The thermodynamic and geochemical significance of the Stiff and Davis stability index

The Stiff and Davis stability index (S&DSI) has never had a clear thermodynamic or geochemical definition. In the present work, an analysis of its formulation allowed it to be defined as the saturation index (SI) of a calcium carbonate polymorph, investing it with clear thermodynamic significance. This analysis also allowed the K coefficient, an essential component of the S&DSI, to be defined; until now, no clear definition has been available. Finally, the experiment performed by Stiff and Davis that led to their announcement of the S&DSI was reproduced by geochemical modelling. The results obtained indicate that calcite was probably the calcium carbonate polymorph used in the construction of this index.