Scanning electron microscopy was the principal technique used to characterize the sand before and after each experiment. Polished sections of the starting material revealed that some of the labradorite (<2%) grains underwent minor alteration to lower -T phases such as albite, prehnite and Na/Ca zeolite. In some sections, minor amounts of mica and opaque minerals were also observed. Quartz grains are inclusion-free and showed no sign of any alteration. Examination of loose sand grains showed that most of the labradorite grains in the starting material have a prismatic habit and exhibit good cleavage, while quartz grains exhibited excellent concoidal fracture. The surfaces of quartz and feldspar grains before the experiments were devoid of any significant surface irregularities such as etch pits.

In all cases, the post experimental material is no longer a loose sand but is an indurated cohesive mass. In all experiments except the one done at 25°C, there is some textural evidence for dissolution of primary material and precipitation of secondary phases. Pitting of feldspar grains is common, with the pits sometimes occurring preferentially along cleavage planes (Figure 8a, b). Precipitation of secondary mineral phases is evidenced by the presence of fibrous or mesh-like coatings on feldspar grains (Figure 8 c, d).

Due to the extremely fine-grained nature and fragility of these coatings, microprobe analysis could not be used to determine their composition. However, EDX measurements using the SEM indicate that they are composed of Ca, Si and Al. X-ray diffraction analysis was conducted on loose grain mounts from several experiments, but the paucity of the secondary precipitate precluded a positive identification. The precipitates reported in other similar hydrothermal reaction studies [Divis and McKenzie, 1975; Hajash and Bloom, 1991; Zuddas and Michard, 1993] suggest that the secondary mineral observed in this study is a smectite or smectite-illite. The presence of etch pits and secondary mineral growth is not widespread, but rather appears to form in patches where dissolution and re-precipitation or alteration are favored. Since SEM observations are made on the surfaces of in situ chunks, it is unclear whether dissolution and growth occur in zones of fast or slow fluid flow (large or small pores) or whether another factor altogether is responsible for the localization of chemical activity. It is noteworthy that the pitting and precipitates are not necessarily found in close proximity to one another. Generally, dissolution features and secondary mineral precipitates are most obvious in the higher temperature runs.

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