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| Click on image to enlarge. Photo © Daniel R Snyder |
Above: Above: Fine-grained metamorphosed basalt from Pre-cambrian Mid-Continent Rift. Note abundant corroded plagioclase. No relict pyroxene is evident. Most of the black masses are magnetite (sample is strongly magnetic). Round black mass at lower left is a chlorite vesicle filling. The light rim is calcite. Mamainse Point, Ontario (eastern end of Lake Superior). XPL. Imaged area 1.3mm by 2mm. Photo made with 4x objective.
Below: Higher-magnification image of a portion of image above. Note predominance of small grains with upper first-order and second-order interference colors. Black masses are magnetite (compare with PPL image below). XPL. Imaged area 0.5mm by 0.8mm. Photo made with 10x objective.
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| Click on image to enlarge. Photo © Daniel R. Snyder |
Below: As above, PPL.
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| Click on image to enlarge. Photo © Daniel R. Snyder |
Below: Anheadral to subhedral grains of crystalline replacement mineral(s) surrounded by partially-altered plagioclase. Dark areas are magnetite. Left image XPL, taken with 10x objective, digitally enlarged to match right image. Right image PPL, taken with 20x oblective. Imaged area of each panel 0.24mm(W) by 0.32mm(H).
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| Click on image to enlarge. Photo © Daniel R. Snyder |
Below: Higher-magnification image of a field of mineral grains characteristic of this specimen. PPL. Imaged area 0.13mm by 0.2mm. Thus, the long dimension of the imaged area is 200 microns. Photo made with 40x objective.
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| Click on image to enlarge. Photo © Daniel R. Snyder |
I will write more about the difficulty of making positive identification of the green mineral. It could be any of several hydrous calcium-aluminum silicates. If YOU know what it is, please comment!
More Text coming
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