As a special sedimentary grain type, the coated grain(with an ooid model) has been known for two centuries due to its fascinating special fabric and genesis developments. The leading factors in forming the coated grain consist mainly of:(1) microorganism movement field;(2) chemical sedimentary effect;(3) hydrodynamic force environment and topography condition;(4) abundant core material supply;(5) embedding condition; and(6) humic acids condition in water medium. With the development of the coated grain genesis, the single factor theory cannot reasonably explain the exact formation of the surface sediment of coated grain. Here, we find a new way to study the coated grain on the basis of traditional research methods. The Wenquan area on the northeast edge of the Qiangtang Basin is one of the few areas where the coated grain is developing, and is a rare "natural laboratory" for the study of the coated grain and the thermal spring sediment. The oolitic sinter of the area has the triaxiality modality of pea polymer, and is obviously different from the karst travertine and the normal lacustrine ooid. We found that the hot spring water in the Wenquan area has higher partial pressure of CO2(PCO2) and saturation index of the calcite(SIc) than normal. Macrocosmically, the oolitic sinter is shaped like a pea, and its grains and gap fillings are light yellow. Microcosmically, the sinter grain forms six types of fundamental lamina, and those six types are developed to be four grain types with different combinations. The C-axis of the mineral grain of sinter cement(calcite) is normal to the lamina face, and grows on it with distinct generation formations. In short, the grain type of oolitic sinter is the oncoid, with the grain development caused by the factors such as the shallow water of strong hydrodynamic force, the special hydrochemistry condition, and the extensive algae activities(diatom).
