After a short history of the discovery of the incommensurate phase of quartz, its properties are briefly presented. New results obtained mainly by birefringence measurements are presented on various irreversible phenomena (global hysteresis, time dependent pinning, and memory effect) which show the great influence of chemical impurities on the properties of the inc. phase of quartz. A simultaneous measurement by birefringence and γ‐ray diffraction shows that these effects are produced by defect pinning of the inc. modulation.

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