Mineral Stability

Every geologic process - crystallization of a magma, burial of sediments, metamorphism, weathering - is at its core a change in the mineralogical make-up of Earth materials, driven by changing conditions of temperature, pressure, and chemical composition. ^[1] To understand which minerals form under a given set of conditions, and why some minerals persist long after those conditions have changed, it is necessary to understand the concept of mineral stability.

What Stability Means

Stability is always a comparative concept - one configuration is stable relative to another if it represents a lower energy state. ^[1] A book on the floor is more stable than the same book held two metres above the floor, because the floor position has lower gravitational potential energy. Importantly, spontaneous processes always move from higher-energy to lower-energy configurations - a book dropped from above falls to the floor; it does not spontaneously leap upward. ^[1]

A higher-energy configuration does not, however, always spontaneously convert to a lower-energy one. A book placed on a bookshelf has higher gravitational potential energy than a book on the floor, yet it does not leap off the shelf on its own - it is metastable with respect to the floor position. The floor still represents the lower energy state, but getting there requires a nudge: a small addition of energy to dislodge the book from the shelf. This energy input is the activation energy. ^[1] Minerals behave exactly this way. A metastable mineral may persist for millions or even billions of years unless activation energy is supplied - through heating, deformation, or chemically reactive fluids - to trigger the conversion to a more stable form. ^[1] Most igneous and metamorphic minerals at Earth’s surface are, strictly speaking, metastable: they formed at conditions very different from the cool, low-pressure weathering environment in which they now reside. ^[1]

Gibbs Free Energy

The appropriate energy measure for evaluating mineral stability is Gibbs free energy, whose symbol is G. ^[1] This thermodynamic metric is quantified in energy per mole (using either joules or calories). By definition, a single calorie provides the thermal energy necessary to elevate one gram of water from 15°C to 16°C, while one joule corresponds to exactly 0.2390 calories. ^[1]

To compare the stability of different minerals on a common basis, mineralogists use the free energy of formation from the elements (ΔG_f) - the energy difference between a set of elements in their standard states at 298 K and 1 atm pressure, and those same elements chemically bonded into a mineral at the temperature and pressure of interest. ^[1] Given two minerals with the same chemical composition - such as quartz and tridymite (both SiO₂) - the stable form at any specific temperature and pressure is whichever has the lower ΔG_f. Because ΔG_f changes with both temperature and pressure, the identity of the stable SiO₂ polymorph changes with conditions - quartz under some conditions, tridymite under others. ^[1]

Mineral Reactions and Reaction Free Energy

Mineralogical changes can be written as chemical reactions, and the direction in which a reaction proceeds is governed by the free energy of reaction (ΔG_reaction), defined as: ^[1]

If ΔG_reaction is negative (< 0), the products have lower free energy than the reactants - the reaction is thermodynamically favoured and can proceed spontaneously. If it is positive (> 0), the reactants are more stable and the reaction runs in reverse. If it is exactly zero, the system is at equilibrium and no net reaction proceeds in either direction. ^[1]

Two examples illustrate this. A simple polymorphic transition involves tridymite converting to quartz - the same composition, different structure. A more complex reaction that occurs in metamorphic rocks during prograde heating is the breakdown of muscovite and quartz to produce K-feldspar and sillimanite, releasing water in the process. Both reactions are balanced with equal numbers of atoms of each element on both sides. ^[1]

Stability of Mineral Assemblages in Rocks

Rocks are not made of single minerals - they contain assemblages of multiple minerals. The stable assemblage at any specific temperature and pressure is the one whose combined free energy of formation is the lowest of all the possible mineral combinations that could be built from the elements present in the rock. ^[1] In practice, identifying this lowest-energy assemblage precisely is difficult. The ΔG_f values of all relevant minerals across all their possible compositional variations are rarely known with sufficient accuracy to allow exact calculations. ^[1]

Moreover, there can be no guarantee that the mineral assemblage in any particular rock actually represents the lowest-energy configuration. Mineralogical changes do always proceed in the direction of lower energy, but the rate at which they proceed depends on whether activation energy is available. Tectonic activity, erosion, and burial continuously change the temperature and pressure conditions to which rocks are subjected, yet the mineralogy rarely updates continuously to reflect every change - particularly at lower temperatures where atomic mobility through crystal lattices is very slow. ^[1] Understanding which mineral assemblages are thermodynamically stable - and under what conditions - is exactly what phase diagrams are designed to communicate.

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