Natural boric acid with not more than 85% H₃BO₃ content on a dry-weight basis falls under heading 2528. Material exceeding that purity threshold is considered a chemically defined compound and is classified in the chapter covering inorganic chemicals rather than here as a natural mineral product. Importers should obtain laboratory analysis to confirm purity before declaring the subheading.

Heading 2528 explicitly excludes borates separated from natural brine — those products are classified elsewhere. Only borates derived from solid mineral deposits (ores and concentrates) and natural boric acid meeting the purity limit are covered here. Documentation of the extraction method and source material is important for correct classification.