Most Effective Methods in Identifying, Etching, and Dissolving Limestones

Abstract

Limestones of different composition occur in Kentucky's subsurface, in addition to being exposed on over 50 percent of the surface. Limestones are of great importance in the geologic record worldwide. This sedimentary rock is comprised largely of calcium carbonate (CaCo3) composed of algae, fecal matter and skeletal fragments of thousands of different types of marine organisms of various sizes. These remains were compacted and cemented together over millions of years. Seventy samples from four locations were slabbed into cubes of fairly similar size and weight, then tested with various concentrations of different acids.

Geologists’ preferred method for testing for limestones is applying a 10 percent concentration of hydrochloric acid. Because limited published information was available, an experiment to test different concentrations of multiple acids was warranted to determine the most efficient methods for identifying, etching, and dissolving limestones. Hydrochloric, muriatic, acetic, sulfuric and nitric acids in concentrations of 40, 20, 10, 5, and 3 percent were selected. Rocks were placed into glass beakers, and 100 ml of each acid was poured slowly onto the various rocks. Reaction was observed and noted during the next four hours and over the course of seven days.

Hydrochloric acids were very efficient in reacting with the limestones. They ranked first, third, seventh, and eleventh. The 38.5% hydrochloric acid ranked first of all acids tested and 20% third. They had a long and very violent reaction when added to the limestones. Nitric acids ranked second, fifth, ninth, and twelfth. The 40% concentration reduced the limestone weight by 47.21% while exhibiting a moderately violent reaction. Muriatic acids ranked fourth, sixth, tenth, and thirteenth. The 31.5% acid concentration produced a forceful reaction dissolving 43.22% of the limestone in a week. Sulfuric acids with concentrations of 40, 10, and 5 percent did not perform well they ranked fifteenth, sixteenth, and 17th of all 17 acids tested. Sulfuric acid with a concentration of 20 percent ranked eighth. Acetic acid with a concentration of 3 percent was the only acetic acid used in the experiment. It was also the weakest of all acids used, but ranked fourteenth in efficiency.

AAPG Datapages/Search and Discovery Article #90258 © 2016 AAPG Eastern Section Meeting, Lexington, Kentucky, September 25-27, 2016