Synthetic ‘deweylite’

Abstract

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

Characterisation of different commercial reactive magnesia

“…Numerous studies were carried out and most of them confirmed the findings by Bakharev (2005). Vandeperre et al (2008a) reported that PC and magnesia hydrated independently in the first month, whereas in the study of Speakman and Majumdar (1971), 'deweylite', which is defined as a poorly crystalline magnesium silicate hydrate, is said to form naturally in hydrating Portland cements high in magnesia. When supplemental cementitious materials are used, interactions between magnesia and the other additives are revealed and various hydration products are confirmed.…”

Experimental study of the carbonation of partially serpentinized and weathered peridotites

“…This material is composed of a very fine-grained mixture of hydrous and possibly poorly ordered Mg-silicate phases with serpentine-, talc-, and stevensite-like compositions. Beinlich et al (2010) referred to this olivine breakdown product as a deweylite assemblage, following previous descriptions in the literature (Faust and Fahey, 1962;Speakman and Majumdar, 1971;Bish and Brindley, 1978). The formation of deweylite is generally attributed to low-temperature alteration or weathering of serpentinites at or close to the Earth's surface (e.g., Lapham, 1961;Wenner and Taylor, 1971).…”

CO2 sequestration and extreme Mg depletion in serpentinized peridotite clasts from the Devonian Solund basin, SW-Norway

“…This alteration product bears chemical resemblance to sepiolite but it may also be an extremely fine-grained intergrowth of several mineral phases. Such an intergrowth has been described in the literature as deweylite and is vaguely defined as a compositionally variable mixture of poorly crystalline serpentine and talc or serpentine and stevensite (Faust and Fahey, 1962;Speakman and Majumdar, 1971;Bish and Brindley, 1978). Generally, the deweylite assemblage is interpreted to form at low temperatures, i.e., <180°C (Speakman and Majumdar, 1971) or even $50°C (Faust and Fahey, 1962).…”