Magnesium silicide (Mg 2 Si) undergoes a transformation from a cubic (fluorite) type lattice to a hexagonal lattice at pressures above about 25 kilobars and temperatures above 900°C. There is an increase of density of about 15 percent associated with this structure change. Both the parent and product substances are semiconductors, but of different resistivities. The high pressure phase may be recovered and is indefinitely metastable at normal ambient conditions.