Alizarin crystals: An extreme case of solvent induced morphology change

The organic compound alizarin (1,2-dihydroxy-9,10-anthraquinone) normally crystallizes as very long needles. However, if alcohol is used as solvent, a completely different, triangular shape is obtained. Due to disorder and twinning of the crystals single crystal X-ray diffraction could not be used to establish a possible difference in crystal structure. Differential scanning calorimetry, IR and Raman spectroscopy and powder X-ray diffraction show that the two forms of alizarin are isostructural. From this it follows that solvent influence rather than polymorphism causes the difference in crystal habit. Surface examination, using optical, scanning electron and atomic force microscopy provides information on the mechanism of habit change, which is introduced by the blocking of growth of specific crystal faces in the alcohol solutions.