Water Well Journal
Issue link: http://read.dmtmag.com/i/592194
field geoscientist and drilling contractor. It is important as a geoprofessional to understand the assembly of the forma- tion and system in which work is per- formed. Robert Compton in his book, Geology in the Field, expresses a valid rationale for field identification of geo- logic material: Several important rock-forming min- erals may be distinguished in the field by staining procedures similar to those used in the laboratory. Smooth, clean outcrop surfaces have been found to give results almost as good as sawed and lap-ground surfaces, and freshly broken surfaces have given adequate results. (Compton 1985) The use of various stains and acids for identification of rock samples is not a new idea. Workers have used these methods for more than a century to identify various mineral assemblies. J.A.D. Dickson explained in his 1966 paper, "Carbonate Identification and Genesis as Revealed by Staining" in the Journal of Sedimentary Petrology: Carbonate minerals are stained over a set period of time with alizarin red-S and potassium ferricyanide only if they will react with dilute hydrochlo- ric acid solution, with which the stain is prepared. The rate of solution of carbonates in the acid control the intensity of color development . . . Staining with alizarin red-S differenti- ates carbonate minerals into two groups. Aragonite, calcite, witherite, and cerussite, which dissolve rapidly in dilute hydrochloric acid, are stained, while dolomite, siderite, mag- nesite, and rhodochosite, which react much more slowly with acid, remain unstained. . . . The color produced on carbonates varies according to the rate at which any specific carbonate reacts with the hydrochloric acid. (Dickson 1966) Dickson added the increase of con- centration of hydrochloric acid modifies the produced color by an increase in the reaction rate. A color of bright red to pink to pale mauve can be realized as the rate of reaction increases. (Dickson 1966) Field Methods The following field methods are a qualitative and not a quantitative method for determining the characteris- tics of the material. These methods are to assist field workers in determining various rock types, and by no means take the place of skilled observation and analysis by laboratory techniques for the determination of these materials. During the drilling process, cutting samples are collected at specific and regular intervals. Once the cuttings are collected, sorted, and containerized, a small cross sample of cuttings is removed from each sample interval. These cuttings represent a cross section of the drilled interval. During the drilling process, cuttings are coated with drilling and formation material. In order to prevent misidentifi- cation of cuttings, it is important to wet each cutting sample to be identified with distilled water to remove the surface coating. This functions as a multifold process. Color determinations are made, based on wetted samples as well as the reac- tion of the material to water. Shale, clay, silt, and gypsum, among others, all react in the presence of water. During this process, it is often best to have a fresh surface to observe reactions. Acid Test Once all other descriptive measures have been performed, the cutting sam- ples can be processed with hydrochloric acid. The use of a diluted solution of hy- drochloric acid (10% HCl) is a regularly practiced method for identification of carbonates or the removal of carbonate cement from various samples to reveal the true matrix of the sample. Example of the staining process of calcite for the identification of limestone. 30 November 2015 WWJ waterwelljournal.com FIELD NOTES from page 29