Comment
Figure shows the first and second heating processes. There is some time lag between 1st and 2nd ones. References are also different between both cases. Base lines of them are not the same because of deferences in heat capacities. Sulfidation temperature range covers over 305-635℃. Peak at 497℃, second peak at 606℃, ΔH=423J/g.
The enthalpy of formation
　Product is an orthorhombic GeS.　The enthalpy of formation of GeS calculated is ΔHf=10.6Kcal/g. Considerably lower than Ref. Phase transition, at 666℃（heating）, at 654℃ （cooling）, -99.7J/g.