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In the system $MgO-SiO_2-H_2O$, Talc[$Mg_3Si_4O_{10}(OH)_2$] has been synthesized hydrothermally at 200 MPa, $600^{\circ}C$ from the oxide mixture of the bulk composition of talc. The oxide mixture of the bulk composition of anthophyllite$[Mg_7Si_8O_{22}(OH)2]...

In the system $MgO-SiO_2-H_2O$, Talc[$Mg_3Si_4O_{10}(OH)_2$] has been synthesized hydrothermally at 200 MPa, $600^{\circ}C$ from the oxide mixture of the bulk composition of talc. The oxide mixture of the bulk composition of anthophyllite$[Mg_7Si_8O_{22}(OH)2]$ converted to talc, enstatite $(MgSiO_3)$, quartz at 200 MPa, $750^{\circ}C$ with excess of $H_2O$. In low to medium pressure metramorphism, enstatite-talc assemblage is metastable relative to anthophyllite with the reaction talc + 4 enstatite=anthophyllite (Greenwood, 1963). The high temperature stability of talc is bounded with the dehydration reaction to anthophyllite rather than that to enstatite(Greenwood, 1963; Chernosky et al., 1985). Therefore our experiment result assemblage, enstatite-talc-quatz at 200 MPa, $750^{\circ}C$ from oxide mixture of bulk compostion of anthophyllite is metastable assemblage. The hydrothermal experiment performed at 41 to 243 MPa, 680 to $760^{\circ}C$ with the starting material composed of synthetic talc, enstatite and quartz. Talc or enstatite grows during the runs and no extra phases including anthophyllite nucleated. Based on the increase or decrease of the each phase from run products, one of the possible reactions is talc=3 enstatite+quartz+H_2O$. The reversal bracket of the reaction is 699 to $700^{\circ}C$ at 100 MPa. Talc is stable up to $740^{\circ}C$ at 200 MPa and enstatite grow at $680^{\circ}C$, 40 MPa and at $760^{\circ}C$, 250 MPa. Though the high temperature limit of talc around 200 MPa is bounded thermodynamically by the reaction, 7 talc=3 anthophyllite+4 quartz+4 H_2O$, talc persisted throughout the previous reaction up to the reaction, talc=3 enstatite+quartz+$H_2O$.

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