Mineralogical Phase Rule

mineralogical phase rule [‚min·rə′läj·ə·kəl ′fāz ‚rül]

(mineralogy)

Any of several variations of the Gibbs phase rule, taking into account the number of degrees of freedom consumed by the fixing of physical-chemical variables in the natural environment; it assumes that temperature and pressure are fixed externally and that consequently the number of phases (minerals) in a system (rock) will not usually exceed the number of components.

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Mineralogical Phase Rule 

a form of expression of the phase rule that demonstrates that, under conditions of thermodynamic equilibrium, the number of minerals (or phases) making up a rock cannot exceed the number of its components (usually chemical elements or oxides). The mineralogical phase rule was first formulated by the Norwegian geologist V. M. Goldschmidt in 1911.

The mineralogical phase rule follows from Gibbs’ phase rule, which defines the number of degrees of freedom n of the balanced state of a system consisting of different components C in different phases P— that is, n = C – P + 2. Of the total number of independent intensive parameters (the temperature, pressure, and chemical potentials of the components), the parameters that can assume arbitrary values within certain limits without disrupting the phase state of the system have degrees of freedom (n). Rocks form at arbitrary values of temperature and pressure, and the number of degrees of freedom in their formation cannot be less than 2 (n ≥ 2; P ≤ C). In abyssal zones, rocks form under the influence of the ascending stream of volatile components, which change the values of the chemical potentials of certain components that D. S. Korzhinskii called completely mobile (C_m). They are included in the number of degrees of freedom of the system, which leads to a new expression of the mineralogical phase rule, which is as follows: n ≥ 2 + C_m; P ≤ C – C_m.

REFERENCES

Korzhinskii, D. S. Fiziko-khimicheskie osnovy analiza paragenezisov mineralov. Moscow, 1957.

V. A. ZHARIKOV and A. A. MARAKUSHEV