Normalization of mineral saturation indices

[Bron]

I have read several texts that suggest mineral saturation indices should be normalized by the sum of the stoichiometries of the positive and negative components in the mineral formula. I was wondering if GWB does this before reporting its saturation indices? If not and provided this is indeed a common action to undertake for geochemists when reporting such figures, may I suggest an option be added within the program for this standardisation? My apologies if this has been discussed before on the forum - I could find no relevant thread. Thank you.

[Brian Farrell]

Hi Bron,

 

GWB makes no such correction to the Saturation Index at this time. SI values are reported in the standard fashion as the log (Q/K). I imagine this is partly to avoid confusion, and partly because there are a number of pitfalls (besides the stoichiometry of the mineral formula chosen) in comparing saturation indices of different minerals, at least as an indicator of the amount of mineral likely to form at equilibrium. We can look into whether this might be a beneficial option, however.

 

The issue of interpreting saturation indices is discussed further in Chapter 6 of Geochemical and Biogeochemical Reaction Modeling.

 

Regards,

 

Brian Farrell

Aqueous Solutions LLC

[Craig Bethke]

Hi Bron,

 

Every decade or so, someone suggests scaling saturation indices in some way, so they can provide a measure of the amount of a mineral that might precipitate, or so saturation indices of various minerals can be compared to each other, or both. Unfortunately, the SI is no more than a measure of the instantaneous energy that would be liberated (or consumed) if the precipitation reaction would begin to move forward. Trying to find a scaling that would add to this meaning is I'm afraid a fool's errand. I think if you were to take out pencil and paper and play around with the idea you mention, you'd agree.

 

HTH,

Craig