Rev. Rul. 56-582

The word `lime' as used in the penultimate paragraph of Revenue Ruling 55-700, C.B. 1955-2, 569, means calcium oxide (CaO) manufactured by calcination of calcium carbonate (CaCo 3 ). Since such calcination is a chemical process, any natural deposit, including dolomite, which contains calcium carbonate and is used, or sold for use, by the mine owner or operator for manufacturing calcium oxide (lime) by calcination is `chemical grade limestone' for percentage depletion purposes. Accordingly, dolomite so used or sold is within this classification, regardless of the purpose for which the lime thus manufactured from it is used or sold, including, for instance, soil treatment; whereas uncalcined dolomite used or sold for use for soil treatment or manufacture (without calcination) of fertilizers is not `chemical grade limestone' within such classification.