Apatite multi-generations in the Três Estradas Carbonatite, Southern Brazil: physical and chemistry meaning and implications to phosphate ore quality

Abstract Carbonatites were recently discovered in Southern Brazil, which increased the interest to evaluate the economic potential of these uncommon rocks, especially the Três Estradas Carbonatite. Carbonates are the dominant minerals of fresh rock followed by apatite, but the weathering process makes apatite abundant. We focused on apatite from the carbonatite using conventional petrography and electronic microscopy associated with microprobe, micro-Raman and Fourier-transform infrared spectroscopy. Results demonstrate the existence of four types. The primary type is associated with the rock crystallization and the subsequent three others are associated with weathering processes. The alteration mechanism was favorable for initial carbonate leaching and subsequent increase of phosphate with late precipitation of three new apatite generations. The deduced model involves long exposure during polycyclic climate changes, intercalating periods of warm dry with humid climate. The apatite types differ chemically and morphologically and have distinctive characteristics that are suitable to be used to differentiate them. These properties should be considered in future planes of industrial processes to transform apatite into single superphosphate, a basic input for fertilizer production.摘要 巴西南部近期发现了碳酸岩（carbonatite）矿床，这引发了学界对这类罕见岩石经济开发潜力的评估热潮，其中尤以特雷斯埃斯特拉达斯碳酸岩（Três Estradas Carbonatite）为研究重点。新鲜碳酸岩的主要矿物为碳酸盐类矿物，其次为磷灰石（apatite）；而风化作用使得磷灰石的含量大幅提升。本研究以该碳酸岩中的磷灰石为研究对象，采用常规岩石学分析、电子显微技术，并结合微探针、显微拉曼光谱（micro-Raman）及傅里叶变换红外光谱（Fourier-transform infrared spectroscopy）开展表征分析。研究结果表明该碳酸岩中的磷灰石可分为四种类型：原生型与岩石结晶过程相关，其余三种则均由风化作用形成。其蚀变机制表现为：早期碳酸盐矿物被淋滤移除，随后磷酸盐含量显著升高，并于后期沉淀形成三代全新的磷灰石。本研究推导的蚀变模型表明，该区域经历了多旋回气候变化下的长期暴露过程，其间暖干期与湿润期交替出现。不同类型的磷灰石在化学成分与形貌特征上均存在显著差异，这些独特的标识性特征可用于区分各类型磷灰石。该类特征在未来将磷灰石转化为普通过磷酸钙（single superphosphate）——化肥生产的基础原料——的工业流程规划中具有重要参考价值。