Differential thermal and X-ray analysis of opal

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. © 1963 Nature Publishing Group.