Ostwald s step rule

We might take a purist s approach and attempt to use kinetic theory to describe the dissolution and precipitation of each mineral that might appear in the calculation. Such an approach, although appealing and conceptually correct, is seldom practical. The database required to support the calculation would have to include rate laws for every possible reaction mechanism for each of perhaps hundreds of minerals. Even unstable minerals that can be neglected in equilibrium models would have to be included in the database, since they might well form in a kinetic model (see Section 26.4, Ostwald s Step Rule). If we are to allow new minerals to form, furthermore, it will be necessary to describe how quickly each mineral can nucleate on each possible substrate. 

Ostwald s step rule holds that a thermodynamically unstable mineral reacts over time to form a sequence of progressively more stable minerals (e.g., Morse and Casey, 1988 Steefel and Van Cappellen, 1990 Nordeng and Sibley, 1994). The step rule is observed to operate, especially at low temperature, in a number of min-eralogic systems, including the carbonates, silica polymorphs, iron and manganese oxides, iron sulfides, phosphates, clay minerals, and zeolites. 

Nordeng, S. H. and D. E Sibley, 1994, Dolomite stoichiometry and Ostwald s step rule. Geochimica et Cosmochimica Acta 58, 191-196. 

Closely allied with the concepts of metastable equilibrium and suspended transformations is Ostwald s rule (Ostwald s step rule or law of successive reactions). Essentially Ostwald s rule states that in all processes it is not the most stable state with the least amount of free energy that is initially obtained but the least stable state lying nearest to the original state in free energy (Ostwald 1897). It is easy to see how this rule and the concept of suspended transformations can explain the production of a metastable polymorph through crystallization from a melt or solution. 

In crystallization processes involving a material that displays polymorphism, it is quite common for an unstable polymorph to appear first and then transform into a stable form. This observation is summarized by Ostwald s step rule, sometimes referred to as the Law of Successive Reactions, which says that in any process, the state which is initially obtained is not the stablest state but the least stable state that is closest in terms of free energy change, to the original state. What this means, therefore, is that a crystallization process, the initial solid phase, can be the least stable polymorph that will then transform into successively more stable forms until the stable form, at the conditions of the system, is reached. With some systems this can mean the formation of an... 

During the 19th century. Gay Lussac observed that, during crystallization, an unstable form is frequently obtained first that subsequently transforms into a stable form [13]. This observation was later explained thermodynamically by Ostwald [13,16-19], who formulated the law of successive reactions, also known as Ostwald s step rule. This... 

Ostwald s step rule [13,16-19] is illustrated by Fig. 12. Let an enantiotropic system (Fig. 12a) be initially in a state represented by point X, corresponding to an unstable vapor or liquid or to a supersaturated solution. If this system is colled, the Gibbs free energy will de-... 

---