142 Subsurface Geologic Methods 



Samples from rotary tools can rarely be used satisfactorily for percentage 

 determinations unless caving intervals have been cased. 



Siliceous limestone, tripolitic or cotton chert, and calcareous shale 

 may lose up to half their weight but retain unit volume after solution. 

 Such samples should be weighed before and after acidification, if the 

 percentage of residue compared to the original volume is needed. If the 

 volume of the vial is used as the unit of the original sample, the percentage 

 of nonporous residues may be scaled or observed through the glass vial. 

 Pure limestone or dolomite samples from cores or outcrops may leave 

 only a few grains of residue, and it may be necessary to use two, three, or 

 even five units of the original sample to obtain sufficient residue for 

 examination and determination. 



Solution of Samples 



Samples are generally dissolved in commercial hydrochloric (muri- 

 atic) acid. It is inexpensive, easily obtained, and effective. The acid 

 should be diluted with water to at least fifty percent but to no less than 

 ten percent. Warming will hasten the reaction, but undesirable precipi- 

 tates may form. Many complex reactions occur between caved material, 

 constituents of the indigenous material, and the impurities in the muriatic 

 acid. Iron, gypsum, and other precipitates frequently coat, stain, and 

 contaminate many types of residues. Many samples will not dry clean 

 if left in the spent acid and precipitates longer than six to eight hours. 



Chemically pure (CP) hydrochloric acid has advantages for special 

 work where outcrop samples are used, where precipitates or impurities 

 are undesirable, or when solution is extended over several days. Acetic 

 acid is best for liberation of delicate, fragile, lacy material or for micro- 

 scopic organisms. Delicate residues may be preserved by using very dilute 

 hydrochloric acid, but the time required for solution is lengthened. 



Beakers are the best receptacles for solution of the samples. They 

 are preferred because the lip facilitates washing and decanting, residues 

 may be easily removed, and a glazed spot is provided on the side for 

 identification of each sample. Molded tumblers or other cheap glassware 

 may be used, but the breakage due to heat while drying samples equals or 

 exceeds the greater original cost of heat-resistant glassware. 



The procedure for residue preparation is simple. Samples of unit 

 quantity are placed in a glass receptacle properly identified on a slip of 

 paper under a pyrex dish or by any consistent regular arrangement. Sam- 

 ples are then digested in acid, washed, dried, labelled, and stored for 

 examination. The use of several stainless-steel trays, or other type of tray, 

 holding forty to fifty beakers, facilitates the bulk movement of samples 

 to the hood for acid application, washing, drying, or other operations 

 involving the handling of large numbers of samples. 



The first application of acid should be small to prevent foaming 

 caused by the rapid effervescence of powder and fine material. The foam- 



