Abstract: Synthesis of Dolomite and Related Mineral Assemblages at Ambient Temperatures and Pressures
Herschell J. McCunn
From a study of existing areas of Recent dolomitization in conjunction with the "Oil Shales" of the Green River Basin, it was determined that it is possible the formation of dolomite and associated mineral occurrences is the result of the natural mixing of two very different types of waters and the salts they contain in solution. One type is formed from the evaporation of sea water. The salts involved are magnesium sulfate, magnesium chloride and calcium sulfate. The second type is the "fresh" waters originating from the land. The salts contained in these waters are sodium bicarbonate, sodium carbonate and trona. Trona is a mixture of the two sodium salts.
These are all metasalts and are found in nature in desert, erg or arid tidal flat areas. They are stable in the host waters but become very unstable when mixed together or if the pH of the host waters is altered.
A series of simple informal experiments was done to observe the results of combining the various salts in both fresh and salt water. The mixing of the above named salts produced dolomite, protodolomite, calcite, magnesian calcite, aragonite, nesquehonite, hydromagnesite, magnesite and a possible calcium-enriched magnesite. Sodium sulfate and halite formed in solution with carbon dioxide given off as gas.
In addition to the minerals formed many sedimentary features also appeared which mimic those observed in the naturally occurring rock. Aragonite needles can be precipitated from a milky solution by adding sodium bicarbonate to salt water saturated with gypsum. Unconsolidated lime sand can be cemented with aragonite using the same mixture and allowing it to set for several weeks.
Replacement of one mineral by another can be effected by varying the pH of the host liquid. Aragonitic shell materials can be converted to gypsum in a matter of hours by placing the shells in varying solutions containing magnesium sulfate, magnesium chloride, hydromagnesite or nesquehonite and acetic or carbonic acid. Aragonite will slowly leach away and be replaced by spar calcite when placed in a solution containing hydromagnesite and sodium carbonate.
AAPG Search and Discovery Article #90955©1995 GCAGS 45th Annual Meeting and Gulf Section SEPM, Baton Rouge, Louisiana