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 (Cu2S) 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 (Cu1.96S), 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.