Plášil Jakub, Petříček Václav, Škoda Radek, Meisser Nicolas, Kasatkin Anatoly V
Institute of Physics ASCR, v.v.i., Na Slovance 2, Prague 8, 18221, Czech Republic.
Department of Geological Sciences, Masaryk University, Kotlářská 2, Brno, 61137, Czech Republic.
J Appl Crystallogr. 2021 Nov 2;54(Pt 6):1656-1663. doi: 10.1107/S1600576721009663. eCollection 2021 Dec 1.
In this work, the structures of chemically related uranyl-oxide minerals agrinierite and rameauite have been revisited and some corrections to the available structure data are provided. Both structures were found to be twinned. The two minerals are chemically similar, and though their structures differ considerably, their unit-cell metrics are similar. Agrinierite was found to be twinned by metric merohedry (diffraction type I), whereas the structure of rameauite is twinned by reticular merohedry (diffraction type II). The twinning of the monoclinic unit cells (true cells) leads to pseudo-orthorhombic or pseudo-tetragonal supercells in the single-crystal diffraction patterns of both minerals. According to the new data and refinement, agrinierite is monoclinic (space group ), with = 14.069 (3), = 14.220 (3), = 13.967 (3) Å, β = 120.24 (12)° and = 2414.2 (12) Å ( = 2). The twinning can be expressed as a mirror in (101) (apart from the inversion twin), which leads to a supercell with = 14.121, = 14.276, = 24.221 Å and = 2 × 2441 Å, which is centered. The new structure refinement converged to = 3.54% for 6545 unique observed reflections with > 3σ() and GOF = 1.07. Rameauite is also monoclinic (space group ), with = 13.947 (3), = 14.300 (3), = 13.888 (3) Å, β = 118.50 (3)° and = 2434.3 (11) Å ( = 2). The twinning can be expressed as a mirror in (101) (apart from the inversion twin), which leads to a supercell with = 14.223, = 14.300, = 23.921 Å and = 2 × 2434 Å, which is centered. The new structure refinement of rameauite converged to = 4.23% for 2344 unique observed reflections with > 3σ() and GOF = 1.48. The current investigation documented how peculiar twinning can be, not only for this group of minerals, and how care must be taken in handling the data biased by twinning.
在这项工作中，对化学相关的铀酰氧化物矿物阿格林矿和拉莫矿的结构进行了重新研究，并对现有的结构数据进行了一些修正。发现这两种结构均为孪晶。这两种矿物在化学上相似，尽管它们的结构差异很大，但它们的晶胞参数相似。发现阿格林矿通过度量准面孪晶（衍射类型I）形成孪晶，而拉莫矿的结构通过网状准面孪晶（衍射类型II）形成孪晶。单斜晶胞（真实晶胞）的孪晶导致这两种矿物的单晶衍射图中出现假正交或假四方超晶胞。根据新数据和精修结果，阿格林矿为单斜晶系（空间群 ）， = 14.069 (3)， = 14.220 (3)， = 13.967 (3) Å，β = 120.24 (12)°， = 2414.2 (12) Å（ = 2）。孪晶可表示为(101)面中的镜面（除了倒反孪晶），这导致一个超晶胞，其 = 14.121， = 14.276， = 24.221 Å， = 2 × 2441 Å，为 心。对于6545个 > 3σ()的独立观测反射，新的结构精修收敛到 = 3.54%，拟合优度GOF = 1.07。拉莫矿也为单斜晶系（空间群 ）， = 13.947 (3)， = 14.300 (3)， = 13.888 (3) Å，β = 118.50 (3)°， = 2434.3 (11) Å（ = 2）。孪晶可表示为(101)面中的镜面（除了倒反孪晶），这导致一个超晶胞，其 = 14.223， = 14.300， = 23.921 Å， = 2 × 2434 Å，为 心。拉莫矿的新结构精修对于2344个 > 3σ()的独立观测反射收敛到 = 4.23%，拟合优度GOF = 1.48。当前的研究记录了孪晶可能是多么奇特，不仅对于这组矿物而言，还记录了在处理受孪晶影响的数据时必须多么小心。
