When minerals in polished sections are subjected to heat treatment, they are affected at temperatures which may be considered as characteristic of each mineral.An electric furnace was built small enough to be placed on the stage of a microscope. Through the pyrex window at the top, the polished surface of the mineral may be observed continuously while the temperature rises inside the furnace. The temperature at which a colored film forms on the polished surface may be used to identify the mineral. This heat treatment also brings out a striking contrast between minerals which are otherwise difficult to distinguish under the microscope. This method is very helpful, for mode analysis of ore minerals in polished sections. It is rapid, does not require a perfect polish and permits the determination of very small grains of minerals.