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Allanite and epidote occur in the parent rocks of weathered regolith at the Coweeta Hydrologic Laboratory in North Carolina and exhibit different responses to weathering. Petrographically, epidote and allanite are identical at Coweeta, and only with additional analytical techniques (e.g., EDS or LA-ICP-MS) can the two be distinguished. Allanite is more abundant in unweathered bedrock but weathers readily below the weathering front. In contrast, the much less abundant epidote persists into the solum with only minimal evidence of weathering. The rapid dissolution of allanite is likely influenced by its metamict nature. Both epidote and allanite at Coweeta dissolve by interface-controlled dissolution kinetics, evidenced by etch pits on grain surfaces. Etch pits appear to be either large "negative crystals," or small, shallow, and elongated. The incipient stage of allanite weathering is characterized by Al mobility and Fe precipitation as goethite. During the initial stage of allanite weathering, carbonate precipitates, but with progressive weathering the carbonate is dissolved. Based on electron microprobe analyses and point-count data of the Ca-bearing phases in the Coweeta bedrock, accessory (<1%) allanite contains a minimum of approximately 25% of the total bedrock Ca by volume, whereas garnet and plagioclase contain 5% and 70%, respectively. Although allanite and epidote are Ca-hosts, only allanite is present in significant quantities, and is weathering sufficiently rapidly, to serve as an important source of Ca in pore and stream waters at Coweeta. Allanite weathering should, therefore, be evaluated in weathering studies of crystalline silicate bedrock, especially where other lines of evidence indicate the need to invoke additional Ca sources. [ABSTRACT FROM AUTHOR]

Studies the factors affecting the formation of a protective surface layer during silicate-mineral weathering. Silicate weathering; Behavior of elements normally considered immobile; Volume ratio of product to reactant.

An introduction is presented in which the editor discusses the crystallography on planet Mars.

Various approaches have been used to estimate when and how long liquid water was present at the surface of Mars. The olivine dissolution-lifetime application suggested by Stopar et al. (2006) and Olsen and Rimstidt (2007) is here adapted and tested at the scale of individual etch-pits using published data from an experimental system in which the volume of mineral removed and the duration of the mineral removal episode are known. Different assumptions about the specific geometry of etch-pits on olivine result in surface-area estimates that vary by less than a factor of two from the simple hemispherical pit used in the calculations. Given that other sources of uncertainty in mass-time relationships of silicate-mineral dissolution during natural weathering can be up to four orders of magnitude, the effects of differing geometric assumptions about the shapes and surface areas of the etch-pits are negligible. Using compiled experimentally determined forsterite dissolution rates and the imaged etch-pit sizes from experiments recovers the duration of the experiment that produced the imaged etch-pits to within less than a factor of two. This suggests that extensively etched olivine surfaces imply a dominance of the etch-pit walls over the bulk surface between the etch-pits during olivine corrosion. The approach adopted here recovers the timescales of experimental etch-pit production on olivine at STP and extreme undersaturation of the solution with respect to olivine in experiments where pH is known. Continued progress in understanding the fundamentals of olivine dissolution kinetics will narrow the ranges of uncertainty in mineral-lifetime estimates at Mars' surface in support of constraining the compositions and duration of potentially habitable aqueous solutions on Mars [ABSTRACT FROM AUTHOR]