[alutar19999]

Having trouble getting my head around these. For the bulk composition at the point labelled '1', at 1400°c, how would i get the total solid/liquid composition? And the proportion of minerals to melt? (Assuming equilibrium crystallisation)  Any helpful links would be appreciated, thankyou!

http://imgur.com/ZnjpPDo

[mjc123]

Seen this?

http://www.tulane.edu/~sanelson/eens212/ternaryphdiag.htm

[alutar19999]

I used that to help me get some of it.

In this image, I understand how 34% melt is calculated. But Im not sure how the specific %s for En and Fo (43 and 23%) are calculated.

http://imgur.com/1YyZ45T

[mjc123]

In principle, as for the simple TPD, by the lever rule. Project the line AB to C. Liquid and solid are in the ratio BC:AB, and in the solid Fo and En are in the ratio CEn:FoC. (Note that the composition of En is not where the line PA meets the axis, but at Fo₇₀Qz₃₀. It's one of those compound-melts-non-congruently cases.)
Alternatively, knowing the total composition, you can work out algebraically what it must be; thus if you have x% solid Fo
Fo = 34*0.38 + x + (66-x)*0.7 = 66
Qz = 34*0.27 + (66-x)*0.3 = 22
Both equations give x = 23
I have printed out the diagram in large size and measured the lines and I think it is badly drawn; I got (different) wrong answers by different methods at first. To get the given answer, I think the initial point B has to be at Fo₆₆An₁₂Qz₂₂, rather than Fo₆₅An₁₂Qz₂₃ as it appears to be, placing C at Fo₈₀Qz₂₀  rather than Fo₇₈Qz₂₂.