Abstract
PUBLISHED differential thermal analysis curves of opal show a marked rounded endotherm with the peak near 130° C, although one example shows a very small peak at this temperature1. Curves we have obtained from the differential thermal analysis of some thirty naturally occurring opaline silicas can be broadly classified into three groups with: (1) a very small, or no apparent endotherm between 100° C and 200° C; (2) a prominent rounded endotherm starting at about 90°, with a peak temperature in the range 125°–140° C; (3) a strong, sharp endotherm, starting at 90° C, with a peak temperature of 140° C.
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References
Schrämli, W., and Becker, F., Ber. deutsch. keram. Ges., 37, 227 (1960).
Flörke, O. W., Ber. deutsch. keram. Ges., 32, 369 (1955).
An extensive bibliography on opal is given in Dana's The System of Mineralogy, 3, by Frondel, C. (John Wiley and Sons, Inc., New York, 1962).
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JONES, J., SEGNIT, E. & NICKSON, N. Differential Thermal and X-Ray Analysis of Opal. Nature 198, 1191 (1963). https://doi.org/10.1038/1981191a0
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DOI: https://doi.org/10.1038/1981191a0
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