An Atlas of Speleothem Microfabrics
L. Bruce Railsback, Department of Geology, University of Georgia.
PRIMARY SPELEAN ARAGONITE
Aragonite is the second most abundant mineral in speleothems, and it is second only only to calcite, the other polymorph of CaCO3. Aragonite differs from calcite in form, in that it almost always exists in speleothems as very elongate crystals, usually in botryoidal clusters. It also differs from calcite in atomic structure, in that each Ca2+ or other cation is surrounded by nine, rather than six, oxygens. The resulting large cation site allows cations larger than Ca2+, such as Sr2+, Ba2+, and Pb2+, to substitute for Ca2+ in significant abundance (Railsback, 1999). The relatively large cation site also allows more extensive inclusion of the uranyl cation, UO2+2 (Kitano and Oomori, 1971; Meece and Benninger, 1993) than in calcite, so that more uranium may be present for radiometric dating. Another geochemical difference between aragonite and calcite is that, for any given water composition from which they precipitate and temperature at which they precipitate, aragonite will be enriched in 13C (Romanek et al., 1992) and 18O (Grossman and Ku, 1986) relative to calcite.
Aragonite is less stable thermodynamically than calcite at earth-surface conditions, but aragonite nonetheless forms in many earth-surface environments. Conditions favoring the formation of aragonite rather than calcite include higher temperatures (Burton and Walter, 1987), faster precipitation (Given and Wilkinson, 1985), and higher Mg/Ca ratio of the precipitating aqueous solution (Folk, 1974; Berner, 1975). These differences have allowed interpretation of the presence and abundance of aragonite in speleothems in terms of paleotemperatures (e.g., Moore, 1956), extent of evaporation (e.g., Railsback et al., 1994), and evolving solution chemistry (e.g., González and Lohmann, 1988).
Aragonite's thermodynamic instability relative to calcite means that most aragonite converts to calcite with time. For example, Frisia (1996) documented conversion of aragonite to calcite in stalgmites from a cave in southeastern France. Calcite resulting from transformation of aragonite can often be recognized by relict textures and/or by high concentrations of Sr2+ inherited from the precursor aragonite. Frisia (1996) generalized that conversion of aragonite to calcite is common in speleothems more than about 20,000 years old, but rates of conversion vary greatly with local to microenvironmental conditions. The diagenetic section of this atlas illustrates some results of this conversion, as well as some examples of aragonite formed, rather than destroyed, by diagenetic processes.