Proto‐Djurleite, a metastable form of cuprous sulphide

In a study of thin single crystals of cuprous sulphide we found that the high‐temperature hexagonal form of chalcosite (Cu 2 S) can be metastable with respect both to temperature (supercooling) and to composition. In reversible equilibrium with the metastable chalcosite a second metastable phase is formed, resembling djurleite (Cu 1.96 S), which we propose to call proto‐djurleite. When cooled to a critical temperature the crystal suddenly reverts to a mixture of the stable low‐temperature forms of chalcosite and djurleite. The boundaries of the metastable two‐phase region have been determined between its upper temperature limit, which is 93 °C, and 75 °C.