| Summary: | A sample of uranyl carbonate mineral andersonite, Na<sub>2</sub>Ca[(UO<sub>2</sub>)(CO<sub>3</sub>)<sub>3</sub>]·5−6H<sub>2</sub>O, originating from the Cane Springs Canyon, San Juan Co., UT, USA was studied using single-crystal and powder X-ray diffraction at various temperatures. Andersonite is trigonal, <i>R</i>−3<i>m</i>, <i>a</i> = 17.8448(4), <i>c</i> = 23.6688(6) Å, <i>V</i> = 6527.3(3) Å<sup>3</sup>, <i>Z</i> = 18, <i>R</i><sub>1</sub> = 0.018. Low-temperature SCXRD determined the positions of H atoms and disordered H<sub>2</sub>O molecules, arranged within the zeolite-like channels. The results of high-temperature PXRD experiments revealed that the structure of andersonite is stable up to 100 °C; afterwards, it loses crystallinity due to release of H<sub>2</sub>O molecules. Taking into account the well-defined presence of H<sub>2</sub>O molecules forming channels’ walls that to the total of five molecules p.f.u., we suggest that the formula of andersonite is Na<sub>2</sub>Ca[(UO<sub>2</sub>)(CO<sub>3</sub>)<sub>3</sub>]·(5+<i>x</i>)H<sub>2</sub>O, where <i>x</i> ≤ 1. The thermal behavior of andersonite is essentially anisotropic with the lowest values of the main thermal expansion coefficients in the direction perpendicular to the channels (plane (001)), while the maximal expansion is observed along the <i>c</i> axis—in the direction of channels. The thermal expansion around 80 °C within the (001) plane becomes negative due to the total release of “zeolitic„ H<sub>2</sub>O molecules. The information-based structural complexity parameters of andersonite were calculated after the removal of all the disordered atoms, leaving only the predominantly occupied sites, and show that the crystal structure of the mineral should be described as complex, possessing 4.535 bits/atom and 961.477 bits/cell, which is comparative to the values for another very common natural uranyl carbonate, liebigite.
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