This is Simon’s blog reporting on the development of a new single-clinopyroxene HPx-EOS.
We have liftoff! Cpxwing is set-up in terms of end-members, and it is pretty big:
As it turns out, there is no straight forward (or unique) way to define the variables that are required to describe this monster. Because we are trying to accommodate a large number of components, we require eight compositional variables to describe the range of the full model. Here, both Fe and Mg have the choice of four different sites to occupy. To take care of this we had to part with the conventional definition of ‘x’, which normally describes the Fe/Mg ratio. In fact, it is this type of ordering that causes us the most headache: cpxwing needs no less than five ordering parameters to fly.
Since the model involves a large number of end-members as shown in the figure above, we had to define 91 W’s, or interaction energies. Luckily though, most W’s were already available: we could either take them directly from other published cpx models or calculate them from dependent end-members. Some fiddling with these numbers will be required sooner or later, though.
Currently, Eleanor & Simon are testing cpxwing against older models. From this early testing it appears that cpxwing is able to reproduce high P–T equilibria remarkably well. For the next step we will look at coexisting clinopyroxene pairs at subsolvus conditions. Further calibration against experimental data will then be used to fill in the gaps.