Pyrite structure

A lightly nontrivial cubic structure.
Seems none of these are are very hard,
but the harder ones with common elements are are not "uselessly" weak and soft for structural applications either.

• Fe₂S Pyrite - 5.0g/ccm - Mohs 6 to 6.5 - optically metallic, electrically conductive, mechanically nonmetallic
• Ni₂S Vaesite - Mohs 4.5 to 5.5 (6.25?) - optically not metallic not transparent

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• Mn₂S Hauerite - 3.46g/ccm - Mohs 4 - optically not metallic not transparent
• Cu₂S Villamanínite (copper_sulfide) - 4.5g/ccm - Mohs 4.75 - maybe optically metallic?
• Co₂S Cattiereite - 4.82g/ccm - Mohs 4 - optically metallic