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| Click on image to enlarge. Photo © Daniel R. Snyder |
Above: Ophitic-texture pyroxene and plagioclase in a metabasalt are being replaced by hydrous minerals of the greenschist facies. The large orange and brown cluster was a pyroxene grain enclosing the now-corroded plagioclase laths. The pyroxene has been replaced by uralite, a massive amphibole usually composed mainly of actinolite (less commonly of hornblende). The scattered plagioclase laths show varying degrees of replacement by epidote, and probably by other minerals too small to identify. Extensive replament has occurred in the two adjacent, parallel plagioclase laths at right of center (See second image below for greater detail). Greenstone cobble in glacial gravel, southeastern Michigan, Specimen No. 49. Provenance likely northern Ontario. XPL. Imaged area 1.3mm by 2mm.
Below: PPL image of above. Green color of uralite is characteristic of actinolite (as well as hornblende).
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| Click on image to enlarge. Photo © Daniel R. Snyder |
Below: Higher-magnification image of area right of center in top photo. Note the cluster of small epidote grains that have replaced much of the two adjacent plagioclase laths. The replacement of plagioclase by epidote in this location was likely speeded by a channel between the two grains (visible as a narrow strip of uralite) that provided access by percolating fluids. XPL. Imaged area 0.5mm by 0.8mm.
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| Click on image to enlarge. Photo © Daniel R. Snyder |
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