A fast and accurate technique has been developed for the determination of the total volatile content of rocks. The loss on fusion (LOF) results are comparable to the conventional time-consuming wet-chemical procedure but require no additional effort or cost when conducted as part of the usual sample preparation procedure for quantitative X-ray fluorescence spectrometric analysis (XRF). The technique utilizes the weight loss of the rock when fused with a suitable blend of lithium tetraborate and lithium nitrate. A simple mass balance model is used to represent the thermogravimetric results and to identify the nature of the high temperature mass alteration reactions taking place in the fusion melt. Under the sample preparation conditions used in this study, sulfide appears to be retained quantitatively in the fusion disc sulfate, a phenomenon permitting the determination of sulfide in prepared by a conventional fusion procedure in the form of
rock samples.