Abstract Gadolinite [REE2Fe2+Be2Si2O10] is a common mineral in certain types of rare element and rare earth element (REL-REE) pegmatites. Changes in pegmatite environment during and after gadolinite formation may be devised by studying its crystal-chemical properties and a thorough observation of microfeatures in the mineral matrix. Post-crystallization processes in pegmatite might trigger alteration mechanisms in gadolinite like in other REE-rich pegmatite minerals, whereby various late-magmatic or metasomatic events may affect mineral chemistry. Three gadolinite samples originating from various pegmatite occurrences in southern Norway offer an excellent opportunity in studying post-crystallization evolution of the pegmatites; by determining their crystallographic, chemical, and micro-textural features, imprints of the related processes in the pegmatites have been characterized in this study. Relevant mineral information was collected in recrystallization experiments of fully or slightly metamictized gadolinite samples and subsequent XRD analyses. Micro-Raman spectroscopy, electron microprobe analysis (EMPA), and scanning electron microscope–backscattered electron–energy-dispersive X-ray spectroscopy (SEM-BSE-EDS) analyses were employed to retrieve micro-chemical properties and related micro-textural features of the mineral matrix. With a reference to the gadolinite supergroup, a general alteration path can be envisaged outlining the pegmatite evolution and suggesting the occurrence of the secondary REE mineral phases: altered gadolinite domains prove Ca enrichment with a tendency toward the hingganite composition, while a slight fluorine increase and sporadic secondary fluorite occurrence imply a significant role of fluorine as a complexing agent in the dissolution-reprecipitation mechanism of metasomatic alteration in the mineral. Micro-Raman spectra show improved vibration statistics for the altered gadolinite domains, which could be linked to the substitution of rare earth elements (REE) by Ca and a possible increase of structural ordering within the gadolinite structure, being at the same time an indication of structural healing of metamictized domains by metasomatic processes. A study of microfeatures in the complex silicates like gadolinite proves to be an excellent tool to trace post-crystallization processes in a pegmatitic environment. With a slight redistribution of radionuclides during an alteration in gadolinite, a moderate precaution has to be taken when selecting gadolinite for U-Th-Pb dating.

中文翻译：

---

伟晶岩形成和蚀变演化印记的钆石晶体化学和微观特征

摘要 钆[REE2Fe2+Be2Si2O10]是某些类型的稀有元素和稀土元素(REL-REE)伟晶岩中的常见矿物。伟晶岩形成期间和之后伟晶岩环境的变化可以通过研究其晶体化学性质和对矿物基质中微观特征的彻底观察来设计。伟晶岩的后结晶过程可能会像其他富含 REE 的伟晶岩矿物一样触发钆石的蚀变机制，从而各种晚期岩浆或交代事件可能影响矿物化学。来自挪威南部各种伟晶岩矿点的三个钆石样品为研究伟晶岩的结晶后演化提供了极好的机会；通过确定它们的晶体学、化学和微观结构特征，本研究已经表征了伟晶岩中相关过程的印记。相关矿物信息是在完全或轻微透镁化钆沸石样品的再结晶实验和随后的 XRD 分析中收集的。采用显微拉曼光谱、电子探针分析 (EMPA) 和扫描电子显微镜-背散射电子-能量色散 X 射线光谱 (SEM-BSE-EDS) 分析来检索矿物基质。参考钆沸石超群，可以设想一般的蚀变路径，概述伟晶岩的演化并暗示次生 REE 矿物相的出现：蚀变的钆沸石域证明 Ca 富集并趋向于菱镁矿成分，而氟的轻微增加和零星的次生萤石出现意味着氟作为络合剂在矿物交代蚀变的溶解-再沉淀机制中发挥着重要作用。微拉曼光谱显示了改变的钆沸石域的改进振动统计，这可能与稀土元素 (REE) 被 Ca 取代和钆钆结构内结构有序性的可能增加有关，同时表明通过交代过程对交混结构域进行结构愈合。对钆石等复杂硅酸盐微观特征的研究证明是在伟晶岩环境中追踪结晶后过程的极好工具。在钆石蚀变期间放射性核素的轻微重新分布，