LABORATORY PROCEDURES

Insoluble Residues

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 100^oC. 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 100^oC, 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 HCl 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 HCl (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.