Compact and hard authigenic calcite was discovered in the surface sediments at the water depth of 530 m on the eastern slope of the Okinawa Trough. It has brown color and biological shapes, and occurs as infilling materials in microbiological crusts, most of which were formed in foraminifera and gastropoda, and a few of which were formed in lamellibranch. Less calcite occurs as fecal pellet. Analyses of mineral and chemical composition as well as carbon and oxygen isotopes were carried out with X-ray diffractometer, electron microprobe, neutron activation and mass spectrograpgh respectively. The analytical results indicate that the iron content of calcite ranges from 0.19%～13.43%(mostly over 9%), with an average of 7.18%, much higher than the content of magnesium, suggesting ferrocalcite calcite; magnesium content is from 3.36% to 5.56%, averagely 4.9%. The calcite contains relatively high Si, Al, K, Na and Sr (1800 μg/g). Ferrocalcite coexists with magnesian calcite as solid solution. There are obvious correlations between most elements. It is worth mentioning that Fe is correlated not only with Si, but also with Si, Al, K and Na. The facts that the Al/Si ratio of ferrocalcite is similar to that of clay minerals and that no clay peaks have been found on the X-ray spectral lines indicate that Al, Si, K and Na in the calcite exist in the form of amorphous clay, which plays a role of filter in the enrichment of iron. δ13C(2.3‰) and δ18O(1.4‰) of the calcite fall between shallow water calcite and deep sea calcite, whereas δ13C(1.5‰)and δ18O(0.3‰) of the shells fall within the range of shallow water mollusk and foraminfera. Crystal and chemical characteristics offerrocalcite, suchas hexagonal system, relatively high content of Sr and isotopic composition of carbon and oxygen, show that the biochemical process play an important role in the formation of ferrocalcite. In addition to physical and chemical conditions in the sedimentary environment, the formation of ferrocalcite presumably has close relation to shells in which beneficial microenvironment has been formed. 