Thermal behavior of vergasovaite, ideally Cu3O(SO4)(MoO4), and its synthetic analog has been studied by high-temperature single-crystal X-ray diffraction in the temperature range of 300–1100 K. According to EMPA results, the empirical formulas are (Cu2.36Zn0.61)Σ2.97O[(Mo0.91S0.08V0.04)Σ1.03O4](SO4) for vergasovaite and Cu2.97O[(Mo0.92S0.09)Σ1.01O4](SO4) for its synthetic analog. The mineral is stable up to 950 ± 15 K; at 975 K, the unit-cell parameters and volume increase abruptly due to topotactic transformation of vergasovaite to cupromolybdite, Cu3O(MoO4)2. The transformation is accompanied by loss of sulfur (and excess copper) without destruction of the crystal. The thermal expansion of the vergasovaite structure is strongly anisotropic, being minimal along the [O2Cu6]8+ chains comprised of vertex-sharing OCu4 tetrahedra. This peculiar thermal behavior can be explained by the anisotropy of bond-length evolution in the Cu1O6 and Cu3O6 octahedra and the flexibility of the S-O-Cu and Mo-O-Cu bond angles. Synthetic Zn- and V-free analogs demonstrate negative thermal expansion at 425–625 K and melt at as low temperature as 700 K with no indication of transformation or recrystallization at least below 1200 K.The topotactic transformation observed in vergasovaite may have important implications for the design of novel materials and for understanding the alteration processes of copper minerals.

中文翻译：

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Vergasovaite 向铜钼矿拓扑转变并保持晶体形状

Vergasovaite（理想情况下为 Cu3O(SO4)(MoO4)）及其合成类似物的热行为已通过 300–1100 K 温度范围内的高温单晶 X 射线衍射进行了研究。根据 EMPA 结果，经验公式对于 vergasovaite 为 (Cu2.36Zn0.61)Σ2.97O[(Mo0.91S0.08V0.04)Σ1.03O4](SO4)，对于 Cu2.97O[(Mo0.92S0.09)Σ1.01O4](SO4)它的合成类似物。该矿物在 950 ± 15 K 下保持稳定；在 975 K 时，由于 Vergasovaite 拓扑转变为铜钼矿 Cu3O(MoO4)2，晶胞参数和体积突然增加。该转变伴随着硫（和过量的铜）的损失，但不破坏晶体。vergasovaite 结构的热膨胀具有强烈的各向异性，沿着由共享顶点的 OCu4 四面体组成的 [O2Cu6]8+ 链最小。这种奇特的热行为可以通过 Cu1O6 和 Cu3O6 八面体中键长演化的各向异性以及 SO-Cu 和 Mo-O-Cu 键角的灵活性来解释。合成的不含 Zn 和 V 的类似物在 425–625 K 时表现出负热膨胀，并在低至 700 K 的温度下熔化，至少在 1200 K 以下没有转变或重结晶的迹象。在 vergasovaite 中观察到的拓扑转变可能对新型材料的设计以及了解铜矿物的蚀变过程。