|Open-File Report 2005-1001|
|OFR 2005-1001 Home / Procedures / East-Coast Database / GIS Data Catalog|
U.S. Geological Survey
Open-File Report 2005-1001
USGS East-Coast Sediment Analysis: Procedures, Database, and GIS Data
The insoluble residue of a sediment is that portion which remains after digestion in hydrochloric acid (HCl) of a prescribed strength under specific conditions. Because carbonate rocks may contain significant amounts of chert, anhydrite, and siliciclastic sand, silt, and argillaceous material, the determination of insoluble residues is useful for comparing sediments from different localities and for expressing their degree of purity or contamination by detrital and authigenic constituents (Milner, 1962).
To perform a typical analysis, approximately 15 g of wet sediment are placed in a pre-weighed labeled 600-ml beaker and dried at 100oC. The beaker is placed in a desiccator to cool and is then re-weighed to determine water content. If the salinity of the interstitial water is known, corrections for the salt content of marine samples can be calculated from these water contents. Cool dilute (10%) hydrochloric acid (HCl) is slowly added in small increments (30 - 50 ml ) until all effervescence has stopped and further addition of the dilute acid solution produces no reaction. After effervescence has ceased, the excess acid is carefully decanted, preferably by siphon. The insoluble residue is washed at least five times with distilled water, and after each washing the supernatant liquid is decanted. Care must be taken to ensure that enough time is allowed for complete settling (at least 24 hours) and that no sediment is lost during decantation. Alternatively, the insoluble residue can be washed onto a labeled pre-weighed paper filter and thoroughly rinsed. Finally, the insoluble residue is dried at 100oC, weighed, and the percent calcium carbonate is calculated as a percentage by weight of the original bulk sample. The difference between 100% and the value obtained is the percent calcium carbonate. Grain-size analyses may be performed on the residues, or they may be examined petrographically.
This procedure will remove calcite and aragonite. If significant
amounts of dolomite are present, 6N
should be used and the suspension must be heated to near boiling to
dissolve the dolomite. Although dry sample weights are necessary, water
contents need be determined only if the samples are marine and salt
corrections must be applied, or if appreciable amounts of hygroscopic
water are present. Although commercial grade
(muriatic acid) may be used, gypsum formation due to the presence of
sulfate ions is a concern (Krumbein and Pettijohn, 1938). The
preparation of dilute
HCl and digestion portions of
this analysis should be performed under a fume hood; gloves and safety
glasses should be worn.