within the framework of a different system, as a result of such substitutions, the crystal structure is preserved. Examples of this are various ammonium salts (NH ₄ NO ₃ , NH ₄ Br, NH _{4 C1).}
There are also polymorphic substitutions in which the system of the crystal is different, while the coordination number and the type of chemical bond are preserved. For this, the example of ZnS can be shown, where the Zn cation occupies half of the number of tetrahedral spaces with the ends facing the same direction in all five types of systems. As a second example, polymorphic modifications of silica - SiO _{2 can be shown. In particular, quartz, cristobalite, tridymite and their forms formed at high and low temperatures} differ from each other only in the order of interconnection of (SiO _{4 ) tetrahedra.} From all this, the coordination number of atoms remains unchanged at four.
There are also polymorphic substitutions that differ in the type of crystal system, coordination number, and type of chemical bond. An example of this is polymorphic modifications of iron. These modifications are crystallized in the volume-centered cubic and side-centered cubic system, and they differ from each other by the coordination number of iron and the type of dense arrangement of atoms. Carbon modifications of graphite with diamond are also distinguished by their crystal system and type of chemical bonding, which are very different from each other