Occurrence and properties of lisetite, CaNa₂Al₄Si₄O₁₆, a new tectosilicate in the system Ca-Na-Al-Si-O.

Lisetite (CaNa₂Al₄Si₄O₁₆, orthorhombic, Pbc2₁, a 8.260(1), b 17.086(1), c 9.674(1) A, Z = 4, D_calc = 2.73) occurs in strongly retrograded clinopyroxene-rich layers in the Liset eclogite pod in the Selje district of the Western Gneiss region, Norway. Chemically identical to 50% anorthite (Ca₂Al₄Si₄O₁₆) + 50% of the pure-Na equivalent of nepheline (Na₄Al₄Si₄O₁₆), the mineral is a tectosilicate (M.A. 87M/3960) with all Al in tetrahedral sites (M.A. 87M/3960) but is neither a feldspar nor a feldspathoid. The strongest XRD powder reflections are 3.198(100), 4.162(44), 3.468(36), 3.534(31), 2.954(31) A. In the type thin-section, lisetite is anhedral, untwinned, uncleaved, transparent, and colourless with refractive indices and birefringence similar to those of plagioclase. It occurs in clusters of approx 100-mu -sized grains, and is easily distinguished from the abundant plagioclase in the type sample by its rich collection of mu -sized mineral inclusions and by a characteristic double corona texture. This corona includes a zone of a new K-poor, Ca-rich nepheline-structural mineral. 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 + or - 4 kbar at 400oC and 20 + or - 4 kbar at 800oC where highest-P greenschist-, amphibolite-, or granulite-facies parageneses form.-J.A.Z.