期刊名称:Proceedings of the National Academy of Sciences
印刷版ISSN:0027-8424
电子版ISSN:1091-6490
出版年度:1972
卷号:69
期号:11
页码:3225-3227
DOI:10.1073/pnas.69.11.3225
语种:English
出版社:The National Academy of Sciences of the United States of America
摘要: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
关键词:phase transformation ; cristobalite ; freezing ; melting ; water II