一种特殊类型硅质岩——浮石状燧石

一种较为少见的特殊硅质岩产于南京及巢湖地区的下二叠统孤峰组地层中。它的显著特征是岩石中存在大量菱形和不规则状孔隙，孔隙周边和孔隙间主要由微晶石英组成。通过对该岩石的成分和结构等特征的研究，认为它是一种交代碳酸盐岩成因的硅质岩，并定名为浮石状燧石。

NECTIC CHERT, A VERY PECULIAR TYPE OF SILICEOUS ROCK

There is a very peculiar type of bedded siliceous rock occurred in the Gufeng Formation of the Lower Permian in the Hushan of Nanjing, jiangsu Province and the Pingdiangshan, Anmenkou of the northern Chaohu, Anhui Province. The Gufeng Formation is dominated by a suite rock, usually grey of light yellow in color, is intercalated with bedded radiolarian chert or shales. It is characyerized by soft and porous nature, and looks like shle or diatomite at the surface outcrops. It can be seen under the microscope that the rock contains a large amount of rhombic and irregular cavities surrounded by siliceous cement consisting of microquartz. Sometimes small amount of calcite presents in the cements either as scattered crystals or as small irregular aggregates. Occasionally, residual calcareous bioclasts can also be found in the rock. Observation with SEM closed by fine anhedral micoquarrtz crystal. Only a few appear to be euhedral crystals. The chemical analysis date demonstrate that these porous siliceous rocks from the Gufeng Formation in northern Chaohu have a higher content of SiO2 (＞90%) and a very small amount of impurities. X-ray determination of the porous rock indicates that it consisted almost entitely of quartz. The preliminary study suggests that this is a siliceous rock originated by replacement of sedimentary carbonates, such as dolomitic limestone. The pores correspond to the casts of euhedral dolomite rhombs contained in a dolomitic limestone and at a later time dissolved out. Since the rock similar to the “chert nectique” (Cayeux,1929) in composition and feature of porosity, the authors proposed to name it nectic chert.