In sedimentary rocks rich in Fe-oxide, such as red beds, white bleached spots that are free of Fe-oxide minerals are often observed. The spot formation has been explained by localized reduction reactions in relation to organic substances, such as fluids including hydrocarbon and organic debris as a precursor, and the recent major prevalent approach is the microbial activity in sediments. However, the evidence for microbial activities within the spots is rarely found, and it remains debatable whether the entire spots are of microorganism origin. We discovered bleached spots in zebra rock, which is the sedimentary rock from northern Australia with rhythmic Fe-oxide bands, and explained that the spots were formed by pH changes induced by the decomposition of primary pyrite. Our comprehensive petrological studies further demonstrate that the spots developed during the infiltration of the acidic fluid in relation to the Fe-oxide band formation, and the primary pyrite as a precursor is currently preserved as a pseudomorph composed of dickite and aggregation of hematite and goethite. This study demonstrates that the bleached spots in zebra rock were formed by inorganic chemical reactions, indicating that the proposal in other studies that spots of this kind can be used as a biomarker to find life on Mars is not always available.

在富含氧化铁的沉积岩中，如红床，经常观察到不含氧化铁矿物的白色漂白斑点。斑点的形成可以通过与有机物质相关的局部还原反应来解释，例如流体，包括烃类和有机碎屑作为前体，最近主要流行的方法是沉积物中的微生物活动。然而，很少发现斑点内有微生物活动的证据，整个斑点是否来自微生物仍有争议。我们在斑马岩中发现了白化斑点，斑马岩是澳大利亚北部的沉积岩，具有有规律的氧化铁带，并解释说这些斑点是由原生黄铁矿分解引起的 pH 变化形成的。我们全面的岩石学研究进一步表明，在酸性流体渗透过程中形成的斑点与氧化铁带的形成有关，而作为前体的原生黄铁矿目前保存为由地开矿和赤铁矿和针铁矿聚集体组成的假晶型。这项研究表明斑马岩中的白化斑点是由无机化学反应形成的，这表明其他研究中提出的这种斑点可以用作在火星上寻找生命的生物标志物的提议并不总是可用的。