Act2 and Tact Calculate stability diagrams on activity, fugacity, and temperature axes. Calculate solubility diagrams. Project traces of React runs onto.

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Presentation transcript:

Act2 and Tact Calculate stability diagrams on activity, fugacity, and temperature axes. Calculate solubility diagrams. Project traces of React runs onto diagrams. Scatter plots: data points (“spreadsheet data”) can be overlain on diagram.

The Basis The basis is the set of aqueous species, minerals, and gases with which we choose to Write chemical reactions. Express composition. The number of basis entries is the number of components in the system, which is fixed by the laws of thermodynamics. By convention, we will choose as the basis Water, the solvent. Each mineral in equilibrium with the system. Each gas at known fugacity. Important aqueous species.

Steps in Running GWB Models SWAP. Prepare your calculation by swapping into the basis the species with which you wish to write reactions, including any coexisting minerals and gases at known fugacity. For an activity- activity diagram, the basis includes the main species and axis variables. SET. Set constraints on the system, perhaps including temperature, total concentrations, species’ activities, gas fugacities, etc. GO. Select Run → Go to let the program trace the reaction path, calculate and display the diagram, or balance the reaction. REVISE. Change the basis, add new components, or alter any of the specifications for your model. Then select Run → Go to render the new result.

Basis Species in the LLNL Database

thermo.tdat Data compiled by Wolery, Johnson, others at LLNL. Log Ks at 0, 25, 60, 100, 150, 200, 250, 300°C. Log K of 500 means no data at that temperature. Other datasets: PHREEQ-C, Minteq, Wateq, … HCO3- charge= -1.0 ion size= 4.5 A mole wt.= elements in species O H C 47 basis species Fe+++ charge= 3.0 ion size= 9.0 A mole wt.= g 4 species in reaction H2O Fe H O2(aq) redox coupling reactions Al(OH)4- charge= -1.0 ion size= 4.0 A mole wt.= g 3 species in reaction H Al H2O additional aqueous species “−” sign indicates left side Fe H 2 O = Fe ++ + H O 2 (aq) Log Ks at principal temperatures: °C

Carnotite type= vanadate formula= K2(UO2)2(VO4)2 mole vol.= cc mole wt.= g 3 species in reaction K UO VO minerals CO2(g) mole wt.= g 3 species in reaction H2O H HCO gases K2O mole wt.= g 3 species in reaction H K H2O 41 oxides thermo.tdat (cont’d) File → View → thermo.tdat

Task 1 — Activity-activity diagrams Redox-pH diagram for Fe. Vary dissolved iron activity. Account for complexing ligands which speciate over axes.

10 –8 a Fe = 10 –6 10 –4 pH log a O 2 (aq) Magnetite Fe ++ FeOH ++ Fe +++ FeO 100°C Hematite

Fe ++ Fe +++ FeOH °C pH log a O 2 (aq) H2SH2SHS − Pyrite SO 4 −− HSO 4 − S

Task 2 — Stability diagrams Kaolinite can react in presence of H 2 O to form almost any aluminosilicate mineral by cation exchange and adding or losing silica. Calculate stability of calcic aluminosilicates vs. Ca ++ /H + exchange and temperature, in the presence of quartz.

Beidellit-Ca Beidellit-H Grossular Heulandite Kaolinite Laumontite Prehnite T (°C) log a Ca++ /a H+ 2

Task 3 — Solubility diagrams Diagram the solubility of gibbsite Suppress stable phases to consider metastable minerals

–8 –7 –6 –5 60°C Al +++ AlOH ++ Al(OH) 2 + Al(OH) 3 Al(OH) 4 − Boehmite Diaspore Gibbsite pH log a Al +++

Craig M. Bethke and Brian Farrell © Copyright 2016 Aqueous Solutions LLC. This document may be reproduced and modified freely to support any licensed use of The Geochemist’s Workbench® software, provided that any derived materials acknowledge original authorship.