material. Large fragments of chert or other insoluble material considered indig- 

 enous may be picked out with forceps from the sieve and added to the residue. 



Amount of Sample 



The volume or weight of sample used to make a residue depends on the 

 purpose of the study, the type of samples used, and individual judgment. Seven 

 grams is an ample amount of sample for ordinary uses. This weight is an aver- 

 age for the volume contained in a one-dram vial, 45 by 15 millimeters. The 

 same volumes of 10 different homogenous samples ranging from very fine to 

 very coarse fragments of limestone, shale, sand, and chert were weighed, and 

 the average of 7 grams was determined. The volume-weight of 7 grams reduces 

 considerably the time for the preparation of residues. Small samples of less- 

 than-unit volume must be weighed if percentage determinations are desired. 

 The use of a small scoop sized for a unit volume or a tip balance saves time. 



Many workers do not use percentages, but the percentages of residue are 

 valuable in many instances for correlation and identification of beds. 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 2, 3, or even 5 units of the original sample to obtain sufficient 

 residue for examination and determination. 



Solution of Samples 



Samples are generally dissolved in commercial hydrochloric (muriatic) 

 acid. It is inexpensive, easily obtained, and effective. The acid should be diluted 

 with water to at least 50 percent but to no less than 10 percent. Warming will 

 hasten the reaction, but undesirable precipitates may form. Many complex re- 

 actions occur between caved material, constituents of the indigenous material, 

 and the impurities in the muriatic acid. Iron, gypsum, and other precipitates, 

 in many instances, coat, stain, and contaminate many types of residues. Many 

 samples will not dry clean if left in the spent acid and precipitates longer than 

 6 to 8 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 



79 



