In 1819 g. E. Mitscherlich set, what solid compounds of similar crystalline structure can form mixed crystals, ie. enter unit cell of another component. Such crystals are called Mitscherlich isomorphic, and the ability to form mixed crystals - the isomorphism. The ability to isomorphism in large degree determined by the similarity of the size factor (the radius of the atoms and the distance between them in the crystal) and the similarity of the crystal type. It is first installed.. Mendeleev, whose work has made a great contribution to the study of the phenomenon of isomorphism. In 1890 g. Vant-Goff introduced the new name of isomorphous mixtures - solid solutions. Currently use both terms. Thanks to the advent of x-ray diffraction method, based on the study of x-ray diffraction by crystals, it was found, what should distinguish three types of solid solutions: replacement, introduction and subtraction (rice. 4.12).

Rice. 4.12.The crystal lattice of solid solutions:

and) replacement; b) introduction; in) subtraction

Solid solutions of substitution formed by substituting structural units of the crystal lattice component - solvent ions, the atoms or molecules of another component, in this case, the lattice type is saved. Obligatory condition of isomorphism is an analogy of chemical structure of the components, the equality of signs of the charges of the ions, the similarity of the symmetry and dimensions of crystalline cells.

In solid solutions of introduction the lattice is formed by component-solvent, and penetrate the atoms or ions of the second component are placed between lattice sites. This type of solid solutions are formed, for example, upon dissolution of the nonmetals (With, H, N, B) in metals; during the introduction of sodium and aluminum in the structure of SiO₂ (b-cristobalite).

Solid solutions of the subtraction are the result of vacancy, unoccupied locations in the structure of the crystal lattice. For example, mineral pyrrhotite contains excessive amounts of sulfur compared to the, which corresponds to the formula FeS. The lack of a certain number of iron atoms in the mineral structure found by x-ray. The composition of pyrrhotite varies Fe₆S₇-Fe₁₁S₁₂.

Mutual solubility of components in solid state can be unlimited (continuous solid solutions) and limited. In the latter case there are narrow solid solutions (from a few percent to a few hundredths of a percent solute) and wide (up to several tens of percent).

Alloys and minerals, forming a continuous and limited solid solutions, extremely common. It, for example, copper and gold, silver and platinum, copper and Nickel. Continuous solid solutions form the plagioclase, olivines.

4.8.1. Continuous solid solutions

Consider the phase diagram of two component system, forming continuous solid solutions, in the example of plagioclase, rice. 4.13.

Plagioclases are calcium-sodium feldspars of different composition. The composition of minerals can continuously change from pure sodium member - NaAlSi₃O₈ (albite), to pure calcium member - Caal interdiffusion₂Si₂O₈ (anorthite).

The System NaAlSi₃O₈-Caal interdiffusion₂Si₂O₈ Bowen studied for the first time in 1913 g. The chart has the liquidus line - ALKB, characterizing the composition of saturated melt depending on the temperature and the line is the solidus - AEFB, characterizing the composition of the solid solution. Above the liquidus - field of the liquid phase, below the solidus line of the field of the solid phase.

Between the curves of liquidus and solidus - the area of heterogeneous

Rice. 4.13. Phase diagram two component system
“albite-anorthite”, forming continuous solid solutions

equilibrium liquid and solid solutions. So, the G-spot in this area corresponds to a heterogeneous system, consisting of a liquid melt of the composition of l’ and of plagioclase of composition n’, in equilibrium at a given temperature. Comparison of compositions of equilibrium phases shows, what solid solution is enriched in the more refractory component than equilibrium with it liquid melt. This rule is common to heterogeneous systems with solid solutions. It's called the first rule of Gibbs-Roseboom.

The compositions and relative amounts of coexisting liquid and solid phases are determined by the lever rule. The length of the line Gl is proportional to the amount of released crystals, and the length of the line Gn - the quantity of remaining liquid.

The progress of crystallization in systems of this type has the characteristic that. When the cooling liquid D to the liquidus at point To begin to appear crystals of composition F’. The crystals are more refractory component, than liquid, equilibrium with them, therefore, the liquid enriched with component NaAlSi₃O₈ and the temperature of solidification is reduced (along the liquidus line from point K to point L). In accordance with the changing composition of the solid phase (from point F’ to point E’), which is in equilibrium with the liquid melt. At the same time, the previously precipitated crystals interact with the surrounding rich melt, enduring temperatures of a gradual change in composition from F’ to E’ in non-equilibrium conditions. Thus, as you change the liquid composition from K to L along the line of liquidus, the composition of the crystals (as an equilibrium formed under these conditions, and previously formed), continuously changes from F to E. The last portion of the melt solidifies at the point L, and the composition of the crystals, the resulting (at point E), similar to the composition of the liquid melt D.

The second interaction, in the above-mentioned, between the cooling melt and the previously precipitated crystals, enriched Sa, slow and can't go to the end. As a result the formation of layered plagioclase from the edge of the area, containing albite more, than in the Central part of the crystal.

Some solid solutions are characterized by the presence of the extreme point on the chart, in which the liquidus and solidus in contact. At the points of extremum of the compositions of equilibrium solid and liquid phases are identical (the second rule of Gibbs-Roseboom). Chart extremum is shown in figure. 4.14.

Continuous solid solutions can be divided into components by the method of fractional recrystallization. So, for example, share on Ag and Au components in the industry (the method of zone melting). The mineral is electrum, representing a solid solution of Ag-Au with content 30-50% melt, then cooled to 1000°C. Crystallizing a solid solution refractory gold enriched compared to the melt. Phase diagram of the system Ag-Au similar to that shown in figure. 4.13. Enriched with gold crystals are separated and melted again. Second time cool is already up to 1025°C and get crystals with a high content of gold and separate them again. By repeating this process many times, get almost pure gold.

Rice. 4.14. Phase diagram of extreme type with continuous solid solutions

System, characterized by phase diagrams with extreme points, not always possible to separate by recrystallization, since at the points of extremum of the compositions of the phases are the same.

Let us illustrate this on the example system galenit-akermanite, rice. 4.14. The minimum on the curves of solidus and liquidus corresponds to the content of akermanite in mineral 70%. Solid solutions with a content of less akermanite 70% can be separated by recrystallization of pure helinet and inseparable crystallizing a solid solution of extreme composition (70% akermanite). From solutions, containing more 70% akermanite, you can allocate net akermanite, getting the rest of the same solid solution 70% akermanite.

Many minerals and alloys, forming at high temperatures solid solutions, with decreasing temperature decay on the mechanical mixture. Therefore, in different temperature intervals of the system can be described in many types of charts. For example, the system albite (Ab) orthoclase (In_r) at temperatures above 800°C forms a solid solution with the lower extreme point, rice. 4.15. At a temperature of 660°C, they formed a solid solution sanidine, decomposes to two solid phases, representing a mechanical mixture of two limited solid solutions a and b (the shaded region). The composition of the equilibrium phases a and b at constant temperature is set on the node, as shown in figure.

Rice. 4.15. Binary system albite-orthoclase with the collapse of
solid solutions

4.8.2. Limited solid solutions

There are two types of diagrams limited solid solutions: eutectic and peritectic types.

Eutectic diagram with limited solubility areas different from a simple eutectic (rice. 4.5) the presence of fields of limited solid solutions a and b. For example, a-solid solution of copper in silver and b-solid solution of silver in copper, rice. 4.16.

The decrease in temperature from point N to the field of liquid melt to liquidus line leads to a solid solution, the composition of which is determined by the point S. When selecting the initial point in the hypereutectic region allocated b-solid solutions, the composition of which is determined similarly.

Below the solidus line of the system is a mixture of solid solutions of a and b,which are in Noda (just, as shown in figure. 4.13). VIORE starting point in hypereutectic field are b-solid solutions, the composition of which is determined similarly.

Peritectic phase diagram areas with limited solubility are more complicated. Because the nature of the system of minerals of this type are widely distributed, consider as an example the phase diagram of the binary system NaAlSiO₄-NaAlSi₃O₈, rice. 4.17. Three shaded areas are fields of solid phases. It solid solutions: nepheline, containing up to 34% NaAlSi₃O₈; Carnegie, stable at high temperatures and albite, forming a solid solution, containing up to 4% NaAlSiO₄.

The triangle KEF is a two-phase region, in which carnegieite crystals associated with crystals of nepheline, slightly different in content component NaAlSi₃O₈.

Consider the cooling of melt on isolate, starting at point a, content NaAlSi₃O₈ 40%. Liquid of this composition is cooled to the liquidus at point b, where it begins crystallization of terragator (carnegieite) composition With. During the cooling process, the composition of the melt changes along the line of the liquidus to D, and the composition of the coexisting melt carnegieite changes to the point E. Below this temperature

Carnegie this composition can not exist; at this temperature invariant conditions reaction takes place:

melt (D) + Carnegie (E) ® nepheline (F)

Part of the melt is consumed by reaction, the amount of fluid decreases. The temperature again begins to fall, when Carnegie will nepheline. The melt composition changes from D to G, and the composition of nepheline - from K to H. Upon reaching the eutectic melt composition (G), along with the nepheline composition N crystallized albite composition J. The system is invariant until the complete solidification of the melt. Further, the temperature of the mixture of two solid phases is lowered again, the compositions of the solid solutions do not change significantly.

This Chapter describes the main physico-chemical laws of phase equilibria and main types of phase diagrams of one-component and two-component heterogeneous systems. Binary systems with limited solubility in the liquid state will be described in the section “Molecular solutions” tutorials. Phase diagram triple and more complex multi-component condensed systems, of interest to material scientists, geologists and petrologists, covered in detail in the literature in materials science, Mineralogy and petrography. In the list of references, given at the end of the textbook, there are a number of references to such publications.