The more that physicists and chemists look at water, the more they find that it really is pretty mysterious stuff. For example, magnetic fields can change the rate of scale formation in boilers - a process that is not really understood and is somewhat controversial, even though it has found industrial application. Simpler systems would be easier to study and this is just what Ichiro Otsuka and Sumio Ozeki of Shinshu University in Japan have done. They have found that while a magnetic field does not change the physical properties of pure water, it does change the vibrational spectrum and electrolytic potential of water with dissolved oxygen.
The team also found that magnetic treatment of aerated water affected the formation of calcite (CaCO3). While magnetic treatment had no effect on CaCO3 crystal formation when they used vacuum-distilled water, when the water was exposed to air or oxygen (O2) both X-ray diffraction images and scanning electron microscope (SEM) images of the crystals showed marked differences - indicating in particular the formation of aragonite. The team concludes that only when water dissolves O2, not ionic species, is the water magnetized by changing the magnetic flux over the water, but admits that "the phenomena are still puzzling from the viewpoint of modern science".
Further reading
Ichiro Otsuka and Sumio Ozeki 2006 J. Phys. Chem. B 110 1509.