| Summary: | The crystal structure of a naturally occurring layered double hydroxide mineral—desautelsite from San Benito County, California, USA—was refined using single-crystal X-ray diffraction data in the space group <i>R</i>-3<i>m</i>, <i>a</i> = 3.1238(2) Å, <i>c</i> = 23.528(3) Å, <i>V</i> = 198.83(4) Å<sup>3</sup>, and <i>Z</i> = 3/8. The Mg and Mn cations are disordered occurring in one <i>M</i> site with occupancy Mg<sub>0.77</sub>Mn<sub>0.23</sub>. According to the electron microprobe analysis supported by Raman spectroscopy, the empirical formula is Mg<sub>6.20</sub>(Mn<sup>III</sup><sub>1.78</sub>Al<sub>0.01</sub>Fe<sup>III</sup><sub>0.01</sub>)<sub>Σ1.80</sub>(OH)<sub>16</sub>(CO<sub>3</sub>)<sub>0.90</sub>·5.35H<sub>2</sub>O that shows higher content of interlayer (H<sub>2</sub>O) molecules in comparison to the ideal formula that also agrees with the structure refinement. The Raman spectroscopy of two samples indicated O–H vibrations (3650/3640 cm<sup>−1</sup>, ~3500 sh cm<sup>−1</sup>), symmetric C–O (1055/1057 cm<sup>−1</sup>), Mg–O–Mg (533/533 cm<sup>−1</sup>) and Mn–O–Mn (439/438 cm<sup>−1</sup>) stretching vibrations and lattice vibrations (284/287 cm<sup>−1</sup>). Summing up our data and that of the current literature, we show a correlation (<i>R</i><sup>2</sup> = 0.91) between the averaged effective ionic radius (<i>x</i>) and <i>a</i> unit cell parameter (<i>y</i>) of hydrotalcite group minerals, <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mi>y</mi><mo>=</mo><mn>1.9871</mn><mi>x</mi><mo>+</mo><mn>1.4455</mn></mrow></semantics></math></inline-formula>. Desautelsite follows this correlation, being the species with one of the largest <i>a</i> unit cell parameters among the group. The correlation can be applied for control of cation intercalation during synthesis.
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