The Conversion of Calcite to Fluorite (1956)

r: A method has been discovered whereby calcareous objects can be changed, by what is believed to be a molecule-by-molecule replacement, from a carbonate to a fluoride composition without any change in gross structure. Evidence supporting this conclusion, and details of the method, are presented in this paper, as well as some suggestions for practical application. The conversion of calcite to fluorite JOHN F. GRAYSON Magnolia Petroleum Company Dallas, Texas In February, 1955, while a siliceous sample was being processed in the course of palynological work at the Field Research Laboratories of the Magnolia Petroleum Company, an interesting phenomenon was observed. This phenomenon, as detailed below, has been explained and demonstrated to numerous visitors to our Laboratories, chiefly geologists, biologists, chemists, and paleontologists from various industrial, academic and government institutions. The expression of surprise was so unanimous that it was decided a short article would be of interest to other scientists and may, as is true in our work, assist in the understanding of this interesting chemical process. The curious phenomenon noted was that after the standard hydrofluoric acid treatment for the removal of the siliceous matter, a large amount of coarse debris remained in the sample. This debris was removed from the sample by sieving. Upon examination, the debris was found to be composed of shell fragments. This was surprising because, until this time, it had been assumed that the hydrofluoric acid, being a halogen acid, would perform the same function as hydrochloric acid and would dissolve the calcium carbonate. Since hydrofluoric acid had not eliminated the calcareous matter, an attempt was made to dissolve it by treatment with hydrochloric It was rather startling to find that upon treatment with hydrochloric acid no reaction occurred. The sample was washed with distilled water, and an X-ray analysis was made. The analysis showed that the shell fragments were no longer composed of calcium carbonate, but were now composed of calcium fluoride. The following equation shows the reaction that had occurred: CaCO3 + 2HF -> CaF, + CO2 + H20 This simple chemical equation in itself is not spectacular. Probably chemists have known about it for years, although it was not observed in a quick survey of chemical textbooks. However, it is suspected that the implications and possible applications in the fields of biology and geology have not been considered. The field of palynology, with which the author is concerned, offers a good example of how the knowledge of this reaction will help to eliminate processing difficulties. Now that this reaction is understood, it is known that samples containing calcium carbonate must first be treated with hydrochloric acid or nitric acid if a higher concentration of polospores is desired. (Polospores is a new term here proposed as a convenient general name used to designate pollen and/or spores. Derivation: Pollen and/or spores.) The concentration of polospores on a microscope slide is probably the greatest processing problem palynologists encounter. Given a sufficient flora with which to work, the palynological results that can be obtained are limited only by the knowledge and the ability of the palynologist. During the palynological investigations of the past several years, a group of very small foraminifera has been discovered. These are commonly referred to as microforaminifera. They are abundant in some samples, absent from others, and some palynologists probably havt never seen them. Wilson and Hoffmeister (1952) discuss these microforaminifera and state that they have tests that are not destroyed by either hydrochloric or hydrofluoric acid. From the author's experience, this is true only when the hydrochloric acid treatment follows the hydrofluoric acid, micropaleontology, vol. 2, no. 1, pp. 71-78, text-figs. 1-9, table 1, january, 1956 71 This content downloaded from 207.46.13.64 on Sat, 03 Sep 2016 03:57:36 UTC All use subject to http://about.jstor.org/terms