Kyanite Andalusite Sillimanite phase diagram
The three Al2SiO5 polymorphs - Kyanite, Andalusite and Sillimanite - help estimate the pressure and temperature of metamorphism.
Kyanite indicates rather high pressure.
Andalusite indicates low temperature and pressure.
Sillimanite indicates high temperature.
Pressure and temperature are estimated more accurately when two of these minerals are present in the same rock.
More on this subject:
Richardson (1969): 620 °C, 6000 bars
Holdaway (1971): 501 °C, 3800 bars
Bohen (1991): 530 ± 20 °C, 4.2 ± 0.3 kbar
Richardson, S. W., Gilbert, M. C. and Bell, P. M. (1969) Experimental determination of the kyanite-andalusite and andalusite-sillimanite equilibria; the aluminia silicate triple point. American Journal of Science, 267.259-272.
Holdaway, M. J. (1971) Stability of andalusite and the alumino-silicate phase diagram. American Joumal of Science, 271,97-131
Steven R. Bohlen, Art Montana, and Derrill M. Kerrick (1991) Precise determinations of the equilibria kyanite <--> sillimanite and kyanite <--> andalusite and a revised triple point for Al2SiO5 polymorphs, American Mineralogist, April 1991, v. 76, p. 677-680.