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
