Dino

Tom, You are correct. So, the problem that I'm trying to simulate is that after glass has dissolved for quite a long time, we see in some cases analcime precipitate. When analcime precipitates, the glass reaction rate increases dramatically. We have always interpreted this as analcime consuming the rate controlling species orthosilicic acid. I also interpreted this to mean that analcime had to precipitate faster than the glass dissolved. What you are saying suggests that analcime has to precipitate slower than the glass, but still consume silicic acid. Is that correct? Denis

Tom, I'm not sure how that helps. Unless Q/K > 1, the analcime will not ppt. Once Q/K = 1, analcime will stop dissolving and start ppt'ng as Q/K gets greater than 1. Since I only put in 0.01 ng, it doesn't take long for the analcime to be completely gone. The glass continues to dissolve until analcime is saturated. If you look at the output, with the current formulation, analcime does ppt and mass steadily accumulates. However, the code has a really hard time converging unless I relax epsilon. I'm just unsure how relaxing epsilon affects the results. If there is no problem with relaxing epsilon, I should be OK. Denis

Tom, To continue the dialogue on the modeling of two kinetic minerals, I have been successful in modeling glass dissolving and a mineral phase precipitating. This is not totally satisfactory as I am unable to prevent dissolution even of a ng of mineral until saturation. However, for the time being, I am willing to live with that. However, as the glass dissolves and saturation of the second mineral is approached, the code begins to decrease the step size until it can converge. Another several small steps (5 or 6 at 0.00001 or so) are calculated before convergence is a problem again. This continues and, most of the time, the calculation completes. It often helps to relax the value of epsilon by 3 to 5 orders of magnetude, maybe as low as 10^7. Even with this, the calculation proceeds very slowly, often taking several hours to complete. In the output, I can often see spikes in the reaction rate and other output that suggests residual instability in the calculation. I am a bit concerned that the calculation is accurate. In an email, I'll send you the script and the database. When you get the time to take a squint at it, I'd appreciate your feed back, especially if you have a suggestion on how to make the code run better. Denis

Tom, You are correct. When I use the built-in rate, the problem converges. However, to get some analcime to precipitate, I need to start with some present. If I start with a very small amount, say 1 ng, the problem still works. However, this amount must first dissolve. This means that the problem is not quite what I wanted, but it may be good enough. The reason for using the script was to make sure that no analcime dissolved. The script I sent you had Q/K < 1.03, but the same problem with convergence occurred when Q/K was constrained to be 1 or greater. As I think about it, the problem might lie in the case that there needs to be some analcime present before some can precipitate. Hence my observation that I needed some small amount present for the reaction to occur. Thnaks for your help. I think I am on the right track for now. Denis

Tom, I have successfully gotten a glass to dissolve in solution with a known reaction rate law. I am trying to see if I can reproduce the observed effect of analcime precipitation on the dissolution rate of the glass. I have set up a script to read the information for the glass, analcime precipitation, and altered log K values. What seems to happen irrespective of what mineral I choose is that as the system gets close to the point at which the second mineral should begin to precipitate, the program cannot converge. It aborts when the step size gets too small. I've tried to decrease the step size, use 'log' instead of 'linear,' and some others. The same result at the same calculated time. Presumably what should happen is that the rate decreases as the silica from the glass increases (the glass has a cross affinity to chalcedony) and when the competing phase precipitates, the dissolution rate of the glass should increase. Any hints on how to proceed? I'll send you the files under a separate email. Denis

Tom, After some playing around, including the suggestion you had about keeping the reaction script simple at the start, I got a script to work. It turns out that the script only applies to the rate equation. Unlike the command mode or the input script, it cannot contain any values for the constants. The rate constant, surface area, etc must either be added in the input script or manually. Denis

Hi Tom The version is Release 8.0 published October 27, 2009. I am not using a custom and built-in. I am putting in both kinetics as custom rate laws. I have the glass kinetics in the input script. The Analcime rate is embedded in the scrpt: If QoverK < 1 then 20 else 40 20: kinetic Analcime rate_law = "pre_exp*(1-Q/(K))*exp(-act_en/(8.314*TK))" pre-exp = .00551 act_eng = 77000 surface = 1000 nucleus = 100 critSI = .3 sparea = 100 Goto 60 10: kinetic Analcime rate_law = 0.0000 60: Return rate However, I don't think I am getting to the script. I use the call on the command tab with no analcime in the reactants list kinetic Analcime rate_law = "c:\active files\gwb files\analcime.bas" (have to use the quotes because GWB does not understand spaces in the command line and I get a response that one of the words is a command that is not in the command list). This command leads to an entry in the reactants list for analcime, but not with the custom rate law or other input like the surface area, etc. I've tried to put the '.bas' file in a different directory with no success. So, I'm missing something. Denis

Tom, There appears to be a typo in the Reaction Modeling Guide. On pg 61, "...The mineral's rate of precipatation, should it be oversaturated, however, is zero..." I believe this should be "undersaturated" since the script sets the rate to zero when Q/K < 0. Correct? In this regard, I find that when I need a model with glass dissolution kinetics and analcime kinetics, some analcime dissolves even when I have '0' in the starting mass of analcime. This leads to a positive rate and a negative volume. I believe the solution is to use a script like the one on pg 61. Correct? Thanks, Denis