Water Well Journal
Issue link: http://read.dmtmag.com/i/592194
WWJ November 2015 31 Twitter @WaterWellJournl Once the acid is applied to the sam- ple, the sample will react rapidly by effervescing or fizzing if limestone or calcium carbonate is present. If the sample is dolomite, the reaction will be weak or not at all. Partially dolomitized limestone can react with acid but at a lesser intensity. Hydrochloric acid ap- plied to anhydrite will have no reaction. With the application of acid to the sample, it is important to observe the reaction of the acid on the sample to ensure the reaction is with the sample material and not the residue from the drilling process. Sometimes the residue will be heavily laden with calcium carbonate and the sample will be a non- carbonate material. The acid will effer- vesce on the residue and may inadver- tently lead to a misidentification of the sample. It is important to remember acids have a low pH and are corrosive. Proper handling and storage techniques should be used when working with acids. Staining Test There are numerous staining tech- niques for various mineral identification methods. For the purpose of this discus- sion, we will only be using stain for identification and differentiation of limestone and dolomite through the use of alizarin red-S stain. In addition, the application of stain can be used to dif- ferentiate cementing material in various sandstones. The application of stain to rock sam- ples is a multipart process. The follow- ing steps when applied consistently gen- erally yield satisfactory results on drill cuttings. • Saturate sample with distilled water to remove drill debris coating. • Apply several drops of alizarin red-S stain to sample. • Allow for reaction for two to three minutes. • If calcite is present, sample will turn reddish-pink. If the sample is dolomite or partially dolomitized limestone, it will not turn color or may only slightly turn color in a few places. (SciOptic 2015) In applying the process in the field to cutting samples, I have found relatively good results from the use of alizarin red-S stain mixed with a diluted hydro- chloric acid. This process appears to work well when dealing with partially dolomitized limestone and calcium car- bonate cemented caliche and sandstone. In some formations, such as in cal- cium carbonate cemented caliche and sandstone, the identity of the matrix material can become obscured through digenetic processes. Through the application of diluted HCl and alizarin red-S stain and the use of magnification under microscope or hand lens, the matrix of the material can potentially be identified. Identifying Sulfate Rock Differentiating anhydrite, gypsum, or other sulfate-related rocks from lime- stone and dolomite can be challenging in the field. Even more complicated is determining cementing characteristics of sandstones or rocks altered through diageneses. Using basic chemistry, anhydrite and gypsum can be processed to reveal their true sulfate identity. The sulfate ion in the presence of hydrochloric acid and barium chloride with the application of heat will precipitate a white barium sul- fate powder or barium sulfate crystals. To determine the presence of anhy- drite or gypsum in a cutting sample, the following process has generally yielded satisfactory results. It should be noted the sample will be destroyed during processing. • Place sample in crucible and crush into a fine powder or crush with stir rod. Identifying precipitated sulfate ions in a gypsiferous cemented sandstone. FIELD NOTES continues on page 32