Alstonite, BaCa(CO3)2, is a mineral described almost two centuries ago. It is widespread in Nature and forms magnificent cm-sized crystals. Notwithstanding, its crystal structure was still unknown. Here, we report the crystal-structure determination of the mineral and discuss it in relationship to other polymorphs of BaCa(CO3)2. Alstonite is trigonal, space group P31m, with unit-cell parameters a = 17.4360(6), c = 6.1295(2) Å, V = 1613.80(9) Å3 and Z = 12. The crystal structure was solved and refined to R1 = 0.0727 on the basis of 4515 reflections with Fo > 4σ(Fo) and 195 refined parameters. Alstonite is formed by the alternation, along c, of Ba-dominant and Ca-dominant layers, separated by CO3 groups parallel to {0001}. The main take-home message is to show that not all structure determinations of minerals/compounds can be solved routinely. Some crystals, even large ones displaying excellent diffraction quality, can be twinned in complex ways, thus making their study a crystallographic challenge.

钙霞石BaCa（CO 3）2是近两个世纪前描述的一种矿物。它在大自然中广泛分布，并形成厘米级的宏伟晶体。尽管如此，其晶体结构仍是未知的。在这里，我们报告了矿物的晶体结构测定，并讨论了其与BaCa（CO 3）2的其他多晶型的关系。Alstonite是三角，空间群P 31米，具有晶胞参数一个= 17.4360（6），C ^ = 6.1295（2）埃，V = 1613.80（9）埃3和Ž = 12的晶体结构解析和精至R 1= 0.0727，基于45次反射，其中F o >4σ（F o）和195个精确参数。通过沿平行于{0001}的CO 3基团隔开的Ba-主层和Ca-主层沿c的交替而形成了奥氏体。带回家的主要信息是要表明，并非所有矿物/化合物的结构确定都可以常规解决。某些晶体，甚至是具有优异衍射质量的大晶体，也可以通过复杂的方式进行孪晶，从而使他们的研究成为晶体学上的挑战。