Dear Mr. Alves,

see the physical-chemical constants in:

Vieillard, P., Tardy, Y., Nahon, D., 1979. Stability fields of clays

and alumium phosphates: parageneses in lateritic weathering

of argillaceous phosphatic sediments. American Mineralogist

64, 626–634. Also consult: Stoffregen and Alpers (1987), Schwab et al. (1993, 1996) 

and further literature in:

Dill H. G. 2001. The geology of aluminium phosphates and sulphates of the

alunite group minerals: a review.-Earth-Science Reviews 53, 35–93.

Best regards

H.G.Dill

Professor Dill,

many thanks for answering.

However it's not evident How can aPO4, aAl, Eh or pH influence the variscite-wavellite transition. That means, what happens (or what might happen) in a system where variscite ceases to form and wavellite begins?

Thanks,

Pedro