Chemical Affinities with Changing Basis Species

Description

Calculate chemical affinities of formation reactions of species using basis species that change with the conditions.

Usage

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  mosaic(bases, bases2=NULL, blend=FALSE, ...)

Arguments

bases

character, basis species to be changed in the calculation

bases2

character, second set of changing basis species

blend

logical, use relative abundances of basis species?

...

additional arguments to be passed to affinity

Details

mosaic can be used to calculate the reaction affinities when the basis species listed in bases change in relative abundance over the range of conditions, due to e.g. ionization, complexation or redox reactions. Chemical activity or predominance diagrams constructed by assembling sub-diagrams corresponding to the predominant basis species have sometimes been described as “mosaic diagrams” in the literature. This is a way to “speciate the basis species”. For example, the speciation of sulfur (SO4-2, HSO4-, HS- and H2S) as a function of Eh and pH affects the formation affinities, and therefore relative stabilities of iron oxide and sulfide minerals.

The function calculates the affinities using each basis species listed in bases in turn, changing them via swap.basis. The first species listed in bases should be in the current basis definition. The arguments in ... are passed to affinity to specify the conditions. If blend is FALSE (the default), the function returns the affinities calculated using the single predominant basis species in bases at each condition. If blend is TRUE, the function combines the affinities of the formation reactions weighted by the relative abundances of the basis species at each condition. This tends to produce curved boundaries.

The basis species listed in bases should all be related to the first basis species there (i.e. all share the same element). A second, independent set of basis species can be provided in bases2 (for example CO3-2, HCO3-, CO2, if the first set of basis species are the sulfur-bearing ones listed above). The function then works recursively, by calling itself instead of affinity, so that the inner loop changes the basis species in bases2. In this way, all possible combinations of the two sets of basis species are used in the calculation.

Value

A list containing A.species (affinities of formation of the species with changing basis species) and A.bases (affinities of formation of the basis species in terms of the first basis species), each having same structure as the list returned by affinity. If bases2 is provided, the list also contains A.bases2 (affinities of formation of the second set of basis species, in terms of the first one in that set).

References

Garrels, R. M. and Christ, C. L. (1965) Solutions, Minerals, and Equilibria, Harper & Row, New York, 450 p. http://www.worldcat.org/oclc/517586

See Also

demo("mosaic"), extending the example below by addition of carbonate species in bases2, with blend set to TRUE, and using thermodynamic data from Garrels and Christ, 1965.

Examples

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# Fe-minerals and aqueous species in Fe-S-O-H system
# speciate SO4-2, HSO4-, HS-, H2S as a function of Eh and pH
# after Garrels and Christ, 1965 Figure 7.20
pH <- c(0, 14, 200)
Eh <- c(-1, 1, 200)
T <- 25
basis(c("FeO", "SO4-2", "H2O", "H+", "e-"))
basis("SO4-2", -6)
species(c("Fe+2", "Fe+3"), -6)
species(c("pyrrhotite", "pyrite", "hematite", "magnetite"))
# the basis species we'll swap through
bases <- c("SO4-2", "HSO4-", "HS-", "H2S")
# calculate affinities using the predominant basis species
m1 <- mosaic(bases, pH=pH, Eh=Eh, T=T)
# make a diagram and add water stability lines
diagram(m1$A.species)
water.lines("pH", "Eh", T=convert(T, "K"), col="seagreen", lwd=1.5)
# show lines for Fe(aq) = 10^-4 M
species(c("Fe+2", "Fe+3"), -4)
m2 <- mosaic(bases, pH=pH, Eh=Eh, T=T)
diagram(m2$A.species, add=TRUE, names=NULL, dotted=3)
title(main=paste("Iron oxides and sulfides in water, log(total S) = -6",
  "After Garrels and Christ, 1965", sep="\n"))
# we could overlay the basis species predominance fields
#diagram(m1$A.bases, add=TRUE, col="blue", col.names="blue", dotted=3)

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