Single crystals of natural rhodochrosite show a large phonon-driven thermal expansivity along the trigonal $c$ axis $({\alpha }_{\parallel }=\frac{1}{c}{\left(\frac{\partial c}{\partial T}\right)}_p=1.53\times {10}^{-5}{\text{K}}^{-1}\text{at}T=120\text{K})$ and a negligible expansivity perpendicular to it $({\alpha }_{\perp }=\frac{1}{a}{\left(\frac{\partial a}{\partial T}\right)}_p=0.057\times {10}^{-5}{\text{K}}^{-1}\text{at}T=120\text{K})$. Impurities in minor concentrations, particularly calcium, seem to be responsible for this one-dimensional thermal expansivity. Interestingly, at lower temperatures ($T\lesssim$ 50 K) where the magnetic interactions switch on, ${\alpha }_{\perp }$ awakens and gives rise to an anisotropic three-dimensional magnetic thermal expansivity, most probably associated with a prominent isotropic magnetoelastic coupling. This is confirmed by magnetostriction experiments. Our results show that expansivity can be easily fine-tuned (from positive to negative) by small amounts of impurities naturally present in carbonate minerals, in contrast with much more sophisticated methods used in functional materials.

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• Magnetic order
• Thermal expansion