Abstract
Cerussite, hbox {PbCO}_3, like all members of the aragonite group, shows a tendency to form twins, due to high pseudo-symmetry within the crystal structure. We here demonstrate that the twin law of a cerussite contact twin may be established using only ^{207}Pb-NMR spectroscopy. This is achieved by a global fit of several sets of orientation-dependent spectra acquired from the twin specimen, allowing to determine the relative orientation of the twin domains. Also, the full ^{207}Pb chemical shift tensor in cerussite at room temperature is determined from these data, with the eigenvalues being delta _{11} = (-2315pm 1) ppm, delta _{22} = (-2492 pm 3) ppm, and delta _{33} = (-3071 pm 3) ppm.
Highlights
Cerussite, PbCO3, is a secondary mineral found in the oxidised zone of lead deposits, and crystallises in the aragonite structure type, usually described using the orthorhombic space group Pmcn (No 62), see e.g. Antao and Hassan (2009)
The ratio of B : A is significantly changed in comparison to the original twin, with the larger crystal containing mostly domain A, with B ∶ A = 1 ∶ 17.1, and the smaller piece having a higher proportion of domain B, with B ∶ A = 1 ∶ 2.1
We have shown that solid-state Nuclear Magnetic Resonance (NMR) spectroscopy may successfully be employed to quantitatively characterise such crystal twinning
Summary
PbCO3 , is a secondary mineral found in the oxidised zone of lead deposits, and crystallises in the aragonite structure type, usually described using the orthorhombic space group Pmcn (No 62), see e.g. Antao and Hassan (2009). PbCO3 , is a secondary mineral found in the oxidised zone of lead deposits, and crystallises in the aragonite structure type, usually described using the orthorhombic space group Pmcn (No 62), see e.g. Natural single crystals may be found in mineral deposits of cerussite, its propensity to form twins has long been known. Cerussite is studied using Nuclear Magnetic Resonance (NMR) spectroscopy. We have determined the full chemical shift tensor of 207 Pb in the cerussite structure from both single crystal and twinned samples. To properly analyse the NMR spectra from the two-domain twin species, the relative orientation of the two domains needs to be known. As described below, this information could be derived from
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