Opal-C

A variety of Opal

A class of opal consisting of a localized order of α-Cristobalite with a large amount of stacking disorder.

Opal-C shows considerably more short-range order than opal-CT in that the diffraction pattern does not have as broad a hump and the satellite characteristics are weak or generally absent. In addition, there may be as many as 8 recognizable peaks in the diffraction pattern. Although the peaks are very similar to the pattern for crystalline cristobalite, true long-range order has not yet been attained, and the role of the water is still critical to the structure. The fraction of material in ordered domains is higher than for opal-CT, but it is probably less than 50%. Tests other than X-ray diffraction may be necessary to distinguish opal-C from cristobalite. Terms such as "cristobalite-like" and ‘tridymite-like" are often applied to opal-C and opal-CT may be misleading because they are interpreted by readers to imply a true two-phase crystalline–non-crystalline system which is probably not true. This terminology is not to be recommended. Also the term micro-crystalline should not be applied to this material until evidence is found to show that the ordered domains are free of water in any form.