Twinning in Ag2S is observed to form, on heating, well below the 177°C inversion from low-temperature Ag2S, acanthite, to the high-temperature form, argentite. This twinning, premonitory of the inversion, is preserved upon subsequent cooling to room temperature. Therefore, the presence of twinning in Ag2S cannot be used as a minimum-temperature indicator: the high-temperature crystal structure may never have existed.

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