Hello again,

As far as I know the Maskelynite in meteorites is the result of shock processes acting on feldspar (e.g., P-T-t conditions) . Stoffler says, in:

http://www.lpi.usra.edu/meetings/lpsc2000/pdf/1170.pdf

that the pressure is below 45 GPa for its formation, and that it forms from rapid disordering of plagioclase feldspar by impact (P ~35 GPa, T ~200-600 C, t ~ 1-2 seconds). Others seem to state that this is a quenched melt from shock (e.g., Chen and El Goresy):

http://www.sciencedirect.com/science/article/pii/S0012821X00001308

It looks like there was quite a debate a on this material a bit more than a decade ago (diaplectic glass vs. remnants of a shock melt). There's some hint of resolution in:

Sharp, T. G., & DeCarli, P. S. (2006). Shock effects in meteorites. Meteorites and the early solar system II, 653-677.

if you can get your hands on this reference.

I am afraid you have to look at the shock literature for silicates and the equations of states of high-pressure silicate glass and then try to infer formation conditions.

Thanks for your answers. Actually, i`m familiar with the shock literature. Sharp and DeCarli, believe that maskelynite pressure stability has been overestimated ~2 orders of magnitude. But it is a really common mineral in meteorites and several papers have based their shock results on the presence of maskelynite; which is not correct. But, my question is, what is the abdolute true? What are the exact (?) P-T boundaries for this mineral?

I think we did see a glass of potassium feldspar-like composition in some fulgurites we analyzed. This material was isotropic with a feldspar composition, yet only required rapid heating, and not necessarily the corresponding increase in pressure. I'm not sure that would qualify as a maskelynite.

I think the SiO2 P-T-t diagram is better known than maskelynite?

Hard to estimate conditions for a material that is likely not in stable equilibrium.

It can be done through shock experiments. The closest analog I know of is for CaCO3:

http://www.sciencedirect.com/science/article/pii/S0031920101002680

It's not a trivial experiment, but it is repeatable. Effectively you get a P-T-t diagram.