ternary diagrams with contours

Dear R-listers,

I would like to ask if it is possible to create ternary diagrams with
contours. I want to plot a property that depends on a ternary mixture
composition, so I think this would be the best way. I found a similar
question in a past R-help list message but there was no answer.

In case it is not possible to create ternary plots with contours, can anyone
suggest an alternative way to plot a property that depends on mixture
composition? Even suggestions for alternative, preferably free, software are
wellcome.

Thank you very much in advance.

Regards,

Kyriakos.