Conceptual Models > Database selection and modification

Database for silicon compounds

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Nandalee:
Hello everybody,
For a part of my master thesis I am modelling possible silicon and phosphate precipitations for a water using Phreeqc.  My problem is that I can't find the right database for the silicon compounds. My current database tells me that quartz (SiO2) precipitates in my water, but I know that it must be a calcium silicate compound. Does anyone know of a suitable database with different Si compounds?
When modelling for the phosphate precipitates I used minteq.v4 and got useful results.
Also, I wanted to ask how to properly quote a database?
My input file is:
SOLUTION 1
    temp      20
    pH        7
    pe        4
    redox     pe
    units     mg/l
    density   1
    Ca        143
    Cl        124
    K         12.3
    Mg        17.4
    Na        84.7
    S(6)      153
    P         200
    -water    1 # kg
EQUILIBRIUM_PHASES 1
    CO2(g)    -3.4 10
    O2(g)     -0.69 10
SAVE solution 2
END

SOLUTION 3
    temp      20
    pH        7
    pe        4
    redox     pe
    units     mg/l
    density   1
    Ca        143
    Cl        124
    K         12.3
    Mg        17.4
    Na        84.7
    S(6)      153
    Si        200
    -water    1 # kg
EQUILIBRIUM_PHASES 3
    CO2(g)    -3.4 10
    O2(g)     -0.69 10
SAVE solution 4
END

Thanks a lot for your help!

dlparkhurst:
There are a number of databases distributed with PHREEQC. You can check, but my guess is that none of them will show a pure Ca-silicate mineral is supersaturated. If you add aluminum to your system, then I expect there are potential Ca-aluminosilicate minerals that could precipitate. If you know the mineral that you expect to form, then you can search the literature to try to find a log K, but I think the PHREEQC databases would cover the expected range of solubilities of Ca-silicate minerals.

There may also be a difference between the minerals that are thermodynamically most stable and the minerals that form kinetically. You do expect positive saturation indices (within uncertainties) for minerals that precipitate.

As for citing the database, I would give the database, the PHREEQC version number and if possible, the web site and the date of download.

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