End members of Cpx solid solution

[Anna Sapegina]

Greetings colleagues!

Is it possible to turn off buffonite and essenite end members of clinopyroxene in rhyolite-MELTs?
In my samples I have a significant amount of Fe³⁺ that provides aegerine existence (it's approved by recalculation of composition). But MELTs places all of the ferric into buffonite and essenite so aegerine is absent.
Aegerine containig is really important for our study.

[asimow]

Anna,

I cannot think of any way to do that. You can turn off cpx, but you can't turn off components of cpx except by leaving their ingredients out of the bulk composition. But even if you remove all the Al and Ti, MELTS can still make a Fe3+-bearing clinopyroxene by linear combination of Buffonite + Essenite ââ,¬â€œ Al-Buffonite. Your best bet is to work around the tendency of MELTS to incorporate a lot of Fe3+ in clinopyroxene by artificially raising the Fe3+ content of your bulk composition until you saturate with an additional phase, perhaps it will be aegirine. You might also artificially inflate the Na content. I realize you aren't then really predicting phase equilibria in your actual target bulk composition, but the resulting model might still give you a guide to trends that are interesting and useful.

-- Paul

[Paula]

Some versions of the MELTS code, including the rhyolite-MELTS GUI, have a separate "aegirine" phase. But this is just a dependent end member (jadeite + buffonite ââ,¬â€œ alumino-buffonite) that was used in testing during the development of the pyroxene solid solution model (see Sack & Ghiorso 1994c, though note that it is referred to as "acmite" there). Aegirine will never crystallize, unless you suppress pyroxene, because the pyroxene solid solution should be more stable for natural compositions.

[There is also sometimes a "fayalite" phase, that should also be ignored.]

What is the aegirine content of the pyroxene that MELTS predicts?

Paula

[Anna Sapegina]

Thank you for answers!

What is the aegirine content of the pyroxene that MELTS predicts?

Now MELTs predicts no aegerine.
As I understand from the answers It's more about features of Cpx solid solution model is used in MELTs code.

I've attached here the Cpx compositions are predicted by MELTs and initial parameters.

Initial parameters:
Initial Composition: SiO2 46,8800
Initial Composition: TiO2 2,0500
Initial Composition: Al2O3 15,3600
Initial Composition: FeO 15,6000
Initial Composition: MgO 6,1800
Initial Composition: CaO 10,7900
Initial Composition: Na2O 2,4700
Initial Temperature: 1400,00
Final Temperature: 900,00
Initial Pressure: 9000,00
Final Pressure: 9000,00
Increment Temperature: 5,00
Increment Pressure: 0,00
dp/dt: 0,00
log fo2 Path: FMQ

Here is a link to file with Cpx compositions: https://dropmefiles.com/xkaHh