12 January 1973
Vol 179, Issue 4069

About The Cover

Surface of an obsidian sample (X 215), of high silica content; a framework silicate glass in which appsoximately 15 percent of the silica atoms of silica glass has been replaced with aluminium atoms with the charge balancing potassium and sodium ions residing within the open framework (see page 177). [Electron micrograph courtesy of J. Arem and W. Brown, U.S. National Museum of Natural History]