A note on the molecular water content in uranyl carbonate mineral andersonite

Abstract

The crystal structure of andersonite from Jáchymov (western Bohemia, Czech Republic) was refined from single-crystal X-ray diffraction data to an R = 0.0286 against 1657 unique observed reflections. Andersonite from Jáchymov is trigonal, R32/m, with a = 17.8589(6), c = 23.6935(8), V = 6544.4(4) Å 3 and Z = 18. The structural formula derived from the refinement of X-ray data is Na 2 Ca[(UO 2 )(CO 3 ) 3 ](H 2 O) 5.28 . This formula confirms previous structure determination on synthetic material. The maximal H 2 O content in andersonite is discussed also with regard to the published thermal analysis and vibration spectroscopy studies and disunities in chemical formula of andersonite in the literature.

Structure Refinement and Thermal Stability Studies of the Uranyl Carbonate Mineral Andersonite, Na2Ca[(UO2)(CO3)3]·(5+x)H2O

“…The quantity of these interchannel molecules may vary, although not significantly considering the 6c multiplicity of the O W 5 site. Recently [18], it was suggested that the amount of H 2 O in the structure of andersonite may vary in the range of 5.3-5.6. Taking into account the well-defined presence of H 2 O W 1-4 molecules within the structure that give the value of five molecules p.f.u., we suggest the formula of andersonite is Na 2 Ca[(UO 2 )(CO 3 ) 3 ]·(5+x)H 2 O, where x ≤ 1.…”

“…It may also appear coating walls of mines or shafts [14]. The crystal structure of andersonite was first fully determined using its synthetic analog [15] and then refined twice [16,17], including the first attempt made on a natural sample [18]. However, the first less successful attempt to refine the structure of synthetic andersonite, based on poor quality data that resulted in the loss of one of the H 2 O sites, was made by Coda [19].…”