Microsoft EXCEL spreadsheet-based program for calculating equilibrium gas speciation in the C–O–H–S–Cl–F system

doi:10.1016/j.cageo.2003.10.006

Computers & Geosciences

Volume 30, Issue 3, April 2004, Pages 211-214

https://doi.org/10.1016/j.cageo.2003.10.006 Get rights and content

Abstract

A high-temperature gas is a relatively simple mixture of gas species in homogeneous equilibrium. Volcanic gases are composed of the elements C, O, H, S, Cl and F along with trace amounts of other components. We present an Microsoft EXCEL spread sheet and add-in for calculation of homogeneous equilibrium in this system. This EXCEL extension allows users to calculate equilibrium species fugacities at $1 bar$ for any volcanic or laboratory gas mixture in the C–O–H–S–Cl–F system. Homogeneous equilibrium is calculated using the RAND algorithm based on the assumption of ideal mixing of 66 molecular gas species. This is implemented as a hierarchy of class objects written in C++. This computation engine is compiled into a dynamically linked EXCEL add-in which can be accessed by any EXCEL spread sheet. An easy to use spread sheet is provided which implements this add-in.

Introduction

Volcanic gases are the medium by which volatile elements are transferred from the lithosphere to the hydrosphere and atmosphere. As such, they play a pivotal role in global cycling of these elements and the maintenance of life on Earth. Volcanic gases are composed primarily of the elements C, O, H, S, Cl and F. Other elements generally comprise only a trace portion of the volcanic gas phase. At a given temperature and pressure, the equilibrium gas phase in this six-component system will be composed of a mixture of a much larger number of molecular gas species in homogeneous equilibrium. The abundances of each of these molecular species, and thus both their chemical potentials and fugacities, will be a function of temperature, pressure, and bulk composition. In this contribution, we describe the thermodynamics of homogeneous equilibrium for C–O–H–S–Cl–F gases at atmospheric pressure (or below) and an efficient and robust algorithm for its calculation. We provide an easy to use EXCEL add in that implements this algorithm for general use.

Section snippets

Geometry and thermodynamics

We consider an n component system with mole numbers in vector $c_{n}$ . At T and P these n components form a homogeneous mixture of m⩾n gas species in vector ς_m. Components and species are related through an n×m formula matrix $A$ , whose columns consist of the component stoichiometry for a given species. For example, a gas in the C–O–H system with the species CO₂, CO, $O_{2}, H_{2}$ , $CH_{4}, H_{2} O$ and COH would have the formula matrix: $A = 110010121200110002421 .$ As this example shows, it is not necessary that any of the

Algorithm

Smith and Missen (1991) present several efficient algorithms for calculating homogeneous speciation. For equilibrium in gas species at low pressure, we have found the RAND algorithm (Smith and Missen, 1991, p. 123) to be particularly robust and effective. The RAND algorithm as presented in Smith and Missen (1991) is a modification of an approach originally presented by White et al. (1958). This algorithm is an iterative procedure that uses the method of Lagrange multipliers to impose mass

C++ class

The C–O–H–S–Cl–F solution is implemented as a class object COHSClFGasSol in the C++ language. The COHSClFGasSol class is a member of a hierarchy of objects, as displayed in Fig. 1. All objects in this hierarchy are child classes of the Phase object and thus must implement a collection of standard Phase methods. These include methods returning the Gibbs free energy, entropy, temperature, heat capacity, setting temperature or pressure, etc.

The COHSClFGasSol class is a child of the Solution class,

Availability

Shared libraries and Excel add-ins are available for download from the MELTS web site, http://ctserver.uchicago.edu and the Brimstone web site, http://ctserver.ess.washington.edu. Both Microsoft Windows and Macintosh versions are included. The Windows version has been tested under Windows 95, 98, NT and 2000 running Excel 97, 98 and 2000. The Macintosh version has only been tested under OS X, but OS 8/9 versions are also available. Much of the source code is available from the above sites, or

Acknowledgements

The first author is indebted to Denton Ebel for pointing out errors in tabulated ΔG_f values for S₂O, C₂H₂ and HS in the two latest JANAF compilations. Reviewers Alexei Fedkin and Denton Ebel provided valuable and constructive reviews. Undergraduate researchers Lori Greene and Richard Green helped with data input and testing. The initial implementation of this code was written at the Geophysical Laboratory under the NSF sponsored Center for High Pressure Research (CHIPR). Refinements and

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Microsoft EXCEL spreadsheet-based program for calculating equilibrium gas speciation in the C–O–H–S–Cl–F system☆

Abstract

Introduction

Section snippets

Geometry and thermodynamics

Algorithm

C++ class

Availability

Acknowledgements

Geochimica et Cosmochimica Acta

Thermochemistry of sulfide liquids Ithe system O–S–Fe at 1bar

Contributions to Mineralogy and Petrology

Thermochemistry of sulfide liquids Ithe system O–S–Fe at $1 bar$