geothermobarometry

Geothermobarometry is employed to determine the pressures and temperatures operating during the formation of particular mineral assemblages during the genesis of igneous and metamorphic rocks.
image at left - click to enlarge - stability fields of common pelitic minerals: 1= chlorites + chloritoid 2= chlorites 3= chlorites + staurolite + garnets 4= chlorites + garnets 5= andalusite + staurolite + garnets 6= sillimanite + staurolite + garnets 7= kyanite + staurolite + garnets 8= kyanite + garnets 9= andalusite + garnets 10= sillimanite + garnets 11= k-spars + andalusite + garnets 12= sillimanite + k-spars + garnets 13= andalusite + k-spars + cordierite + garnets 14= sillimanite + k-spars + cordierite + garnets Compare the mineral stability field chart above with chart of metamorphic facies & chart of trajectories of metamorphic facies (on different scale). Andalusite is a polymorph of the other aluminosilicate minerals kyanite and sillimanite. Cordierite or iolite is a magnesium iron aluminium cyclosilicate that is particularly common in hornfels produced by contact metamorphism of pelitic rocks. Determinations rely upon: the phenomenon whereby the composition of particular minerals alters under the agencies of temperature and pressure. minerals may be influenced by their growing in contact with other minerals.
Barrovian regional metamorphic zones are regions defined by reactions in which minerals appear or disappear, and these mineral zones can be mapped as isograds. The A(K)FM diagram is used to describe the phase relations of pelites by assuming that excess SiO2, H2O are present, and that a plagioclase (identity unimportant) is also stable . The diagram results in a tetrahedron where A=Al2O3, K=K2O, F=FeO, M=MgO
Barrovian zone	mineral assemblage
chlorite zone	chlorite + mus + qtz + H2O + relict minerals
biotite zone	chlorite + biotite + mus + qtz + H2O
garnet zone	chlorite + biotite + garnet + mus + qtz + H2O
staurolite zone	staurolite + 2 AKFM phases + mus + qtz + H2O
kyanite zone	kyanite + 2 AKFM phases + mus + qtz + H2O
sillimanite zone	garnet + biotite + sillimanite + mus + qtz + H2O
2nd sillimanite zone, or sillimanite + orthoclase zone	sill + or + qtz + H2O + melt and no mus
Examples/ Links: Ti-in biotite geothermometer, Henry et al. 2005 Fe-Mg exchange between garnet-biotite Ca-in-garnet webpage: Pelitic Rocks