本文介绍了一种获取灰岩中弱硅化腕足类化石的酸处理方法。以四川省布拖县浪珠乡万吨山剖面上奥陶统铁足菲克组近顶部的生屑灰岩为实验对象, 通过准备样品、悬吊样品、配酸解液、酸解样品等实验流程, 配合遮盖液(如二乙酸纤维素丙酮溶液)的使用, 可获得保存精美的、弱硅化的腕足类实体化石, 且该化石保存有主突起、铰齿和匙形台等重要的细小结构。相较于传统方法, 该方法可获取硅化程度更低的壳相化石。该方法为灰岩中其他多门类弱硅化保存的化石之酸解分析提供了新思路和参考。
Brachiopods are a group of lophotrochozoan animals, which have a long geological range from the Cambrian to Recent and a considerable geographical distribution in Paleozoic marine environments. Most rhynchonelliformean brachiopod shells are composed of calcite, and preserved in different kinds of sedimentary rocks (e.g. limestone, dolomite, siltstone, mudstone, and shale). An improved technique of acid digestion for extracting weakly-silicified brachiopod fossils from limestone is intro-duced here. This method includes a series of processes, i.e. preparing samples (fresh rock samples cut into cuboid pieces of 40 ? 20 ? 20 cm3 approximately), suspending samples (samples suspended in an acid-resistant container by wire), making an ethanoic acid solution (the concentration of the acetic acid within the range of 0.5%–3% in volume), etching samples (a series of procedures involved to keep acid solution effective for etching samples but harmless to brachiopod shells), and brushing mask liquid (cellulose acetate solution involved here to mask shells from acid etching). This method is effective in extracting weakly-silicified brachiopod fossils from limestone, and is applicable for extract-ing other kinds of fossils with similar preservation. However, this method is time-consuming and complicated compared with the classic acid technique. Sample processing may take two to four months, depending on the host-rock lithology, concentra-tion of the acetic acid solution, and the ambient temperature of acid digestion lab. This improved technique yielded many intact, weakly-silicified, and well-preserved brachiopod fossils with delicate car-dinal process and dental plates, extracted from the upper part of the Tiezufeike Formation (Upper Ordovician) of the Wandunshan section, Butuo County, Sichuan Province. Some of them were photographed under an optical microscope without physical modification and presented here as examples.