Isomorphic substitutions in afghanite: Identification of the mineral

Abstract

a member of the sodalite group, is abundant at lazurite deposits around the world and has been found in a number of volcanic occurrences in Italy. Three genetic types of afghanite mineralization were identified at lazurite deposits in the Baikal region:(1) replacement of lazurite in diopside-and calcitelazurite rocks; (2) joint crystallization with lazurite without relationship with the latter in terms of reaction;(3) primary afghanite metasomatic rocks (diopsideafghanite and phlogopite-afghanite rocks and afghanite calciphyre) formed at the contact between aluminosilicate and carbonate rocks (Ivanov and Sapozhnikov, 1985). At the deposits in Italy, afghanite was found in silicified limestone ejections in pumice and it occurs as small crystals in cavities from large fragments of metasomatized igneous rocks hosted in pumice (Hogarth, 1979;Leoni et al., 1979;Parodi et al., 1996).Despite the fact that afghanite in Italy (Leoni et al., 1979) was identified somewhat later than in the Baikal region, variations in the chemical composition and unit-cell dimensions were studied for the former . No such investigations were carried out for the Baikal afghanite.The aim of this study is to determine the unit-cell dimensions and chemical composition of afghanite samples from the Baikal lazurite deposits by X-ray diffraction and an electron microprobe.The parameters of the hexagonal unit cell were calculated from X-ray powder diffraction patterns recorded with a DRON-3 diffractometer ( Table 1). The unit-cell dimensions of the afghanite from all aforementioned types of afghanite mineralization preliminarily estimated from precisely measured reflections of 440 ( d ≈ 1.60 Å) and 4.0.12 ( d ≈ 1.50 Å) (silicon, a = 5.430 Å, as an inner standard) allowed us to select four samples that completely characterize the studied mineral series. The unit-cell dimensions of the selected afghanite samples were calculated by the least-squares method on the basis of uniquely indexed reflections located in the 2 θ range of 45 ° to 62 ° (Cu radiation) (Tables 1, 2). It should be noted that perfectly faced crystals from lazurite-afghanite and afghanite calciphyres (the 2nd and 3rd types of afgharite mineralization) yielded good X-ray powder diffraction patterns, which ensured easy calculation of the unit-cell dimensions. The determination of the unit cell dimensions of afghanite from zones of replacement of lazurite met with difficulties in picking out pure mineral from the diopside-afghanite rock, where the afghanite content is low, or from the zones of replacement of afghanite by sodalite (Fig. 1). In the latter case, the mineral (sample 1 Abstract -Samples of afghanite, (Na,Ca,K) 8 (Al 6 Si 6 O 24 )(SO 4 ,Cl,CO 3 ) 3 · H 2 O, from the Malaya Bystraya and Tultui lazurite deposits in the Baikal region have been studied with X-ray diffraction and an electron microprobe. The unit-cell dimensions a and c of the examined afghanite samples range from 12.729 to 12.762 Å and from 21.385 to 21.415 Å. Isomorphic substitution is exhibited between Na and K; ...