Crystal structure and thermal behavior of Bi $$_6$$ 6 Te $$_2$$ 2 O $$_{15}$$ 15 : investigation of synthetic and natural pingguite

The previously unknown crystal structure of pingguite was determined and refined from laboratory X-ray powder diffraction data using a synthetic sample. Additional single crystal diffraction of natural pingguite confirms that the crystal structure of the synthetic sample is identical to the natural mineral. This new crystal structure calls for a revised chemistry of the rare mineral pingguite to Bi $$_6$$ Te $$_2$$ O $$_{15}$$ instead of the previously reported formula Bi $$_6$$ Te $$_2$$ O $$_{13}$$ . Pingguite contains Te $$^\mathrm{VI}$$ only and not Te $$^\mathrm{IV}$$ as previously reported. Pingguite undergoes an irreversible phase transition around 840 $$^{\circ }$$ C which is characterized by a loss of oxygen and a reduction from Te $$^\mathrm{VI}$$ to Te $$^\mathrm{IV}$$ resulting in a $$\delta $$ -Bi $$_2$$ O $$_3$$ like type structure. In addition, we report the Raman spectroscopic data on the natural pingguite.