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The action of water on y-Ca,SiO,, either alone or in the presence of calcium hydroxide, quartz, or amorphous silica, has been studied mainly a t 150-600" and 5-350 bars. A new compound, of probable composition Ca8Si5Ols, is formed over a wide range of conditions; its appears to have a regularly interstratified structure composed of alternate layers of y-Ca,SiO, and kilchoanite (Ca,Si,O,).The supposed compound, " y-dicalcium silicate hydrate " or " dicalcium silicate hydrate (C)," which has been widely reported as forming under saturated steam pressures around ZOO", is a mixture consisting principally of a disordered form of Ca8Si5018 and calciochondrodite, Ca5(Si04),(OH),.Reaction sequences and equilibria are discussed ; a t least up to 450", the initial and early products are crystalline compounds (kilchoanite, calcio-chondrodite, Ca8Si5OI8) with structures related to that of y-Ca,SiO,.THE system CaO-SO,-H,O has been extensively studied under hydrothermal conditions because of its relevance to the manufacture of autoclaved building materials and to geological problems. Few studies have, however, been made much above 200" on the reaction of water with y-Ca,SiO,, either alone or in the presence of added lime or silica, and the present work was therefore undertaken. I t had two specific aims. First, a supposed compound of composition 2CaO,Si0,,0~3--1~OH2O, generally called y-dicalcium silicate hydrate or dicalcium silicate hydrate (C): has been reported by many workers to form under saturated steam pressures around 200". The status of this product as a distinct compound has been questioned,* and it was hoped to obtain more definite evidence as to its nature. Secondly, there is uncertainty about the stability under hydrothermal conditions at 200-600" of kilchoanite (a polymorph of Ca,Si,O, relative to assemblages such as foshagite [Ca,Si309(0H),] plus dellaite (Ca,Si,O,,H,), and it was hoped to obtain further evidence bearing on this problem.

SummaryHydrothermal studies in the silica-rich region of the CaO-SiO2-H2O system have given information on the stability field of tobermorite. At saturated steam pressures tobermorite is unstable relative to xonotlite (C6S6H) above 140 °C but between 10 000 and 40 000 lb/in.2 (69 and 276 N/mm2), the decomposition temperature is raised to 285±5 °C. Tobermorite is stable at all compositions between C/S = 0·67 and 1·0 up to this temperature and has a formula close to C5S6H5. Variable compositions reported earlier are thought to be mixtures rather than lime-rich or silica-rich tobermorites.Data have also been collected on the equilibrium assemblages below C/S = 0·67 and between C/S = 1·0 and 1·5 involving truscottite, gyrolite, xonotlite, foshagite, and hillebrandite.

StJMMARY. Materials similar to the natural hydrated magnesium silicate mineral deweylite have been synthesized hydrothermally under low-temperature-low-pressure conditions from magnesia-silica gels. Optical, X-ray, infra-red, and DTA examinations have shown that both the natural and synthetic materials are mixtures of badly crystallized talc and badly crystallized serpentine. The mineral is thought to be a coarse mixture of these two components while the synthetic products are intimately interlayered. These materials are believed to be closely related to the hydrated magnesium silicates detected in high-magnesia cement hydration products.AN investigation into the nature of the magnesia-bearing phases formed during the hydration of high-magnesia cements has led to an examination of the mineral 'deweylite'.High levels of MgO in Portland cements result from the use of dolomitic limestones in countries where high-grade limestones are either not available or are in short supply. Provided the amount of magnesia is less than 5 ~o the cement behaves norreally, but expansion and cracking can result when amounts greater than this are present. The volume increase consequent on the hydration of periclase to brucite is believed to be chiefly responsible for this phenomenon. Cements containing up to 15 ~o MgO can be stabilized against expansion, both during room-temperature curing and under hydrothermal conditions, by adding reactive silicate materials such as trass, fly-ash, slag, etc. (Rosa, 1965; Majumdar and Rehsi, I969). Although the mechanism responsible for the stabilization is still open to question it has been suggested (Majumdar and Rehsi, 1969) that the formation of a poorly crystallized magnesium silicate hydrate may play some part in the process. Similarities between the X-ray powder patterns of 'deweylite' and a component of high-magnesia cements stabilized with fly-ash caused the authors to examine the mineral and attempt its synthesis.There appears to be considerable confusion over the naming of the group of poorly crystallized magnesium silicate hydrate and nickel magnesium silicate hydrate minerals. Over the years there has been a proliferation of names including garnierite, gymnite, genthite, deweylite, nepouite, noumeite, etc., several of which refer to materials that are essentially identical. The composition of 'deweylite' is variable; its MgO: SiOe ratio lies between those of talc and serpentine and it can contain up to 5 % NiO. Kato (2961) used DTA, infra-red spectroscopy, electron microscopy, and chemical analysis to examine several minerals of this group. He concluded that nickel-gymnite, genthite, garnierite, and deweylite were all mixtures of I:I and 2:2 9

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