  The crystalline structure of minerals due to the highly ordered assembly of
its constituent atoms, ions or molecules confers a considerable hardness and
brittleness to the materials. As a result, they are generally subject to
fracture. Here we report that microcrystals of natural inorganic salt hydrates
such as sodium sulfate decahydrate (Na2SO4.10H2O) and magnesium sulfate
hexahydrate (MgSO4.6H2O) can behave remarkably differently: instead of having a
defined faceted geometrical shape and being hard or brittle, they lose their
facets and become soft and deformable when in contact with their saturated salt
solution at their deliquescence point. As a result, the hydrated crystals
simultaneously behaves as crystalline and liquid-like. We show that the
observed elasticity is a consequence of a trade-off between the excess
capillary energy introduced by the deformation from the equilibrium shape and
viscous flow over the surface of the microcrystals. This surprising, unusual
mechanical properties reported here reveals the role of the water molecules
present in the crystalline structure. Although many compounds can incorporate
water molecules in their crystalline frameworks, the relationship between the
different hydrates and anhydrate crystal forms are still poorly investigated.
Our results on the floppy behaviour of such crystalline structure reveals some
unexplored properties of water molecules entrapped in the crystalline structure
and can open novel routes for their application in various fields such as
pharmaceutical sciences, thermal energy storage and even the traceability of
water on Mars.
