A kinetic study of clinochlore and its high temperature equivalent forsterite-cordierite-spinel at 2 kbar water pressure

Clinochlore and its higher temperature equivalent assemblage, forsterite-cordierite-spinel-H₂O have been synthesized at 2 kbar water pressure, using oxide mixtures of clinochlore composition as starting material. A time-temperature-transformation (TTT) diagram has been established in the range 600-750°C for periods up to 250 days. Following Ostwald’s rule, the final stable phase assemblages (14 Å chlorite or forsterite-cordierite-spinel) are synthesized only through the prior formation of metastable assemblages 7 Å chlorite-forsterite-oxide mix, 7 Å chlorite-forsterite-talc-oxide mix or talc-forsterite-spinel. From the TTT diagram, the apparent activation energy for the heterogeneous nucleation of the 14 Å chlorite from 7 Å chlorite is calculated as kcal/mole. Rate studies employing the parabolic rate equation x = ktⁿ indicate that the activation energy for this transformation changes from 90±35 kcal/mole below 670°C, to kcal/mole between 670 and 691°C. Such high values suggest that the polymorphic transition of chlorite involves the rearrangement of Si-O bonds which might be the rate-determining process. The higher activation energy above 670°C can be attributed to the “stable” coexistence of forsterite and talc with 14 Å chlorite, which is more aluminous than clinochlore. The most stable Mg-chlorite has the approximate composition (Mg_4.89Al_1.11)(Si_2.89A1_1.11)O₁₀(OH)₈.