Unusual secondary Ca-Mg-carbonate-kerolite deposits in basaltic caves, Kauai, Hawaii

Aragonite, calcite, monohydrocalcite, dolomite, magnesite, hydromagnesite, and talc-like kerolite have been identified in speleothems from basaltic ca ves on the island of Kauai Hawaii. These secondary minerals have crystalliz ed in the presence of freshwater seeping from the basaltic host rock, and t he mineralization process is ongoing at some locations. The deposits are ex tremely variable in macroscopic character, ranging from millimeter-thin, po wdery coatings to hard crusts several centimeters thick. Many deposits are associated with actively forming microbial mats, and stromatolitic structur es are common in the thicker deposits. Formation of the mineral deposits is a result of a combination of direct precipitation from solution by evapora tion and CO, degassing, alteration of precursor minerals, and (or) microbia l processes and physicochemical conditions within the mats. These unusual d eposits provide important insights into the mineralogy and geochemistry of low-temperature alteration and carbonation of terrestrial basalts and the p otential role of microbes in such processes.