Platy phlogopite from blast-furnace slags

furnace slags at atmospheric pressure. Its chemistry is characterized byan excess of Na and K over their normal stoichiometric content, which is attributed to a com-posite association between phlogopite and glass. The diffraction pattern indicates that the phlogopite has a preferred orientation around the a-axis. Scanning electron microscopy shows that this phlogopite consists of numerous mall platelets, which probably have their a-axes aligned. PHLOGOPITE mica, with unusual mineralogical features, has been found in some blast-furnace slags from the Port Talbot works of the British Steel Corporation. The chemistry of these slags belongs largely to the quaternary system CaO-MgO A12Os-SiOz, but additional oxides lead to the crystallization of phases outside this system. Blast-furnace slags are cast into approximately hemi-spherical adles 2 m in diameter and allowed to air-cool for two days. Samples were obtained from the coarse-grained interiors of the solid slags. In thin section this mica forms as a pale blue to colourless pleochroic mineral which has extinction parallel to the trace of the (001) cleavage. It possesses econd-order blue interference colours (6 = 0.017), and is negative biaxial with a low 2V. The phlogopite crystallized as a groundmass mineral between melilite phenocrysts, forming as thin tabular crystals (up to 0.75 x 0.30 mm in size). The otherwise hollow vesicles within these slags often contain similar mica crystals. Mineral chemistry The mica chemistry (Table I) was determined with a CSI Microscan 9 electron microprobe; on-line computer corrections for ZAF effects were applied to all raw data. A natural phlogopite was analysed at the same time as these micas to check for analytical deviations. The composition of this natural phlogopite (Table I) remained approxi