Textural patterns, mineralogy, and chemistry of sandstone-related Calçadinha chalcedony (Piauí, Brazil)

ABSTRACT: Paleozoic sandstones of the Parnaíba Basin, in addition to hosting opal deposits, also have occurrences of chalcedonies with potential for mineral and ornamental handicrafts, in addition to assisting the understanding of the geological evolution of the basin. However, the chalcedonies were not investigated yet, and this study intended to fulfill this gap by the investigation of the chalcedonies of Calçadinha in Piauí. Fieldwork, microtexturals analysis, X-ray diffraction and scanning electron microscopy with energy dispersive spectrometry, chemical analysis, and gemological assessments were developed. Four distinct types of chalcedonies have been distinguished. They stand out for their well distribution of Fe and Mn dendrites, which involves opal nodules, and contains microcavities with well-formed microcrystalline quartz, nontronite, and palygorskite. The mesoscopic features of these chalcedonies and cabochon and free forms cutting show potential for use in mineral crafts and semi-jewels. As expected, the chalcedonies are dominated by high contents of SiO2, besides the low and variable contents of Al2O3, Fe2O3, MgO, and TiO2. Among trace elements that show high Ba contents, bound in barite, seem also to be a geochemical signature of the country sandstones in Parnaíba basin. These chalcedonies were formed during the partial solubilization of SiO2 of sandstones, which was promoted during their tectonic formation in faults and fractures zones.

摘要：帕纳伊巴盆地（Parnaíba Basin）的古生代砂岩不仅赋存蛋白石矿床，还产出具备矿物工艺品与装饰手作开发潜力的玉髓（chalcedony），同时可为该盆地的地质演化研究提供重要支撑。然而此前尚未对该区域的玉髓开展系统调查，本研究旨在通过对巴西皮奥伊州（Piauí）卡尔卡迪尼亚（Calçadinha）地块的玉髓进行研究，填补这一研究空白。本次研究采用了野外踏勘、微结构分析、X射线衍射（X-ray diffraction）、扫描电子显微镜-能量色散光谱联用（scanning electron microscopy with energy dispersive spectrometry）、化学成分分析以及宝石学评价等多种技术方法。研究共识别出四种不同类型的玉髓，其典型特征为铁与锰的树枝状晶体分布均匀，伴生蛋白石结核，内部发育完整的微腔，腔内充填微晶石英、绿脱石（nontronite）与坡缕石（palygorskite）。该类玉髓的宏观形貌特征、弧面素面宝石（cabochon）切割与自由形态加工工艺，均展现出用于矿物工艺品与半宝石饰品的应用潜力。正如预期，该玉髓以高含量二氧化硅（SiO₂）为主要成分，此外还含有含量较低且波动范围较大的氧化铝（Al₂O₃）、三氧化二铁（Fe₂O₃）、氧化镁（MgO）与二氧化钛（TiO₂）。在微量元素中，高钡含量特征显著，且钡元素赋存于重晶石（barite）中，这或可作为帕纳伊巴盆地区域砂岩的地球化学标识。上述玉髓形成于砂岩中二氧化硅的部分溶解过程，该过程因砂岩在断裂与裂缝带的构造活动而得以促进。