X-ray diffraction measurements at high pressures and high temperatures revealed that Si clathrate Ba8Si46 is formed by a solid-phase reaction of an 8:30 molar mixture of SrSi2-phase BaSi2 and Si after BaSi2 undergoes the BaSi(2-)to-EuGe2 and the EuGe2-to-SrSi2 transitions. The volume reduction during the formation of Ba8Si46 is the largest, 7.6%, among the observed transitions. On the other hand, an 8:30 molar mixture of SrSi2-phase SrSi2 and Si does not result in the formation of Sr8Si46 at high pressures and high temperatures; only SrSi2 transforms from the SrSi2 phase into the alpha-ThSi2 phase, and Si remains in the diamond phase.