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I have a green, moderately magnetic, rock with yellow inclusions. The rock is very heavy for its size. What is the most likely minerals in such a rock?

Update: Found on the Maine coast. Rock has been smoothed and polished by waves and tides. Most inclusions are between 1 and 3 cm. Metal knife scratches surface (pic #2). Picture #1 closeup of surface.

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Update: New closeup image

Significantly closer image

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Looks like the rock could be a basalt porphyry. The matrix appears to be basalt (see diagram below for mineral make-up) with phenocrysts (the large crystals) of potasium feldspar. It is really hard to tell for sure with just a photo, but this diagram could help you out with the mineral composition of each type of igneous rock.

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Image source: https://physicalgeology.pressbooks.com/chapter/7-3-classification-of-igneous-rocks/

More information can be found at the above link.

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  • $\begingroup$ While I don't argue that it may be a mafic rock, it is definitely not basalt - most crystals in basalt are not visible by naked eye and here one can clear distinguish at least 3 types of crystals in the groundmass. $\endgroup$ – Gabija Dec 1 '17 at 12:08
  • $\begingroup$ Could it be an eroded peridotite with another mineral forming the inclusions? $\endgroup$ – MikeC Dec 5 '17 at 21:12
  • $\begingroup$ If you think the matrix/groundmass is >15% of the green mineral (likely olivine) then it could be considered peridotite. If not, then consider it a gabbro matrix. I cannot tell because I have a hard time seeing green in a matrix like that. Looking at the photo again I can see Gabija's point that there are visible crystals. $\endgroup$ – wanderweeer Dec 5 '17 at 23:24

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