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Looking into the cobbles
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Looking into the cobbles
Sections
 
Quartzite cobbles cut through the centers of craters. Note the diverging concussion fractures and the white halos below the depressions resulting from intense micro-fracturing.
Thin sections
Spallation occurs also on a microscopic scale. In the photomicrograph (arrow length is 2.2 mm), a spall is completely (2-D) detached from a quarzite grain in the Buntsandstein conglomerate. The image shows pure tension without contact between the neighboring grains (in 2-D). The matrix is opaque from iron-hydroxide.
Cross-sections of millimeter-sized craters in Buntsandstein quartzite cobbles, drawn from photomicrographs. Note the central mounds and the dinstinct open fissures below the crater floor. Both are in proof of tensile stress (from spallation) and exclude pressure dissolution by overburden and tectonic compression, the more so as no material is dissolved, removed, and precipitated below the craters. Pressure dissolution results in an increase of compaction and a decrease in porosity. Evidently, we are observing just the opposite.
Similar fracture pattern below a crater in Buntsandstein quartzite cobble and a crater produced in impact spallation experiments (gabbro target).
Top: Photomicrograph (plane polarized light) of the halo below a crater in Buntsandstein quartzite cobble. The halo results from intense intergranular micro-facturing and intragranular planar deformation features. Distinct PDF concentrations are encircled. Bottom: Photomicrograph (crossed nicols) taken from the encircled area. Decorated intragranular PDFs of varying orientation and decorated intergranular, subparallel micro-fractures (SSW - NNE trending) in quartz.
Photomicrograph (by courtesy of Inka Siegert) of subparallel decorated micro-fractures in Buntsandstein quartzite cobble. The fresh-water fluid inclusions have been analysed thermomicrometrically establishing hydrothermal conditions at about 200°C trapping temperatures and pressures between 0.11 and 0.26 GPa. A similar hydrothermal signature has been reported for the Siljan Ring impact structure (Komor et al. 1988; see I.Siegert, 1997).
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