We made the following statement in this paper in Mineralogical Magazine, 85(2), 125–131 under the heading ‘Extrinsic properties of a mineral’ on page 128:

“What are extrinsic properties of a mineral? Refractive index (dependent on the wavelength of light illuminating the crystal), density (dependent on the value of gravitational acceleration at the location of the mineral), unit-cell volume (dependent on ambient temperature and pressure).”

The word “density” should be replaced by the word “weight”; this example is given correctly on page 127.

Acknowledgements: We thank our numerous colleagues who drew our attention to this miserable lapse in concentration.

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