P--T Stabilities of Laumontite, Wairakite, Lawsonite, and Related Minerals in the System CaAl2Si2O8-SiO2-H2O

Hydrothermal investigation of the bulk composition CaO.Al₂O₃.4SiO₂+excessH₂O has been conducted using conventional techniques over thetemperature ranges 200–450 °C and 500–6000 barsP_fluid. A number of reactions have been studied by employingmineral mixtures consisting of reactants and products in about9: 1 and 1: 9 ratios. The phase relations were deduced fromrelatively long experiments by observing which seeded assemblagedisappeared or decreased markedly in one of the paired run charges. Laumontite was synthesized in the laboratory, probably for thefirst time. Laumontite was grown from seeded wairakite to over99 per cent using a weak NaCl solution. The refractive indicesof the synthetic material are about = 1.504 and = 1.514. Theaverage unit cell dimensions are a₀ = 14.761±0.005 Å;b₀ = 13.077±0.005 Å; c₀ = 7.561±0.003 Å;and ß = 112.02°±0.04°. Within the errorof measurement, the optical properties and cell parameters arein good agreement with those of natural laumontite. The equilibriumdehydration of laumontite involves two reactions: (1) laumontite= wairakite+2H₂O, passing through about 230 °C at 0.5 kb,255±5 °C at 1 kb, 282±5 °C at 2 kb, 297±5°C at 3 kb and 325±5 °C at 6 kb; and (2) laumontite= lawsonite+2 quartz+2H₂O, taking place at about 210 °Cat 3 kb and 275 °C at 3.2 kb. Above 300 °C, the equilibriumcurve for the solid-solid reaction (3) lawsonite+2 quartz =wairakite passes through 305 °C, 3.4 kb and 390 °C,4.4 kb. Equilibrium has been demonstrated unambiguously forthe above three reactions. The hydrothermal decomposition ofnatural laumontite above its own stability limit appears tobe a very slow process. Combined with previously published equilibria determined hydrothermallyfor wairakite, the phase relations are further investigatedby chemographic analysis interrelating the phases, laumontite,wairakite, lawsonite, anorthite, prehnite+kaolinite, and 2 pumpellyite+kaolinitein the system CaAl₂Si₂O₈-SiO₂-H₂O. This synthesis allowed theconstruction of a semiquantitative petrogenetic grid applicableto natural parageneses and the delineation of the physical conditionsfor the various low-grade metamorphic facies in low µCO₂environments. The similar stratigraphic zonations, consistentlyfound in a variety of environments, are recognized to be a functionof burial depth, geothermal gradient, and mineralogical andchemical composition of the parental rocks. Departures fromthe normal sequences are believed to be due to the combinationsof mineralogical variations, availability of H₂O, differencesin the ratio µCO₂/µH₂O, and the rate of reaction.The possible P-T boundaries for diagenesis, the zeolite facies,the lawsonite-albite facies, the prehnite-pumpellyite facies,and the adjacent metamorphic facies are illustrated diagrammatically.