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A solid solution is a single phase which exists over a range of chemical compositions. Some minerals are able to tolerate a wide and varied chemistry, whereas others permit only limited chemical deviation from their ideal chemical formulae. In many cases, the extent of solid solution is a strong function of temperature, with solid solution being favoured at high temperatures and un-mixing and or ordering favoured at low temperatures. A commonly regarded solid solution right from the Bronze-age is the term Alloy.

TYPES OF SOLID SOLUTION:

1. Substitutional solid solution: chemical variation is achieved simply by substituting one type of atom in the structure by another. When the atoms of solute substitute for the atoms of the solvent in its lattice, the solution is known as Substitutional solid solution. The solute may incorporate into the solvent crystal lattice substitutionally by replacing a solvent particle in the lattice. Example includes, Brass, in which zinc (solute atom) is introduced into the lattice of copper (Solvent). See also another example of substitutional solid solution in the Aluminium nickel alloy system, both metal are FCC. The relative size factor is 14%. However, nickel is lower in valence then Al and as per relative valence factor solid nickel dissolve 5% Al but higher valence Al, dissolve only 0.04 % Ni. Hence, we may say substitutional solid solutions can be of two types:

1a. Ordered solid solution

1b. Disordered solid solution

1a. Ordered Solid Solution: If the atoms of the solute occupy certain preferred sites in the lattice of the solvent, an ordered solid solution is formed. It may occur only at certain fixed ratios of the solute and solvent atoms. In Cu – Au system, Cu atoms occupying the face-centered sites and Au atoms occupying the corner sites of the FCC unit cell. Ordered Substitutional solid solution Substitutional element replaces host atoms in an orderly arrangement e.g., Ni-Al, Al-(Li, Zr)

1b. Disordered Solid Solution: chemical variation is achieved by omitting cations from cation sites that are normally occupied. If the atoms of the solute are present randomly in the lattice of the solute, it is known as disordered solid solution. Most of the solid solutions are disordered solid solutions

2. Interstitial solid solution: chemical variation is achieved by adding atoms or ions to sites in the structure that are not normally occupied. When the atoms of the solute occupy the interstitial spaces in the lattice of the solvent, it is known as interstitial solid solution. If the size of the solute is less than 40% that of solvent, interstitial solid solution may be formed. The solute may incorporate into the solvent crystal lattice interstitially, by fitting into the space between solvent particles. Only H, Li, Na and B form interstitial solid solution. A typical example of this is steel, where carbon atoms are present in interstitial positions between iron atoms with maximum percentage of 2. Atomic radius of carbon is 0.071 nm which is less than 59% of 0.125 nm radius of iron atom.

The interstitial solid solution can as well be classified into three. These are of three types:

2a Intermetallic Compound or Valency Compound: These are generally formed between chemical dissimilar metals and are combined by following the results of chemical valence. They have strong bonding their properties are essentially non-metallic. These are formed where metals are far away in periodic table. i.e., poor ductility, poor electrical conductivity. e.g., MgPb2, Mg2Sn, Cu2Se, Ti3A/, Ni3A/ etc.

2b Interstitial Compound: These compounds formed between the transition metal such as Scandium (SC), titanium (Ti), Ta, W, Fe etc. with hydrogen, oxygen, carbon, boron & nitrogen.

These compound are metallic having high melting point and are extremely hard, e.g. TiC, TaC, Fe4N, W2C, CrN, TiH etc.

2c Electron Compound: These are formed by Copper, gold, silver, iron, and nickel with the metal like cadmium, magnesium, tin, zinc and aluminium.

These compounds have definite ratio of valence electrons to atom and therefore called electron compound. Many electron compounds have properties resembling to solid solution.

FACTORS AFFECTING THE EXTENT OF SOLID SOLUTION:

1. Atomic or ionic size: If the atoms or ions in a solid solution have similar ionic radii, then the solid solution is often very extensive or complete. Generally, if the size difference is less than about 15%, then extensive solid solution is possible. For example, Mg2+ and Fe2+ have a size mismatch of only about 7%, and complete solid solution between these two elements is observed in a wide range of minerals. However, there is a 32% size difference between Ca2+ and Mg2+, and we expect very little substitution of Mg for Ca to occur in minerals.

2. Temperature: High temperatures favour the formation of solid solutions, so that end members which are immiscible at low temperature may form complete or more extensive solid solutions with each other at high temperature. High temperatures promote greater atomic vibration and open structures, which are easier to distort locally to accommodate differently sized cations. Most importantly, solid solutions have higher entropy than the end members, due to the increased disorder associated with the randomly distributed cations, and at high temperatures, the Gibb's free energy stabilizes the solid solution.

3. Structural flexibility: Although cation size is a useful indicator of the extent of solid solution between two endmembers, much depends on the ability of the rest of the structure to bend bonds (rather than stretch or compress them) to accommodate local strains.

4. Cation charge: Heterovalent substitutions (i.e. those involving cations with different charges) rarely lead to complete solid solutions at low temperatures, since they undergo complex cation ordering phase transitions and/or phase separation at intermediate compositions. These processes are driven by the need to maintain local charge balance in the solid solution as well as to accommodate local strain.

SOLID SOLUTION
A "solid solution" is a single phase which exists over a range of chemical compositions. Some minerals are able to tolerate a wide and varied chemistry, whereas others permit only limited chemical deviation from their ideal chemical formulae. In many cases, the extent of solid solution is a strong function of temperature, with solid solution being favoured at high temperatures and un-mixing and or ordering favoured at low temperatures. A commonly regarded solid solution right from the Bronze-age is the term Alloy. TYPES OF SOLID SOLUTION: 1. Substitutional solid solution: chemical variation is achieved simply by substituting one type of atom in the structure by another. When the atoms of solute substitute for the atoms of the solvent in its lattice, the solution is known as Substitutional solid solution. The solute may incorporate into the solvent crystal lattice substitutionally by replacing a solvent particle in the lattice. Example includes, Brass, in which zinc (solute atom) is introduced into the lattice of copper (Solvent). See also another example of substitutional solid solution in the Aluminium nickel alloy system, both metal are FCC. The relative size factor is 14%. However, nickel is lower in valence then Al and as per relative valence factor solid nickel dissolve 5% Al but higher valence Al, dissolve only 0.04 % Ni. Hence, we may say substitutional solid solutions can be of two types: 1a. Ordered solid solution 1b. Disordered solid solution 1a. Ordered Solid Solution: If the atoms of the solute occupy certain preferred sites in the lattice of the solvent, an ordered solid solution is formed. It may occur only at certain fixed ratios of the solute and solvent atoms. In Cu – Au system, Cu atoms occupying the face-centered sites and Au atoms occupying the corner sites of the FCC unit cell. Ordered Substitutional solid solution Substitutional element replaces host atoms in an orderly arrangement e.g., Ni-Al, Al-(Li, Zr) 1b. Disordered Solid Solution: chemical variation is achieved by omitting cations from cation sites that are normally occupied. If the atoms of the solute are present randomly in the lattice of the solute, it is known as disordered solid solution. Most of the solid solutions are disordered solid solutions 2. Interstitial solid solution: chemical variation is achieved by adding atoms or ions to sites in the structure that are not normally occupied. When the atoms of the solute occupy the interstitial spaces in the lattice of the solvent, it is known as interstitial solid solution. If the size of the solute is less than 40% that of solvent, interstitial solid solution may be formed. The solute may incorporate into the solvent crystal lattice interstitially, by fitting into the space between solvent particles. Only H, Li, Na and B form interstitial solid solution. A typical example of this is steel, where carbon atoms are present in interstitial positions between iron atoms with maximum percentage of 2. Atomic radius of carbon is 0.071 nm which is less than 59% of 0.125 nm radius of iron atom. The interstitial solid solution can as well be classified into three. These are of three types: 2a Intermetallic Compound or Valency Compound: These are generally formed between chemical dissimilar metals and are combined by following the results of chemical valence. They have strong bonding their properties are essentially non-metallic. These are formed where metals are far away in periodic table. i.e., poor ductility, poor electrical conductivity. e.g., MgPb2, Mg2Sn, Cu2Se, Ti3A/, Ni3A/ etc. 2b Interstitial Compound: These compounds formed between the transition metal such as Scandium (SC), titanium (Ti), Ta, W, Fe etc. with hydrogen, oxygen, carbon, boron & nitrogen. These compound are metallic having high melting point and are extremely hard, e.g. TiC, TaC, Fe4N, W2C, CrN, TiH etc. 2c Electron Compound: These are formed by Copper, gold, silver, iron, and nickel with the metal like cadmium, magnesium, tin, zinc and aluminium. These compounds have definite ratio of valence electrons to atom and therefore called electron compound. Many electron compounds have properties resembling to solid solution.

FACTORS AFFECTING THE EXTENT OF SOLID SOLUTION: 1. "Atomic or ionic size": If the atoms or ions in a solid solution have similar ionic radii, then the solid solution is often very extensive or complete. Generally, if the size difference is less than about 15%, then extensive solid solution is possible. For example, Mg2+ and Fe2+ have a size mismatch of only about 7%, and complete solid solution between these two elements is observed in a wide range of minerals. However, there is a 32% size difference between Ca2+ and Mg2+, and we expect very little substitution of Mg for Ca to occur in minerals. 2. "'Temperature": High temperatures favour the formation of solid solutions, so that end members which are immiscible at low temperature may form complete or more extensive solid solutions with each other at high temperature. High temperatures promote greater atomic vibration and open structures, which are easier to distort locally to accommodate differently sized cations. Most importantly, solid solutions have higher entropy than the end members, due to the increased disorder associated with the randomly distributed cations, and at high temperatures, the Gibb's free energy stabilizes the solid solution. 3. "Structural flexibility": Although cation size is a useful indicator of the extent of solid solution between two endmembers, much depends on the ability of the rest of the structure to bend bonds (rather than stretch or compress them) to accommodate local strains. 4. "Cation charge": Heterovalent substitutions (i.e. those involving cations with different charges) rarely lead to complete solid solutions at low temperatures, since they undergo complex cation ordering phase transitions and/or phase separation at intermediate compositions. These processes are driven by the need to maintain local charge balance in the solid solution as well as to accommodate local strain.