Floppy modes in crystalline and amorphous silicates

1Mineral Physics Group, Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge, CB2 3EQ, United Kingdom 2ISIS Facility, Rutherford Appleton Laboratory, Chilton, Didcot, Oxfordshire, OX11 0QX, United Kingdom 3Department of Engineering Materials, Sheffield University, Mappin Street, Sheffield, S1 3JD, United Kingdo 4Neutron and Condensed Matter Science Branch, AECL Research, Chalk River Laboratories, Chalk River, Ontario, K0J 1J0, Canada 5Cavendish Laboratories, University of Cambridge, Madingley Road, Cambridge, CB3 0HE, United Kingdom (Received 21 August 1996 )