Maryland Department of Natural Resources


Petrologic and chemical investigation of chemical weathering in mafic rocks, eastern Piedmont of Maryland

1974, Cleaves, E.T.

Report of Investigations 25


Microenvironments of chemical weathering were investigated for eight mafic rock weathering profiles by petrologic and chemical methods in the eastern Piedmont of Baltimore and Harford Counties, Maryland. The chemical weathering is a constant volume process, whereby 30 to 60 percent of the original rock mass is removed as dissolved solids in percolating groundwater. In the transition from rock to saprolite, original rock minerals are replaced bys secondary minerals of lesser density, bulk density decreases, and porosity increases. Mineralogically, the transition is accompanied by the replacement of alumina-poor minerals like hornblende and plagioclase by more aluminous ones such as montmorillonite, kaolinite, and gibbsite. The mineral changes reflect the relative enrichment of alumina contrasted to silica, although silica, alumina, and alkali and alkaline earth cations are all leached out of the system.

In poorly-drained domains, the environment is weakly acid to alkaline and reducing. Alteration of the alumino silicate rock minerals yields montmorillonite, kaolinite, aqueous silica and various cations. In contrast chemical weathering in well-drained domains proceeds in an acidic, oxidizing environment and yields kaolinite, gibbsite, hydrous ferric oxides, amorphous matter plus aqueous silica and various cations.