[FAQ] Amphibole and pyroxene output

[kchristle]

I have been modeling an intermediate magma but haven't been able to get amphibole and pyroxene to output properly. In order to get any amphibole to be part of the output I have to deselect pyroxenes from the available phases. Otherwise, all I get is pyroxene with no amphibole phase present. I have widely adjusted the H2O values with no change to this problem. I've tried this with both MELTS for Excel and rhyolite-MELTS 1.1.0 with the same results.

[Paula]

This is an old post, but it's a frequently asked question on and off the forum... The problem is basically there are minor components that are not included in the two amphibole and hornblende solution phases (e.g. Ti, Mn) or not included in the MELTS system as a whole (e.g. F). In natural systems these components tend to stabilize amphibole relative to pyroxene, but not in the MELTS family of models where they are ignored. To quote from a recent proposal:

The problem is that simplified models such as the current three-component reciprocal hornblende model (NaCa2(Mg,Fe2+)4(Al,Fe3+)(Si6Al2)O22(OH)2), even when rigorously constructed using precise thermodynamic data on the pure end-members, are almost never stable in computed assemblages. Small concentrations of numerous minor components ââ,¬â€ vacancies and K in the A-site; Na on M4; Mn, Ti, and Cr on M1-M3; variable amounts of Tschermak substitution; and F and O2ââ,¬â€œ on the hydroxyl site ââ,¬â€ all serve to raise the entropy and lower the Gibbs energy of natural amphiboles. The collective effect of the minor components is what makes amphiboles stable in natural thermodynamic assemblages.

Also see the Rhyolite-MELTS entry in the FAQ on the MELTS website. Anyway, we now have NSF funding to improve and expand the amphibole model (and for F the liquid model) for the MELTS family of algorithms. So watch this space! In the meantime, unfortunately the best we can recommend is to suppress pyroxenes if you are having trouble crystallizing amphibole in MELTS runs.

Paula