400 METAMORPHIC ROCKS/Classification, Nomenclature and Formation Figure 10 The chemical system applied to metapelites is identical with the AKF system However, an essential difference has to be taken into account: pelites plot in the Al rich part of the AKF triangle (Figure 9) and are mainly capable of producing Fe Mg Al silicates plotted at the AF line This rock composition probably contains the most key (or index ) minerals with changing metamorphic grade In order to show the reaction sequence in terms of the Fe Mg Al silicates, it is necessary to consider FeO and MgO as separate components Thus these four components build up an AKFM tetrahedron and for practical use the AFM plane (projected from muscovite or K feldspar) is used for graphical presentation of the mineral phases Figure 11 Possible mineral assemblages in metacarbonates with increasing metamorphic grade H2O-rich pore fluid Quartz has to be added to enable mineral reactions of the carbonates Hence the relevant chemical system comprises CaO–MgO–SiO2– CO2–H2O (Figure 11) The temperature of incoming or breakdown of diagnostic assemblages in the carbonate system is dependent on the fluid composition (XCO2) and a T-XCO2 plot is usually applied to show this effect (Figure 4) The prograde sequence of metamorphic index minerals in calcite–dolomite–quartz marbles affected by an intermediate P/T-gradient is generally: talc, tremolite, diopside, forsterite, and wollastonite The maximum transition-temperature from talcỵcalcite to tremolite is about 500 C The maximum stability of tremoliteỵcalcite is ca 700 C The first appearance of diopside is at ca 600 C when it is replacing the mineral assemblage tremolite ỵ calcite ỵ quartz At ca 800 C, diopside þ dolomite breaks down to forsterite At low P/T-gradients,