The dividing line is mineralogy: heading 2507 is reserved for clays whose dominant mineral is kaolinite. Clays with a different dominant mineral — smectite-group clays like bentonite, illitic ball clays, or refractory fire clays — fall under heading 2508. A mineralogical analysis report is the standard documentation used to support the classification when the clay type is not self-evident from the commercial description.

These three aluminosilicate polymorphs (subheading 250850) are critical raw materials for high-alumina refractories used in steel furnaces, glass tanks, and kilns. They convert to mullite at high temperatures, providing dimensional stability. Importers in the refractory sector should note that South Africa is a significant global producer of andalusite, and supply-chain concentration in these minerals can affect refractory production planning.