Zeolites in Closed Hydrologic Systems

Abstract

Zeolites were first described from vugs and fissures in basaltic flows in the mid-eighteenth century (Cronstedt 1756), and they are now known to be widespread in a variety of geological environments. More zeolite species occur in vesicles and fractures of basaltic rocks than in any other geologic setting. However, zeolites from sedimentary rocks, in particular, represent the most important occurrences both in terms of aerial extent of deposits and in terms of the abundance of certain zeolite species. The introduction of new analytical techniques in the latter part of the 20th century provided a significant stimulus to zeolite research; these techniques allowed identification of large quantities of zeolites in widespread deposits of sedimentary rocks and, in particular, in sediments from saline, alkaline lakes of the western United States (Sheppard and Gude 1968, 1969a, 1973). Based on geological and hydrologic environments, zeolite occurrences in sedimentary rocks can be classified in the following framework: (a) saline, alkaline lakes; (b) alkaline soils and land surfaces; (c) deep-sea sediments; (d) open hydrologic systems; (e) as products of hydrothermal alteration; and (f) burial diagenetic or low-metamorphic environments. From a broader perspective, zeolite deposits can be classified into two main groups, namely closed hydrologic systems and open hydrologic systems. The work of Sheppard and Gude (1968, 1969a, 1973) and later Surdam (1977), discussing not only the mineralogy of the deposits but also the geology, hydrology, and chemistry of the depositional basins, represent milestones in the study of closed hydrologic system formation of zeolites. Surdam (1977) considered closed hydrologic basins in two different tectonic settings: (a) block-faulted regions in arid and semiarid regions; and (b) trough valleys associated with rifting. Examples of the first setting are the closed lakes of the Basin and Range province of the western United States whereas the …