Disordering of Al and Si in Nepheline from Graulai (Germany)

摘要

Sodium and potassium aluminosilicate, nepheline, is the most naturally abundant feldspathoid. This is one of the key rock-building minerals of alkaline complexes, which is important as a raw material for various branches of industry. The proper nepheline has the idealized formula KNa3[Al4Si4O_(16)] where the Na/K and Al/Si ratios can vary over certain limits and their distribution among positions of the structure can be either ordered or disordered. In some types of effusive rocks, a low-potassium variety of nepheline is known with composition approaching Na4[Al4Si4O_(16)]. Different degrees of Si—Al ordering in the tetrahedra depending on the cooling conditions of the nepheline -containing rocks have been noted in a number of papers, in particular, those by Hahn [1], Buerger [2], Simmons [3], Sahama [4, 5]. In particular, Simmons and et al. [3] found a high degree of disorder of Al and Si in the structure of nepheline from the Monte Somma—Vesuvius Complex (Italy). The Si and Al disorder in the tetrahedral positions of the volcanic nepheline structure was attributed, in Sahama's works [4, 5], to the temperature regime of crystallization, which was confirmed by Dollase and co-workers [6], who demonstrated an increase in the degree of Al/Si disorder for nepheline in the series metamorphic— intrusive—volcanic rocks. Meanwhile, in a more recent study on the nepheline crystal chemistry [7], structure refinement for three samples from different deposits led the authors to the conclusion that nepheline is highly ordered irrespective of the origin.