SUMMARY. The system sanbornite-celsian has been investigated by the quenching method, and the results compared with earlier studies. The system is essentially a simple binary eutectic system with little or no solid solution. The eutectic is located at a composition of 69 % sanbornite and 3I % celsian (by weight) and at a temperature of IZ27~ 3 ~ No evidence for substantial solid solution of celsian in sanbornite, as reported in previous work, was found. Paracelsian, a naturally occurring polymorph of celsian, was not encountered in the study.THE compound BaSi205 occurs in nature as the rare mineral, sanbornite. The compound BaAlzSi~Os is the principal component of the barium-feldspar, celsian. Both compounds are important constituents of synthetic products, including fluorescent phosphors, barium crown glasses, electrical porcelains, refractories, and so-called glass-ceramics. Their phase relations with each other, as well as with other compounds in the BaO-A12Oa-SiO2 system, are thus of considerable mineralogical and technological interest. This study is the third of a series included in a comprehensive reinvestigation of the BaO-A1203-SiO ~ system. Previous reports in the series dealt with the polymorphism of celsian (Lin and Foster, 1968) and the binary system celsiansilica (Foster and Lin, i969).Previous work. Bowen (I918) determined the melting point of pure BaSi205 as I426 ~ Rogers (1932) believed that the polysynthetic twinning of natural sanbornite indicated polymorphism. Klasens, Hoekstra, and Cox (1957) and Roth and Levin (1959) detected an inversion in synthetic sanbornite at about 135o ~ Lin and Foster (1968) reviewed the latest findings on the melting and polymorphism of pure synthetic celsian. They selected 176o ~ as the melting temperature of hexacelsian, and 159 ~ ~ as the inversion temperature of celsian to hexacelsian.The system sanbornite-celsian has not previously been systematically studied. It was included however, in several general investigations of the BaO-A120~-SiO ~ system (Thomas, 195o; Toropov, Galakhov, and Bondar, 1954, I955). That by Thomas presented a theoretically possible diagram for the system, based on limited experimental data. Toropov and his co-workers (1955) gave no direct plot for the sanbornite-celsian join. However, their traverses in the ternary system at SiO2-1evels of 36, 39, and 42 % permit the deduction of a sanbornite-celsian diagram that closely resembles that of Thomas. The approximate relations deduced from these previous Present address: