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These materials are siliceous earths formed of small fossilised organisms (diatoms, etc.) and are very light. Their "apparent specific gravity", which must not exceed 1, is to be taken as their effective weight in kg/1,000 cm3, uncompressed, in the form in which they are presented.
The principal siliceous earths are kieselguhr, tripolite, diatomite and moler earth. Although certain earths classified here are sometimes referred to as "tripoli", they must not be confused with the true tripoli known as "rotten‑stone", which, since it results from the natural decomposition of certain rocks, is not diatomaceous. This latter product, which is used as a mild abrasive for polishing, falls in heading 25.13.
The various earths of this heading are sometimes incorrectly called "infusorial earths".
Most of these earths are used for the manufacture of heat‑resisting or heat‑ or sound‑insulating articles of heading 68.06 or 69.01. Thus, sawn blocks of diatomite fall in heading 68.06, if they have not been fired; otherwise, they are classified in heading 69.01.
Some of the products of this heading are used as abrasive or polishing powders.
This heading excludes activated diatomite, e.g., diatomite calcined with sintering agents such as sodium chloride or sodium carbonate (heading 38.02). On the other hand, diatomite calcined (without the addition of other products) in order to eliminate impurities or washed for that purpose in acid, without altering the structure of the product, remains in this heading.
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