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Showerthought: I was calling all the dark, heavy rocks in my creek 'basalt' for years
A guy at the mineral show in Flagstaff pointed out my 'basalt' sample was full of olivine crystals, so it's actually peridotite. Anyone know a good field guide for basic igneous ID?
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rubybarnes16d ago
That "full of olivine crystals" bit is the giveaway. I've seen a lot of basalt with some olivine specks, but if it's truly packed with them, you're right on the edge. Could be something like an olivine basalt instead of a true peridotite. The line gets blurry without a thin section. For a field guide, the old "Roadside Geology" books for your state are a solid, simple start.
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roberts.paul15d ago
Ever read that old paper about the olivine threshold in mafic rocks? @rubybarnes is right, the line is pretty fuzzy without a scope.
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