FEG-TEM image of thinned edge of a cristobalite crystal from volcanic ash sample MRA5/6/99. The platinum (Pt) strip shows as dark grey on the right hand side of the image with the mottled area being attributed to crystal thinning or background scattering from the Pt strip. The dashed black line represents the approximate boundary between the crystal and the strip. Superimposed on the image are the EDS results from a 50 nm transect from rim towards the core of the crystal. Pt = dark blue; Si = red; Al = green; K = pink; Fe = turquoise; Cl = light blue; O = yellow.