2508 - Other clays (not including expanded clays of heading 6806), andalusite, kyanite and sillimanite, whether or not calcined; mullite; chamotte or dinas earths:

Heading 2508 encompasses a variety of non-expanded clays and related mineral substances that do not fall into more specific mineral headings. This category is primarily for clays that are not exclusively used as pigments (covered in other chapters) or as raw materials for specific manufactured products like ceramics (often classified further down the HTS). It includes naturally occurring clays such as bentonite and fire-clay, as well as valuable aluminosilicate minerals like andalusite, kyanite, and sillimanite, which are known for their refractory properties. Calcined forms of these materials are also included, indicating processing that has altered their physical or chemical characteristics for industrial use.

This classification is distinct from other headings within Section V (Mineral Products) and Chapter 25. For instance, while it deals with earthy materials, it is separate from headings covering construction materials like lime and cement (e.g., 2522 and 2523, respectively). It also differs from headings that cover manufactured articles of clay, such as those found in Chapter 69. The focus here is on the raw or semi-processed mineral substance itself, before it is extensively shaped or combined into finished goods.

Within heading 2508, further subheadings provide granular classification based on the specific type of clay or mineral. These subdivisions allow for precise identification of materials such as bentonite (2508.10), fire-clay (2508.30), andalusite, kyanite, and sillimanite (2508.50), as well as the specifically defined refractory materials of mullite (2508.60) and chamotte or dinas earth (2508.70). The "Other clays" subheading (2508.40) serves as a residual category for clays that do not meet the specific criteria of the other subheadings within this heading, requiring careful analysis of the material's composition and intended use to ensure accurate classification.