The Crystal Planet: Magnesium Sulfate Birefringence

When photographing Epsom salt, I expected orderly needles and clear glass. Instead, a planetary limb burst from the salt, painting bands of blue and orange above a crystalline terrain.

I grew magnesium sulfate crystals by evaporating a supersaturated solution over 24 hours, using undissolved grains as nucleation sites. Dendritic fans sprawled across the walls of the glass bowl as rapid supersaturation caused branching, while slower supersaturation at the bottom produced long needles. I photographed the crystals through a DIY polariscope—a polarizer crossed at 90° to my LCD monitor’s linearly polarized white light.

Magnesium sulfate is birefringent: It resolves light into two orthogonal rays due to anisotropy that is caused by asymmetric molecular bonding along different axes. With refractive indices of 1.433 and 1.461 (birefringence Δn = 0.028), one ray travels slower through the crystal and accumulates retardation proportional to the crystal’s thickness. At a second polarizer, the rays recombine and interfere because only components along the polarization axis pass through. The resulting color follows the Michel-Lévy chart, where 0.035-mm-thick magnesium sulfate displays vivid orange.

Here, vibrant colors mark thin crystals of low-order interference, and pastel colors indicate increasingly thick crystals of high-order interference. Dark areas reveal crystals whose optical axis is aligned with the LCD polarizer. There, light remains unsplit and is blocked by the second polarizer. Unexpectedly, the colorful arc of the normally isotropic rim of the glass bowl shows photoelasticity because residual manufacturing stress induces anisotropy. Physics paints birefringent crystals and maps the stress in glass. Different materials, through polarized light, create yet another mesmerizing world I never knew existed.