Nickel sulphide stones are known to be at the origin of delayed fracture in tempered glasses due to phase transformation within the inclusion. It is generally reported that inclusions found in glasses display several phases but only inclusions containing a-NiS phase provoke delayed fracture. This study brings new insight into stone composition through chemical analysis & microstructural characterization carried out on 22 stones collected from glass fractured on building or after Heat Soak Test. This characterization was carried out using SEM, together with EPMA. According to EPMA, the average composition varies between 50.10 & 52.7at.% in Sulphur. The majority of inclusions contain some Fe impurities about 0.2at.%. The iron content varies between 0.11at.% & 1.62at.%. SEM & optical microscopy reveal 3 types of microstructres depending on the measured composition range. This study points out that delayed fracture, during HST and on building, is not only due to the stoichiometric NiS inclusions, but also to overstoichiometric ones. The origin of the composition range in natural stones is discussed with the help of the phase diagram.
