Dehydration Series in the System CaSiO3— SiO2— H2O

Abstract

Hydrothermal studies show a dehydration series involving CSH(I), tobermorite, xonotlite gyrolite, truscottite (reyerite), silica, and wollastonite. The synthetic equivalents of gyrolite and Qruscottite (reyerite) have ideal compositions very close to 2CaO.3SiO2.2H2O and 3CaO.5SiO2.1SH2O respectively. With rising temperature the following reactions take place: (1) CSH(1) or tobermorite + silica = gyrolite (+H2O); (2) gyrolite + silica = truscottite + H2O; (3) gyrolite = tobermorite + truscottite + H2O (high pressure); (4) gyrolite = xonotlite + truscottite + H20 (low pressure); (5) tobermorite = xonotlite + H20; (6) truscottite = xonotlite + quartz + H2O; (7) xonotlite = wollastonite + H2O. Data on reactions (l), (5), and (7) confirm results of previous workers. The water pressure‐temperature curves or bands, except the one involving the breakdown of tobermorite, are steep. This curve crosses the curve representing the breakdown of gyrolite at a quintuple point. None of the reactions was reversed, and the temperatures indicated are approximate and are based on runs of about 4‐weeks duration. For much shorter times somewhat higher temperatures are needed to cause the reactions, and conversely in much longer runs slightly lower temperatures might suffice. The silica was usually amorphous, cristobalite, or quartz with a mineralizer and therefore of higher activity than quartz alone. Up to 2 % alkali fluorides facilitated the formation of gyrolite; this is probably related to the fact that apophyllite in many cases accompanies gyrolite in amygdules. Truscottite (reyerite) forms very easily, and the rarity of reported occurrences, compared with those of xonotlite, gyrolite, and opaline silica, suggests that some workers may have overlooked it.