Crystal Chemistry and Spectroscopy of Aegerines from Oktyabrskii Massif and Alkaline Metasomatic Rocks of Azov Region

UDC 549.642.31 : 548.3 : 543.424.4

V.M. Khomenko

M.Р. Semenenko Institute of Geochemistry, Mineralogy and Ore Formation of the NAS of Ukraine

34, Acad. Palladina Pr., Kyiv-142, Ukraine, 03680


Crystal Chemistry and Spectroscopy of Aegerines from Oktyabrskii Massif and Alkaline Metasomatic Rocks of Azov Region

Language: Ukrainian

Mineralogical journal 2015, 37 (3): 15-27

Abstract: Crystal-chemical peculiarities of the most common Fe-bearing mineral of alkaline rocks, aegirine, were studied on samples from Oktyabrskii massif nepheline sienites and spatially close to them alkaline metasomatic rocks (East Azov region, Ukraine). The analytical data were collected using a set of chemical and spectroscopic methods. Par ti cular attention was paid to complex heterovalent sub sti tutions, their connections with the conditions of rock for mation fixed in paragenetic mineral assemblages, and co lour of the mineral. Aegirine composition was studied using a microprobe method in 11 rock samples. Single crystal po la rized optical absorption- and IR spectra were measured in the ranges 33000—5600 and 7000—1000 cm–1, respectively. The deficiency of cations in the M2 position, partially compensated by an excess of Si, is a common feature of the studied Na-Fe3+ pyroxenes from Oktyabrskii massif and metasomatic rocks. Aegirines from metasomatites are characterized by an increased content of Ti (up to 1.1—1.5 % TiO2) and significant impurity of Zr (up to 0.7—0.8 % ZrO2). Impurity of Mn (up to 1.0 % MnO) is typical for pyroxenes from the albite-aegirine and astrophyllite meta somatites. Variations in the composition of the aegi rines studied occur mainly due to the heterovalent isomorphic substitutions 2Fe3+ (M1) ↔ (Ti, Zr) (M1) + Me2+ (M1) and Fe3+ (M1) + Na (M2) ↔ Me2+(M1) + Ca (M2). Fe2+ con tent was calculated on the basis of the sum of these sub stitutions. It results in 0—0.04 Fe2+ atoms per unit cell, that does not exceed 5—6 % of the total number of Fe ions in the aegirine samples. Pyroxenes from different metasomatic rocks and mariupolites differ also in the ratio Mg/Mn. Optical absorption spectra indicate occurrence of both Fe3+ and Fe2+ ions only in the M1 structural position. Weak greenish-yellow colour of "pure" aegirines is caused by a combination of charge transfer (CT) bands O2– → Fe3+ in UV area and Fe2+ → Fe3+ centered at 13000 cm–1. An in crease in the diopside-hedenbergite component in Na-Fe3+ pyroxene, close to the stoichiometric aegirine, accompa nied by a sharp increase in the relative content of Fe2+. These changes lead to rapid intensification of both CT bands in the aegirine spectra that causes typical intense green colour of the mineral. The same mechanism is responsible for the colour zonation in the studied aegirine crystals.

Keywords: aegirine, crystal chemistry, isomorphic substitu tions, optical spectroscopy, colouration.


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