Identification Of Cao-Mgo Orthosilicate Crystals, Including Merwinite (3Cao-Mgo-2Sio2), Through The Use Of Etched Polished Sections

THIS paper describes a technique of polishing and etching specimens of open-hearth furnace slags or hearth aggregates for identification of the crystalline constituents-lime (CaO), tricalcium silicate (3CaO•SiO2), dicalcium silicate (2CaO-SiO2), monticellite (CaO•MgO-SiO2), or forsterite (2MgO•SiO2), with especial emphasis on the mineral merwinite (3CaO-MgO.2SiO2). With proper standardization, this identification does not require the use of the petrographic microscope. The composition of basic open-hearth slags and furnace bottoms falls, almost without exception, within systems containing CaO, MgO, "FeO," MnO and SiO2, in which the number of basic molecules so greatly exceeds the orthosilicate ratio (two molecules of base to one of silica) that free basic oxides, and combinations between them such as aluminates or ferrites, are present in cooled specimens. Orthosilicates of (CaO + MgO) are the most common in such specimens, since in nearly all cases, except premelt slags, the molecular ratio of (CaO + MgO) to SiO2 is more than 2 to r. When sufficient lime is available it combines with the silica to form dicalcium silicate (2CaO•SiO2), which contains little, if any, MgO, FeO or MnO in solid solution whereas the latter oxides combine to form the oxide solid solution known as periclase. If the lime present is insufficient to form dicalcium silicate (2CaO•SiO2) it combines with MgO to form either merwinite (3CaO•MgO.2SiO2) or monticellite (CaO-MgO•SiO2); these minerals take little if any FeO or MnO into solid solution and the remaining MgO, FeO and MnO combine as periclase. This generalization seems to be valid for basic slags and furnace bottoms, since minerals such as CaO-MnO•SiO2 and CaO-FeO-SiO2 are found only in slags in which the lime-silica ratio is less than 2 and are not observed in specimens from furnace bottoms. The identification of crystalline constituents in such materials, especially of fine crystals in the groundmass, is difficult under the petrographic microscope. They are often masked by their neighbors because of their small size in relation to the thickness of the thin section and because of the presence of opaque or colored constituents. The indices of retraction and the optical sign of the mineral are sometimes difficult to determine because of the small size or because of twinning or of inclusions within the crystal. Moreover, the positive identification of merwinite (3CaO•MgO.2SiO2) from its optical properties is usually difficult in the presence of dicalcium silicate (2CaO•SiO2). CaO, MgO, 3CaO•SiO2 and 2CaO•SiO2 in open-hearth slags have been identified for a number of years in the U.S. Steel Corporation Laboratory by the usual