Fine-grained natural dolomite incongruently dissolves to form aragonite in artificial sea-water solutions, the amount of aragonite produced being a function of the concentration of the sea water. While some aragonite was produced in dilute solutions, maximum yields occurred in solutions ranging from normal salinity to about 2.5 times the normal salinity. In solutions with higher salinities, the yield decreased. Reaction time was relatively short, with detectable aragonite produced in less than 1 day. This aragonite is very fine grained and, since it is newly crystallized, has no strain from the grinding process which might exist in aragonite ground to such a fine-size fraction. By using this aragonite, a study of a dolomitization reaction could be made. Solutions in contact with aragonitized dolomite were allowed to slowly evaporate to near dryness for 7 wk. The solids were then filtered, dried, and analyzed. It was observed that the amount of dolomite increased with respect to aragonite. It is postulated from evidence from electron microprobe, x-ray diffraction, and staining techniques that the aragonite was converted to ordered dolomite.