Detailed high magnification scanning electron microscopic (SEM) study supported by petrographic, mineralogical, and geochemical (major and trace element) analysis of the Gercus Formation clastics (sandstones and mudstones) from northern Iraq shows that the main authigenic components are carbonate (dolomite and calcite), clay and haematite minerals with rare quartz, feldspar, and gypsum. The carbonates and sulphate minerals were the main early diagenetic phase in the Gercus Formation and were produced from the post‐depositional breakdown of ferromagnesian minerals. These latter are unstable in the oxidizing conditions of the interstitial waters below the desert surface. The main authigenic clay minerals are illite, illite‐smectite, and palygorskite. The presence of tangential illite coating and illite‐smectite in the sandstones can be related to their desert origin. Palygorskite is associated with dolomite, calcite, and haematite. These mineral associations may indicate early diagenesis in evaporitic and saline conditions. It also confirms the syndepositional and diagenetic origin for the clayey‐haematitic matrix and pigment of the Gercus Formation. Haematite occurs as extremely fine‐grained crystals mixed with authigenic clays and exhibits a range of shapes (e.g., fibrous to lath‐shaped aggregates, small spherical grains, specularite crystals, blade‐shaped, rosette‐like clusters, and a lump of bright spheroid‐shaped haematite/goethite). The arid to semi‐arid conditions together with the oxidizing and alkaline interstitial water favoured the noted mineral assemblage within the Gercus Formation.

中文翻译：

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粘土矿物成岩作用和红层着色：伊拉克北部格库斯组（中始新世）的SEM研究

由伊拉克北部格库斯组碎屑（砂岩和泥岩）的岩石学，矿物学和地球化学（主要和微量元素）分析支持的详细高倍扫描电子显微镜（SEM）研究表明，主要的自生成分是碳酸盐（白云石和方解石） ），粘土和赤铁矿矿物以及稀有的石英，长石和石膏。碳酸盐和硫酸盐矿物是格库斯组的主要成岩早期阶段，是由铁磁镁矿物质沉积后分解产生的。后者在沙漠表面以下的间隙水的氧化条件中不稳定。自生粘土的主要矿物是伊利石，伊利石-蒙脱石和坡缕石。砂岩中切向伊利石涂层和伊利石-蒙脱石的存在可能与其沙漠起源有关。坡缕石与白云石，方解石和赤铁矿有关。这些矿物质的关联可能表明在蒸发和盐分条件下的早期成岩作用。这也证实了格库斯组黏土-风化基质和颜料的同沉积和成岩成因。赤铁矿是与自生粘土混合的极细晶粒晶体，并呈现出各种形状（例如，纤维状至板条状聚集体，小的球形晶粒，镜石状晶体，叶片状，玫瑰花状簇和一大块明亮的球状体）形的赤铁矿/针铁矿）。