Different saturation indices at different conditions

[maria]

Dear David,

I would like to ask about the difference between saturation indices at different conditions. For example, a mineral can have a SI -1.0 at some specific conditions T, P and if I change these I get a SI for the same mineral -10.3.
So, if I want to explain the difference in the numbers, does a SI of -1.0 mean that the dissolution is faster compared to a SI of -10.3? Am I right? And is the same for positive SIs in terms of precipitation? e.g. a precipitation of a mineral with a SI +10.0 is greater than with a SI of +1.0?

Thank you very much in advance.

Kind regards,
Maria

[dlparkhurst]

An SI of -10 is more undersaturated than an SI of -1, and usually implies that more mineral would dissolve to reach equilibrium. Often, rate expressions are proportional to a factor [1 - SR("mineral")], which imlies faster rates the greater the undersaturation. Although you can infer that the mineral would dissolve faster for the more undersaturated case, it is possible that the mineral is virtually inert, with very little reaction at all.

[maria]

Thank you very much for the explanation. That is very clear.

Kind regards,
Maria