Experimentelle Gesteinsmetamorphose—VII Die Bildung von Talk aus kieseligem Dolomit

Abstract

Mixtures of dolomite, quartz and various amounts of water have been subjected to experimental metamorphism in the temperature range from 250° to 550°C under constant total gas-pressure of 2000 bars. The reaction, 3CaMg(CO 3) 2 + 4SiO 2 + H 2O ⇋ Mg 3[(OH) 2Si 4O 10] + 3CaCO 3 + 3CO 2 , leads to the formation of talc, calcite and CO 2. Only if the developed CO 2 is steadily allowed to leave the system the temperature of reaction is 300°C at 2000 bars H 2O-pressure. However, caloric data and phase rule considerations suggest that this reaction is endothermic and bivariant in the P-T-region where supercritical gas mixtures of CO 2 and H 2O are present. This has been ascertained by experiments carried out in the temperature range from 440° to 510°C. For this temperature range approximate values for the equilibrium constant could be worked out after fugacities of CO 2-H 2O-gas mixtures have been taken into consideration. The bivariancy of the reaction implies that the equilibrium for the formation of talc + calcite + CO 2 from dolomite + quartz + H 2O has not a fixed temperature even if the total gas-pressure is fixed. At 2000 bars total gas-pressure talc and calcite are formed at a temperature somewhere between 300° and 530°C; the temperature being the higher the greater the partial-pressure of CO 2 relative to the partial-pressure of H 2O. Thus, in an extreme case, dolomite + quartz + H 2O can be preserved and does not react until 530°C; but this has not boon the case in rock metamorphism. From the experiments it seems probable that talc could have been formed from dolomite 4-quartz already during high-grade diagenesis, ⩾ 300°C, if CO 2 could have left the system completely. In rock metamorphism the reaction has taken place (a) either at the temperature of the beginning of the greonschist facis (about 400 ±20°C at 2000 bars) or (b) at higher temperatures of the quartz-albite-epidote-biotite subfacies of the greenschist facies. This last occurrence proves that during rock metamorphism a rather high fugacity of CO 2 relative to the fugacity of H 2O can have been present in the pores of the rocks (weight-ratio CO 2:H 2O has been about 2:1). Talc + calcite + quartz react to form diopsido + CO 2 + H 2O. This also is a bivariant reaction which is at present investigated.