These are illustrations of the experiment by Denis Andrault and co-workers. Top: the sample (orange) is placed on the tip of a conically shaped diamond, surrounded by a metallic gasket (grey). The tip of a second diamond is pressed against the sample, creating a pressure that can exceed 1 million atmospheres. The light of two powerful infrared lasers is focused on the sample held under pressure, producing temperatures of 4000K and more, inducing partial melting of the rock. Bottom: The X-ray beam at the ESRF illuminates the sample with a resolution of about a micrometre. The sample is scanned perpendicular to the axis of the X-ray beam. By recording diffraction patterns (at the bottom right of the cell) and fluorescence spectra (beneath the cell) at many different sample locations, it is possible to determine where the sample was molten and not. The measurements also enable determination of the sample mineralogy as well as the concentration of various elements, notably of iron which defines primarily the density difference between the solid and liquid fractions.