Zur Synthese und Stabilit�t der Albit-Modifikationen

Raase, P.

doi:10.1007/bf01134165

Cited by 11 publications

(6 citation statements)

References 21 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…At lower temperature, it is theoretically possible for the ordering transformation to remain continuous (52) or to have a discontinuity (118). The experimental data of Martin (1969), Raase (1971) and Senderov and Shchekina (1976) are consistent with continuous disordering of low albite up to -953 K where there appears to be a distinct increase of ordering to high albite. This temperature is used for the first-order transition given in Figure lea).…”

Section: Albitesupporting

confidence: 66%

Phase Relations of Plagioclase Feldspars

Smith

1984

Feldspars and Feldspathoids

View full text Add to dashboard Cite

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,

show abstract

Section: Albitesupporting

confidence: 66%

Phase Relations of Plagioclase Feldspars

Smith

1984

Feldspars and Feldspathoids

View full text Add to dashboard Cite

show abstract

“…Each of the Tim, T2o and T2rn sites were considered equivalent, thus reducing the problem to a two tetrahedral-site model. Senderov (1980) was then able to conclude, in agreement with the experimental results of Raase (1971) that ordering in albite occurs over a narrow temperature range (680___20 ~ C) and that the transformation is possibly of first order, involving an instantaneous transformation from an incompletely-disordered high albite to an incompletely-ordered low albite, with no intermediate ordering states occurring. Goldsmith and Jenkins (1985) demonstrated complete reversal of the low-high transformation in albite (NaA1Si308) at high pressures, monitoring the progress of order-disorder with the triclinic obliquity only.…”

Section: Introductionsupporting

confidence: 66%

“…Apparently conflicting experimental and theoretical evidence exist for both a discontinuous (e.g. Raase 1971;Senderov and Shchekina 1976;Senderov 1980) and continuous (e.g. Goldsmith and Jenkins 1985;Salje et al 1985) transformation.…”

Section: Relevance To Albitementioning

confidence: 95%

See 1 more Smart Citation

Unit-cell dimensions and tetrahedral-site ordering in synthetic gallium albite (NaGaSi3O8)

Burns

Fleet

1990

Phys Chem Minerals

View full text Add to dashboard Cite

Abstract.The tetrahedral-site order-disorder transformation in gallium albite (NaGaSi308) has been investigated using Rietveld structure refinement. Study of gallium-substituted albite (in contrast to pure albite [NaA1Si3Os]) is facilitated by a relatively rapid order-disorder transformation and the large difference in X-ray scattering efficiencies of gallium and silicon. High albite-structure NaGaSi308, grown in a Na2WO4 flux, was ordered by hydrothermal annealing below 820~ and dry annealing above 820 ~ C, to avoid melting, using a load pressure of approximately 1 kbar. Equilibration of the order-disorder reaction has been verified by three independent reversals of ordering. The transformation between low gallium albite and high gallium albite occurs over the temperature range 890~ 970 ~ C. The gallium content of the Tao site increases continuously with decreasing temperature. The gallium contents of the Tam and T2m sites decrease smoothly with increasing ordering while the gallium content of the 7"2o site decreases, then increases and then decreases again with decreasing temperature. Unit-cell parameters and the triclinic obliquity vary throughout the order-disorder transformation and undergo abrupt changes at 913_+3~ C and 937+3 ~ C. These abrupt changes correlate with changes in the gallium content of the T2o site, the X and Z ordering parameters and the configurational entropy. The order-disorder transformation in gallium-aluminum albite (NaGao.sAlo.sSi3Os) occurs in the temperature range 765 ~ C-850 ~ C, at a temperature intermediate to the transformation in albite (50% order at about 680 + 20 ~ C) and gallium albite.

show abstract

“…It is much steeper than the curve at ~ 1.8 GPa and lies at a T 30 70 ~ lower. There is no compelling reason for suggesting a discontinuous transformation Raase, 1971;Senderov and Shchekina, 1976;Senderov, 1980;Smith, 1983;Kroll and Voll, in Ribbe, 1983). If the transformation were discontinuous it would be hard to understand why two distinct albites have never been found in ordering experiments (particularly below 700 ~ and how albite in contact with a hydrothermal fluid can order continuously to reach values of A131 as low as 1.5 1.3.…”

Section: The High-low Albite Transformationmentioning

confidence: 99%

Alkali feldspars: ordering rates, phase transformations and behaviour diagrams for igneous rocks

1989

View full text Add to dashboard Cite

Homogeneous and heterogeneous phase relationships in the alkali feldspars are reviewed, and behaviour diagrams developed. A1,Si ordering is almost certainly continuous and higher order in both albite and potassium feldspar and has been established reversibly or nearly so down to below 500 ~ in albite and possibly to ~ 200 ~ in potassium feldspar. The degree of order in intermediate albite changes strongly over a range of ~ 75-150 ~ depending on pressure, low albite being stable up to about 620-650 ~ and high albite above about 725 ~ at low pressure. Symmetry is broken at ~ 980 ~ mainly by a cooperative shearing of the whole framework and not by A1,Si ordering alone; there is a thermal crossover near 700~ shearing being dominant above (high albite) and ordering dominant below (intermediate albite).In potassium feldspar symmetry is broken by A1,Si ordering at a temperature of about 500 ~ The change in degree of order with respect to temperature has been followed easily and reversibly in sanidine from ~ 1075 to ~ 550 ~ and to a lesser extent in microcline from 450 to 200 ~ Ordering rates in sanidine down to 500 ~ and ordering rates in microcline between 450 and 200 ~ are almost as fast as in albite. Ordering in sanidine at 500 ~ and below slows and then stops with the development of the tweed orthoclase domain texture. The tweed texture acts as a barrier to further order because the strain energy associated with the (incipient) twin domain texture balances or nearly balances the free energy decrease resulting from ordering. Ordering stops not because of the kinetics of A1,Si diffusion, but because the total driving force is very small or nil. Ordering can readily proceed to completion, with the formation of low microcline, only if the domain-texture barrier is overcome by processes involving fluids or strong external stresses. There is no barrier in albite.The symmetry-breaking process in alkali feldspar changes with composition from mainly shearing in albite to ordering in potassium feldspar. Symmetry is broken equally at a compositional crossover (metastable with respect to exsolution) near Abso 75 at low pressure and progressively displaced towards Or at higher pressures. Ordering in pure albite occurs by a (nearly) one-step path which progressively becomes two-step with substitution of Or. Diagrams showing the near-equilibrium variation of the order parameters at low pressure with composition and T are given, as well as two extreme phase and behaviour diagrams for complete coherent and complete incoherent (strain-free) relationships, These diagrams can be used to understand feldspar relationships and microtextures in hypersolvus and subsolvus rocks, the occurrence of orthoclase, and of intermediate and low microcline.

show abstract

Zur Synthese und Stabilit�t der Albit-Modifikationen

Cited by 11 publications

References 21 publications

Phase Relations of Plagioclase Feldspars

Phase Relations of Plagioclase Feldspars

Unit-cell dimensions and tetrahedral-site ordering in synthetic gallium albite (NaGaSi3O8)

Alkali feldspars: ordering rates, phase transformations and behaviour diagrams for igneous rocks

Contact Info

Product

Resources

About