Reactions in the system CaO-MgO-SiO₂-H₂O. Hydrothermal treatment of some compositions on the joins Ca₃Si₂O₇-Mg₃Si₂O₇ and Ca

Abstract

SUMMARY. The effects of hydrothermal treatments on gel mixtures and synthetic minerals in the C3S2-M8S2 and C2S-M2S regions of the CaO-MgO-SiO~ system have been investigated. The products formed in the temperature range I80-35o ~ at pressures up to 2zo N/mm* were identified, x The reactants generally behaved as simple mixtures of binary silicates and their products agreed well with existing data for reactions in the CaO-SiO2-H20 and MgO-SiO2-I-I20 systems. An unidentified phase that resulted from the hydration of ~kermanite could be a new compound in the quaternary CaO-MgO-SiO~-H20 system. There was no evidence to indicate that a magnesium analogue of kilchoanite can be formed from materials that are isostructural with 7-Ca2SiO,.RESULTS of earlier studies in the Ca2SiO4-SiO2-H20 region of the CaO-SiO2-H20 system by Speakman and Taylor (1965, 1967) and Speakman (I968) showed that a material consisting largely of kilchoanite was formed when a mixture of 7-CasSiO4 and quartz, having an over-all CaO/SiO~ ratio of 1"5, was treated hydrothermally at 18o ~ The thermodynamic stability of kilchoanite formed at low temperatures and pressures has been the subject of much discussion (Speakman and Taylor, 1967; Roy and Harker, 196o; Roy and Johnson, 1965) and although the point has not been finally settled, current opinion favours the view that it is a non-equilibrium phase under these conditions. The explanation for the readiness with which 7-Ca2SiO4 can form kilchoanite is almost certainly connected with the structural and chemical similarities that exist between them. The equilibrium products xonotlite and hillebrandite, on the other hand, have very different crystal structures.The present work was undertaken to investigate the possible formation of a magnesium substituted kilchoanite from forsterite and monticellite, which are isostructural with 7-Ca2SiO~. The study was later extended to include an examination of the products formed from several other compositions on the join Ca~Si2OT-MgaSi2OT. Four compositions were thought to be sufficient to give all the information required to detect and characterize a magnesium analogue of kilchoanite and any other new phases formed in this particular region. More emphasis was placed on the nature of the starting materials and on their influence on the type of products formed than on the number of compositions examined, since the formation of kilchoanite is believed to be a structure-dependent phenomenon and the starting material all important.x I N/mm ~ = IO bars = 9"869 atm. 9 Crown copyright reserved.