Variscite [Al(PO4)·2H2O] is an uncommon secondary phosphate mineral but is important in a variety of environmental and technological applications. It exists in at least one monoclinic (metavariscite) and two orthorhombic polymorphs (“Lucin-type” and “Messbach-type”), but the fine-grained nature of the “Messbach-type” variscite has hampered the determination of its crystal structure. The crystal structure of the latter from Tooele County, Utah, was solved and refined using laboratory powder X-ray diffraction (XRD) data, charge-flipping, and the Rietveld method. Both variscite modifications belong to the family of framework 3D MT structures in which octahedra (M) and tetrahedra (T) are linked by bridging O atoms. Topological analysis reveals that the two structures are polytypes. Based on our results and our structural interpretations, we refer to “Lucin-type” variscite as variscite1O and the “Messbach-type” as variscite2O, to be consistent with modern polytype terminology. The similarity of these two structures suggests that 1O-2O interstratifications may exist in nature, which is consistent with observed broadening of diffraction peaks of the Tooele material. 31P and 27Al MAS/NMR measurements are consistent with the XRD-determined crystal structure, and they show distinct signals for each of the two independent P and Al positions in variscite2O.High-temperature XRD, thermal analyses, and NMR measurements were applied to study the nature of the transformation of variscite2O to a derivative AlPO4 structure above 473 K. Charge-flipping analysis showed that the crystal structure of the new anhydrous AlPO4 phase (AlPO4-var2O in analogy to its parent structure) can be described as a 3D framework of alternating AlO4 and PO4 tetrahedra linked by bridging O atoms. Thermogravimetric analyses revealed almost complete dehydration above ~450 K, and NMR results were consistent with tetrahedral Al and P atoms.

中文翻译：

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高温下斜方晶系 variscite2O 及其衍生物 AlPO4 结构的晶体结构测定

Variscite [Al(PO4)·2H2O] 是一种不常见的二次磷酸盐矿物，但在各种环境和技术应用中都很重要。它存在于至少一种单斜晶（偏压沸石）和两种斜方晶多晶型（“Lucin 型”和“Messbach 型”）中，但“Messbach 型”混晶石的细粒性质阻碍了其晶体结构的确定. 使用实验室粉末 X 射线衍射 (XRD) 数据、电荷翻转和 Rietveld 方法对来自犹他州图勒县的后者的晶体结构进行了解析和改进。两种 variscite 修饰都属于框架 3D MT 结构家族，其中八面体 (M) 和四面体 (T) 通过桥接 O 原子连接。拓扑分析表明这两种结构是多型的。根据我们的结果和我们的结构解释，我们将“Lucin 型” variscite 称为 variscite1O，将“Messbach 型”称为 variscite2O，以与现代多型术语保持一致。这两种结构的相似性表明自然界中可能存在 1O-2O 间层，这与观察到的 Tooele 材料衍射峰的展宽是一致的。31P 和 27Al MAS/NMR 测量结果与 XRD 确定的晶体结构一致，并且它们在 variscite2O 中的两个独立 P 和 Al 位置中的每一个都显示出不同的信号。高温 XRD、热分析和 NMR 测量用于研究variscite2O 转变为 473 K 以上的衍生 AlPO4 结构的性质。电荷翻转分析表明，新的无水 AlPO4 相（与其母体结构类似的 AlPO4-var2O）的晶体结构可以描述为通过桥接 O 原子连接的交替 AlO4 和 PO4 四面体的 3D 框架。热重分析显示在~450 K 以上几乎完全脱水，NMR 结果与四面体 Al 和 P 原子一致。