Crystallographic characteristics of thenardite and ankerite minerals from Libya

Within the cooperation of the Geological Survey of Serbia (former Geoinstitute, Belgrade, Serbia) with the Industrial Research Center (Tripoli, Libya), during the period from 2006 to 2010 many polymineral samples were obtained from central and southern parts of Libya whose exact mineral composition was unknown, and which were intended for analysis by X-ray diffraction method. With the X-ray diffraction method on powdered samples there were determined the presence of numerous mineral kinds, such as: quartz, gypsum, anhydrite, calcite, aragonite, halite, dolomite, celestine, bassanite, palygorskite, ankerite, thenardite, fluorite, hematite and goethite; as well as various mineral groups: clays (illite, kaolinite, montmorillonite), chlorites, micas, feldspars, serpentines, zeolites and amphiboles. In this paper, a more detailed crystallographic analysis of thenardite (Na2SO4) and ankerite [Ca(Fe2+,Mg)(CO3)2] was performed, given the fact that these minerals are quite rare in nature, as well as to indicate the possible way of their formation as marine evaporites. These minerals were identified as dominant in sample 1018/10/1 from southern Libya (Sheet Wādi Eghei, NF 34-1), along with secondary calcite, quartz, and clay minerals. Calculated unit-cell dimensions of thenardite (space group Fddd, No 70) of: a0=9.820(3) Å; b0=12.312(3) Å; c0=5.867(1) Å; α=β=γ=90o; and V0=709.3(2) Å3 are in a very good agreement with the reference data (ICDD-PDF 74-2036: a0=9.829 Å; b0=12.302 Å; c0=5.868 Å; α=β=γ=90o; and V0=709.54 Å3). Calculated unit-cell dimensions of ankerite (space group R-3, No 148) of: a0=b0=4.829(1) Å; c0=16.121(5) Å; α=β=90o; γ=120o; and V0=325.5(2) Å3 are also in a very good agreement with the reference data (ICDD-PDF 84-2066: a0=b0=4.823 Å; c0=16.122 Å; α=β=90o; γ=120o; and V0=324.78 Å3; ICDD-PDF 84-2067: a0=b0=4.831 Å; c0=16.166 Å; α=β=90o; γ=120o; and V0=326.77 Å3). Since ankerite is very similar to dolomite (space group R-3, No 148; ICDD-PDF 74-1687: a0=b0=4.815 Å; c0=16.119 Å; α=β=90o; γ=120o; and V0=323.64 Å3), significantly larger calculated unit-cell dimensions prove that there was a significant (over 60%) isomorphic exchange of smaller Mg 2+ ion with bigger Fe2+.U ovom radu je izvršena detaljnija kristalografska analiza tenardita (Na2SO4) i ankerita [Ca(Fe2+,Mg)(CO3)2], s obzirom na činjenicu da se ovi minerali prilično retko javljaju u prirodi, kao i da ukazuju na mogući način njihovog obrazovanja kao morskih evaporita. Ovi mineralisu identifikovani kao dominantni u uzorku 1018/10/1 iz južnog dela Libije (List Wādi Eghei, NF 34-1), zajedno sa sporednim kalcitom, kvarcom i mineralima glina