Abstract

Lisetite occurs in strongly retrograded clinopyroxene-rich layers in the Liset eclogite pod in the Selje district of the Western Gneiss region, Norway.

Electron-microprobe analyses reveal the almost stoichiometric composition Ca-Na2Al4Si4O16, which is chemically identical to exactly 50% anorthite (Ca2Al4Si4O16) + 50% of the pure-Na equivalent of nepheline (Na4Al4Si4O16). The mineral is a tectosilicate with all Al in tetrahedral sites, but it is neither a feldspar nor a feldspathoid. The crystal structure is orthorhombic with space group Pbc21 and unit-cell parameters a = 8.260(1), b = 17.086(1), c = 9.654(1) Å, V = 1362.5(4) Å3, Z = 4, Dcalc = 2.73. The strongest X-ray powder-diffraction reflections are [d in Å, (I/I0), (hkl)] 3.198(100)(042,231); 4.162(44)(200); 3.468(36)(221); 3.534(31)(141); and 2.954(31)(113).

In the type thin section, lisetite is anhedral, untwinned, uncleaved, transparent, and colorless with refractive indices and birefringence similar to those of plagioclase. Lisetite, which occurs in clusters of ∼ 100-μm-sized grains, is however easily distinguished from the abundant plagioclase in the type sample by its rich collection of micrometer-sized mineral inclusions and by a characteristic double corona texture. This corona includes a zone of a new K-poor, Ca-rich nepheline-structure mineral.

Very localized diffusion of principally the ions Na+, Ca2+, and H+, related essentially to the destabilization of jadeite-rich clinopyroxene, can explain the main features in the sequence of mineral development in this rock. Lisetite grew via intermediate reactions involving paragonite, and perhaps also corundum + albite, in Al-rich Si-poor domains originally represented by eclogite-facies kyanite. The stability field of lisetite is approximately estimated to lie between 10 + 4 kbar at 400°C and 20 ± 4 kbar at 800°C where highest-pressure greenschist-, amphibolite-, or granulite-facies parageneses form.

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