Gaidonnayite from nepheline syenite pegmatite on Siktesøya in the southern part of the Oslo Region, Norway
pp. 303-306

Gaidonnayite, (Na2ZrSi3O9 2H2O), the orthorhombic dimorph of catapleiite, has been found in small vugs in a nepheline syenite pegmatite dike on Siktesøya, Oslo Region, Norway. The mineral occurs as transparent, colourless to very pale green crystals in aggregates up to l mm across. Gaidonnayite probably formed as the result of hydrothermal alteration of eudialite. The crystals display the forms {001}, {010}, {101} and {120}. The average of 18 electron microprobe analyses gives the following empirical formula: (Na1.52 K0.40 ?1.92 (Zr0.92Ti0.07Nb0.02)?1.01Si0.09O9*2H2O. The unit cell dimensions are a= 11.761(8) Å, b = 12.830(6) Å, c= 6.693(4) Å, V= 1009.9(8) Å3. The cell parameters of gaidonnayite show a linear correlation with the potassium content. Gaidonnayite from Siktesøya shows a vivid green fluorescence in short-wave UV light (254 nm). The UV excited fluorescence spectrum is typical of uranyl-activated rninerals.

A. O. Larsen, Norsk Hydro a.s., Research Centre Porsgrunn, N-3901 Porsgrunn, Norway;
G. Raade, Mineralogisk-Geologisk Museum, Sars gate l, N-0562 Oslo 5, Norway.