https://doi.org/10.1351/goldbook.S05806
When the area of the @I03082@ between two phases is proportional to the mass of one of the phases (e.g. for a solid @A00153@, for an @E02065@ or for an @A00176@), the specific surface area (\(a\),\(s\) or preferably \(a_{s}\)) is defined as the surface area divided by the mass of the relevant phase.