The hydrothermal reactions of sodium aluminosilicate gels have been investigated between 160" and 450". Crystal growth has been examined in absence and in presence of additional sodium compounds (NaCl, Na,SO,, Na,CO,, and KaOH) in the mother-liquors. Gel compositions were in the range Na,O,Al,O,, 1-12Si0,.The conditions employed gave rise to a fclspar (albite) ; zeolites (analcite and mordenite) ; a mica (paragonite) ; felspathoids (nepheline, sodalite, nosean, and cancrinite), and a-quartz. Two hydrated phases of nepheline composition were synthesised, and also a range of crystalline phases related to the cancrinite, nosean, and sodalite structures, but without the usual interstitial salts. Rather large laminar crystals, of an apparently new hydrated silicate species, were formed in some preparations, and also large crystals of sodium silicate. Details of two new species obtained in minor yields are recorded.All the compounds have been characterised by optical and X-ray methods, which in some cases reveal interesting differences between synthetic and naturally occurring forms of the mineral, Some ion-exchange reactions of cancrinite, nosean, and sodalite-type minerals, one of the minerals of nepheline composition, and one of the minor species were studied. DESPITE the technical and scientific interest of many naturally occurring aluminosilicates, problems relating to their reproducible laboratory synthesis and their properties have as yet been solved very incompletely. The chemistry of mineral reactions can usually be inferred only in general outline from geochemical reasoning. Much of the earlier literature of hydrothermal reactions has been summarised by Morey and Ingerson (Econ. GeoZ., 1937, 32, 607), and Barrer (Faraday SOC. Discussion, 1949, 5, 326) has discussed the principal synthetic methods in silicate mineral chemistry.Recently, by direct synthesis from gels under carefully specified conditions, much new information on mineral formation has been obtained in this laboratory (Barrer,
