Synthesis and stability relations of wairakite, CaAl2 Si4 O12·2H2O
Abstract
Hydrothermal investigation of the bulk composition CaO·Al2O3·4SiO2 + excess H2O has been conducted using conventional techniques over the temperature range 200 500° C and 500 5,000 bars P fluid. The fully ordered wairakite was synthesized unequivocally in the laboratory, probably for the first time. The gradual, sluggish and continuous transformation from disordered to ordered wairakite evidently accounts for failure by previous investigators to synthesize ordered wairakite in runs of week-long duration. The dehydration of metastable disordered wairakite to metastable hexagonal anorthite, quartz and H2O has been determined; this reaction takes place at temperatures exceeding 400° C, even at fluid pressures of 500 bars or less. The upper P fluid- T boundary of the disordered phase is equivalent to the maximum temperature curve of synthetic wairakite presented by previous investigators. The hydrothermal breakdown of natural wairakite above its stability limit appears to be a very slow process. The equilibrium dehydration of wairakite to anorthite, quartz and H2O occurs at 330±5° C at 500 bars, 348±5° C at 1,000 bars, 372±5° C at 2,000 bars and 385±5° C at 3,000 bars. Where fluid pressure equals total pressure, the thermal stability range of wairakite is about 100° C wide. At lower temperatures wairakite reacts with H2O to form laumontite. Reconnaissance experiments dealing with the effect of CO2 on stabilities of calcium zeolites suggest that wairakite or laumontite may be replaced by the assemblage calcite + montmorillonite in the presence of a CO2-bearing fluid phase. The determined P fluid -T field of wairakite is compatible with field observations in some metamorphic terrains where it is related to the shallow emplacement of granitic magma and with direct pressure-temperature measurements in certain active geothermal areas. Under inferred conditions of higher μCO2/μH2O ratios, essentially unmetamorphosed rocks grade directly into those characteristic of the greenschist facies; moderately high values of μCO2 in carbonate-bearing rocks result in the downgrade extension of the greenschist facies at the expense of zeolite-bearing assemblages.
- Publication:
-
Contributions to Mineralogy and Petrology
- Pub Date:
- December 1970
- DOI:
- 10.1007/BF00389814
- Bibcode:
- 1970CoMP...27..259L