Ostwald’s step rule: rule of thumb or strict physical law?

In his 1897 article, Friedrich Wilhelm Ostwald wrote that “during departure from any state, and the transition to a more stable one, not the under given circumstances most stable state is reached, but the nearest one“. The word “nearest” essentially gave rise to the widespread interpretation that during a phase transition not the thermodynamically most stable but a metastable phase forms first, which is usually referred to as Ostwald’s step rule. It is considered a general rule rather than strict physical law, although its precise physical basis seems not fully understood on a mechanistic level, despite its potential importance for mineral formation under Earth’s surface conditions.

While Ostwald’s step rule is commonly explained through the classical nucleation theory, there are several inconsistencies that are not explained by this theory. One is that a transition to the stable phase cannot be forced by strongly increasing the driving force (supersaturation), and also adding seed crystals may not help. This conundrum particularly applies to the two most abundant minerals in Earth’s sedimentary record, dolomite and quartz (Meister et al., 2014), which are observed not to precipitate directly from aqueous solution as long as the solution remains supersaturated with respect to one of their metastable polymorphs.

Here, an alternative concept is proposed that would be consistent with Ostwald’s (1897) original formulation and with several observations from natural environments and laboratory experiments. The difference lies in the translation of the word “nearest”, not in a thermodynamic sense as “having a similar Gibbs energy”, but kinetically as “having the smallest energy barrier”. In the latter case, Ostwald’s step rule would become an actual physical law, equivalent to the Arrhenius law. This goes along with the concept that not the thermodynamic barrier of nucleation but some kinetic barrier, not affected by supersaturation, is responsible for the efficient inhibition of the phase. Inhibition (giving rise to Ostwald’s step rule) would then not be a matter of nucleation but of growth.

Meister et al. (2014) Early diagenetic quartz formation at a deep iron oxidation front in the Eastern Equatorial Pacific. GCA 137, 188–207.