Phase Relations of Plagioclase Feldspars

Abstract

Albite breaks down to jadeite and quartz at high pressure, and reacts with water at low temperature to give analcime and quartz. Dry anorthite melts incongruently to corundum and liquid above 9 kb, and to grossular plus kyanite plus quartz above 20 kb. Anorthite saturated with water breaks down to a variety of assemblages involving corundum, zoisite, margarite, lawsonite and wairakite with decreasing temperature. Calcic plagioclase saturated with water breaks down to andesine plus zoisite plus kyanite plus quartz.The sub-solidus phase equilibria for plagioclase feldspars are complicated by the frequent occurrence of micrometer intergrowths and the e-superstructure in which the chemical composition and thermodynamic functions vary across a crystal. Furthermore there are higher-order transitions even in albite and anorthite. A few pairs of coexisting plagioclases from metamorphic rocks have compositions which span a single asymmetric solvus, and some decomposition assemblages indicate that the micrometer intergrowths and ~-superstructure are unstable with respect to a mechanical mixture of two plagioclases. Nevertheless, these coherent intergrowths are very persistent, and provide useful information on the cooling history of igneous and metamorphic rocks. Data obtained by crystallographic and microscopic techniques, experimental syntheses, controlled annealing, thermochemical measurements and petrogenetic observations can be interpreted in terms of solid-state theories of spinodal behavior and chemical bonding. Brown (ed.), Feldspars and Feldspathoids,

Thermochemistry of Aluminium/Silicon Ordering in Feldspar Minerals

“…5b). It undergoes a displacive transition near 510 K giving a symmetry change of II + pI with falling temperature (Laves and Goldsmith, 1954;Brown et al, 1963;Bruno and Gazzoni, 1967;Smith and Ribbe, 1969;Laves et al, 1970;Czank et al, 1973;Czank, 1973;Staehli and Brinkmann, 1974;Smith, 1974Smith, , 1984Frey et al, 1977;Adlhart et al, 1980a;Wruck, 1986;Redfern and Salje, 1987a). In the absence of meltin~ an At/Si order/disorder transition would occur at some high temperature (CI + II with falling temperature) and, at a higher temperature still, the crystals would become monoclinic (C2/m), (Fig.…”

“…Both theory and available experimental data are more advanced for alkali feldspars than for plagioclases. For the latter, even qualitative descriptions of the stability relations are in dispute (Grove et al, 1983(Grove et al, , 1986Smith, 1983Smith, , 1984Carpenter, 1985Carpenter, , 1986a. The feldspar structure consists of a framework of Si0 4 and A204 tetrahedra linked in three dimensions, with alkali or alkaline earth ions filling large cavity sites between the tetrahedra.…”

“…Incommensurate superstructures have been found in many stoichiometric compounds, and they typically exist over a small temperature interval between the stability field of a high symmetry "para" phase and a low symmetry commensurate phase (Shiba and Ishibashi, 1978;Bruce et al, 1978;Cowley and Bruce, 1978;Heine and McConnell, 1984 Smith, 1984;Kitamura and Morimoto, 1984; and references therein). In the past, this incommensurate structure has been treated as some kind of metastable, compromise ordered state which develops in crystals that have A~:Si ratios inappropriate for ordering either on the same basis as low albite (ideally with A~:Si = 1:3) or II anorthite (ideally with A£:Si = 1:1), (Smith, 1974(Smith, , 1983(Smith, , 1984Wenk et al, 1980;Wenk and Nakajima, 1980;Horst, 1984;Jagodzinski, 1984; and references therein).…”

“…There is also evidence for significant coupling, not only between the different ordering transitions, but also between the ordering transitions and the miscibility gaps. Schematic phase diagrams are shown in figure 5, but, for a complete description of the structure and chemistry of feldspars, readers are referred to reviews by Smith (1974Smith ( , 1983Smith ( , 1984, Ribbe (1983aRibbe ( , b, c, 1984.…”

The incommensurately modulated structure of an andesine (An38)

“…The electron-density maps of this site do not show essential differences for plagioclases with compositions from An28 to An66 (Smith, 1984). The relative Ca/Na distribution over the split sites should therefore be independent of the Ca content though the split site M(2) is preferentially occupied by Ca (Gerald, Parise & Mackinnon, 1986).…”