Jump to ContentJump to Main Navigation
Geochemical Reaction ModelingConcepts and Applications$
Users without a subscription are not able to see the full content.

Craig M. Bethke

Print publication date: 1996

Print ISBN-13: 9780195094756

Published to Oxford Scholarship Online: November 2020

DOI: 10.1093/oso/9780195094756.001.0001

Show Summary Details
Page of

PRINTED FROM OXFORD SCHOLARSHIP ONLINE (oxford.universitypressscholarship.com). (c) Copyright Oxford University Press, 2021. All Rights Reserved. An individual user may print out a PDF of a single chapter of a monograph in OSO for personal use. date: 23 June 2021

Solving for the Equilibrium State

Solving for the Equilibrium State

(p.61) 5 Solving for the Equilibrium State
Geochemical Reaction Modeling

Craig M. Bethke

Oxford University Press

In Chapter 3, we developed equations that govern the equilibrium state of an aqueous fluid and coexisting minerals. The principal unknowns in these equations are the mass of water nw, the concentrations mi of the basis species, and the mole numbers nk of the minerals. If the governing equations were linear in these unknowns, we could solve them directly using linear algebra. However, some of the unknowns in these equations appear raised to exponents and multiplied by each other, so the equations are nonlinear. Chemists have devised a number of numerical methods to solve such equations (e.g., van Zeggeren and Storey, 1970; Smith and Missen, 1982). All the techniques are iterative and, except for the simplest chemical systems, require a computer. The methods include optimization by steepest descent (White et al., 1958; Boynton, 1960) and gradient descent (White, 1967), back substitution (Kharaka and Barnes, 1973; Truesdell and Jones, 1974), and progressive narrowing of the range of the values allowed for each variable (the monotone sequence method; Wolery and Walters, 1975). Geochemists, however, seem to have reached a consensus (e.g., Karpov and Kaz’min, 1972; Morel and Morgan, 1972; Crerar, 1975; Reed, 1982; Wolery, 1983) that Newton-Raphson iteration is the most powerful and reliable approach, especially in systems where mass is distributed over minerals as well as dissolved species. In this chapter, we consider the special difficulties posed by the nonlinear forms of the governing equations and discuss how the Newton-Raphson method can be used in geochemical modeling to solve the equations rapidly and reliably. The governing equations are composed of two parts: mass balance equations that require mass to be conserved, and mass action equations that prescribe chemical equilibrium among species and minerals.

Keywords:   Governing equations, Newton's method, Newton-Raphson method, Phase assemblage, Reduced basis

Oxford Scholarship Online requires a subscription or purchase to access the full text of books within the service. Public users can however freely search the site and view the abstracts and keywords for each book and chapter.

Please, subscribe or login to access full text content.

If you think you should have access to this title, please contact your librarian.

To troubleshoot, please check our FAQs , and if you can't find the answer there, please contact us .