ABSTRACT

Laboratory experiments at ordinary temperature and pressure for periods up to 240 days on Recent biogenic carbonate materials showed that fresh water and sea water dissolve aragonite and magnesium calcite prior to mineralogical changes.

The observed rates of dissolution of calcium, magnesium, and strontium indicate that these elements are incorporated in aragonite and magnesium calcite in more than one way (in lattice positions, absorbed in lattice interstices, or as inclusions). This suggests the presence of more than one mineral phase in the skeletal material studied; these phases differ in response to solution, in quantity, and probably in chemical composition.

Calcium, magnesium, and strontium are dissolved in proportions different from those in the original solid, that is incongruently. The experimentally established sequence of preference is Mg > Ca > Sr, as a rule

Factors found to determine direction and degree of incongruency in dissolution include mineralogy (number, kind, and relative abundance of phases present), chemical composition, physiologic effects (taxonomy and non-taxonomy related), and chemical composition and volume of waters effective in dissolution.

Incongruent dissolution determines abundance of ions derived from solids in the diagenetic environment and hence, ion availability for precipitation in cement and for inhibition or catalysis in inversion or cement precipitation. Magnesium and strontium contents of shell material have been used by others as temperature and salinity indicators in Recent environments. The present study shows that because of incongruent dissolution these indicators are not dependable for paleo-environmental analysis.