How well suited are current thermodynamic models to predict or interpret the composition of (Ba,Sr)SO₄ solid-solutions in geothermal scalings?

In this study, we report results of the analysis of a particularly interesting scaling sample
from the geothermal plant in Neustadt-Glewe in northern Germany, which contained
19% Galena (PbS) and 81% of a heterogeneous assemblage of (Ba,Sr)SO₄ crystals with
varying compositions, 0.15 < X$_{Ba}$ < 0.53. A main fraction of the sample (~56%) has a
barite content of X$_{Ba}$ ≈ 0.32. We try to relate the solid composition of the (Ba,Sr)SO₄
solid-solution to the conditions at the geothermal plant concerning temperature,
pressure, and solution composition, and discuss it with respect to the challenges in
modelling the composition of (Ba,Sr)SO₄ solid-solutions on the basis of thermodynamic
mixing models. We note that considerable uncertainties are related to the description
of (Ba,Sr)SO₄ formation by means of thermodynamic models. The scaling composition
observed in this study would be in line with endmember solubilities as predicted by
the PhreeqC-Pitzer database for 70 °C and an interaction parameter, a0 = 1.6. According
to such a model, the scaling heterogeneity would reflect bimodal precipitation
behaviour due to various degrees of depletion of the brine with respect to X(Ba)($_{aq}$).
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Minor fluctuations in X(Ba)($_{aq}$): 0.0017 < X(Ba)($_{aq}$) < 0.0042 explain the full range of
observed solid compositions. The choice especially of the interaction parameter seems
to some extent arbitrary. This knowledge gap strongly limits the interpretation of (Ba,Sr)
SO₄ compositions. Thus, it is not possible to distinguish between kinetic and thermodynamic
effects on partitioning or to use the solid-solution composition to draw conclusions
about the precipitation conditions (e.g. Temperature).