High-pressure X-ray diffraction and electron-microscopy experiments show that fayalite (Fe2SiO4) transforms at around 350 kbar to an amorphous phase that can be quenched down to 1 bar. This room-temperature vitrification represents a thermodynamically driven fusion to a glass, as implied by the existence of a maximum above 100 kbar in the metastable extension of the melting curve of fayalite. The diffraction measurements, a thermodynamic analysis of the melting curve of fayalite, and the difference in bulk modulus between fayalite, and the difference in buik modulus between fayalite and Fe2SiO4 liquid are consistent with a reversible switch of silicon in the amorphous phase from a high coordination number at pressure down to 4-fold coordination at 1 bar. ¿ American Geophysical Union 1990