While measuring the optical properties of natural “massicot” it was noticed that the plates are made up of two minerals with distinct optical properties, which correspond to those of the two modifications of artificial lead monoxide. The centers of the plates are like the artificial yellow, orthorhombic modification (litharge), while the borders are like the artificial red, tetragonal one. It is proposed that the name massicot be confined to the tetragonal modification, and that the name litharge be applied to the orthorhombic modification, now recognized for the first time as a distinct mineral species.

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