Basalt weathering

[rudolff16]

Dear all,
The result of petrology of a basalt samples revealed the following mineral composition:
47.70% plagioclase
10.46% Diopside
11.34% Hematite
and
15.67% Hypersthene
the purpose is to investigate the basalt weathering and its effects on groundwater chemistry using inverse modeling. All minerals are found in the database except the Hypersthene, please any help regarding the weathering equation of this mineral in PHREEQC? 

thanks
 

[dlparkhurst]

For inverse modeling, you need to define the new phase with PHASES.

You need a balanced chemical reaction for the dissolution of the mineral. You need not worry about the log_K for the reaction because only the stoichiometry of the mineral is used in inverse modeling.

[rudolff16]

Thank you dlparkurst for you continuous support,
the Hypersthene has the following chemical formula:
MgFeSi2O2
I assumed a reaction with soil water, but I could not find any information in internet about its weathering possibilities of Hypersthene.
can I substitute the Hypersthene with their parent minerals? :  enstatite (MgSiO3) - ferrosilite (FeSiO3)
thanks

[dlparkhurst]

You need a reaction from mineral to aqueous species. However, llnl.dat differs from phreeqc.dat in terms of aqueous species for Si.

For use with llnl.dat, include this data block in your input file:
PHASES
Hypersthene
MgFeSi2O6 + 4 H+  =  2H2O + Mg+2 + Fe+2 + 2 SiO2

For use with phreeqc.dat, include this data block in your input file:
PHASES
Hypersthene
MgFeSi2O6 + 4 H+ + 2H2O =  Mg+2 + Fe+2 + 2 H4SiO4

[rudolff16]

it works! thank you so much