Thorium triiodide is a black, violet-tinged, usually poorly crystallized mass. Formed crystals exhibit strong
dichroism from violet to olive green under the microscope and are
birefringent. Above 550 °C, thorium(III) iodide decomposes to thorium(IV) iodide and thorium(II) iodide.
β-thorium(III) iodide has an
orthorhombic crystal structure with the space group
Cccm (space group no. 66). The
α-form has a pseudoorthorhombic crystal structure. ==Chemical properties==