Thermal Effect in Opal below Room Temperature

• Published in: Proceedings of the National Academy of Sciences - PNAS Vol. 69; no. 11; pp. 3225 - 3227
• Main Authors: Buerger, Martin J., Shoemaker, Gerald L.
• Format: Journal Article
• Language: English
• Published: United States National Academy of Sciences of the United States of America 01-11-1972; National Acad Sciences
• Subjects: Ambient temperature; Cooling; Crystallites; Impurities; Moisture content; Opal; Physical Sciences: Geology; Room temperature; Temperature effects; Water pollution; Water quality
• ISSN: 0027-8424; 1091-6490
• DOI: 10.1073/pnas.69.11.3225

Opal, once believed to be amorphous silica, was shown by Levin and Ott (1932, J. Amer. Chem. Soc. 54, 828-829) to give an x-ray powder pattern of the high-temperature form of cristobalite. The early explanation of this anomalous existence of a phase below its high-low transition temperature is now known to be untenable. One of us suggested that the tiny sizes of the component cristobalite crystals might explain the anomaly; if so, the transition might be expected below ambient temperatures. The record of a du Pont 900 Thermoanalyzer indeed revealed heat effects in opal below ambient temperatures, with an exotherm having a maximum at about -40 degrees on cooling and an endotherm that began about -50 degrees on heating. This was not a latent-heat effect due to the high-low transition of cristobalite, however, for the low-cristobalite pattern persisted to below -50 degrees. Opal normally contains 4-9% water, which is tenaciously held; water loss is nearly linear with temperatures up to about 422 degrees, when water loss is abruptly complete. Water-free opal does not display the thermal effect, but the same opal rehydrated does display it. Water is housed in minute voids, judged to be a few hundred angstrom Angstroms across, between minute particles of cristobalite. This water behaves differently from water in bulk, for its begins to melt at about -50 degrees.