Dioctahedral micas are composed of two tetrahedral sheets and one octahedral sheet to form TOT or 2:1 layers. These minerals are widespread and occur with structures differing by (1) the layer stacking mode (polytypes), (2) the location of vacancies among non-equivalent octahedral sites (polymorphs), and (3) the charge-compensating interlayer cation and isomorphic substitutions. The purpose of the present study was to assess the potential of parallel-illumination electron diffraction (ED) to determine the polytype/polymorph of individual crystals of finely divided dioctahedral micas and to image their morphology. ED patterns were calculated along several zone axes close to the c*- and c-axes using the kinematical approximation for trans- and cis-vacant varieties of the four common mica polytypes (1M, 2M1, 2M2, and 3T). When properly oriented, all ED patterns have similar geometry, but differ by their intensity distribution over hk reflections of the zero-order Laue zone. Differences are enhanced for ED patterns calculated along the [001] zone axis. Identification criteria were proposed for polytype/polymorph identification, based on the qualitative distribution of bright and weak reflections. A database of ED patterns calculated along other zone axes was provided in case the optimum [001] orientation could not be found. Various polytype/polymorphs may exhibit similar ED patterns depending on the zone axis considered.

中文翻译：

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使用选定区域电子衍射从单个云母颗粒中鉴定多晶型和多型

双八面体云母由两片四面体片和一张八面体片组成，形成 TOT 或 2:1 层。这些矿物分布广泛，其结构因（1）层堆叠模式（多型），（2）非等效八面体位点（多晶型）之间的空位位置，以及（3）电荷补偿层间阳离子和同构替代品。本研究的目的是评估平行照明电子衍射 (ED) 的潜力，以确定细分的双八面体云母的单个晶体的多型/多晶型，并对其形态进行成像。使用四种常见云母多型（1M、2M1、2M2 和 3T）的反式和顺式空位变体的运动学近似，沿着靠近 c* 和 c 轴的几个区域轴计算 ED 图案。当方向正确时，所有 ED 图案具有相似的几何形状，但不同之处在于它们在零级劳厄区的 hk 反射上的强度分布。沿 [001] 区轴计算的 ED 图案的差异得到增强。基于明亮和弱反射的定性分布，提出了多型/多晶型识别的识别标准。提供了沿其他区域轴计算的 ED 图案数据库，以防找不到最佳 [001] 方向。根据所考虑的区域轴，各种多型/多晶型可能表现出类似的 ED 模式。基于明亮和弱反射的定性分布，提出了多型/多晶型识别的识别标准。提供了沿其他区域轴计算的 ED 图案数据库，以防找不到最佳 [001] 方向。根据所考虑的区域轴，各种多型/多晶型可能表现出类似的 ED 模式。基于明亮和弱反射的定性分布，提出了多型/多晶型识别的识别标准。提供了沿其他区域轴计算的 ED 图案数据库，以防找不到最佳 [001] 方向。根据所考虑的区域轴，各种多型/多晶型可能表现出类似的 ED 模式。