Andesite: Fine-Grained Intermediate Rock

Andesites are typically medium gray and porphyritic, with phenocrysts of white to light gray plagioclase and/or black augite, hornblende, and/or biotite. Some andesites are reddish and some are vesicular. If you find quartz, you are probably looking at a rhyolite.

This andesite is so packed with plagioclase and hornblende phenocrysts that it almost looks coarse-grained. A close look (photo below) reveals a fine-grained gray groundmass surrounding the phenocrysts.

The orangish coloring on some of the grains is iron staining.

The black phenocrysts are hornblende, the white crystals are plagioclase. The grayish areas between the phenocrysts display some tiny mineral crystals but are mostly too fine-grained to see any crystal grains.
This andesite has a large black hornblende phenocryst and a large dark grayish blob in the center. The blurry boundary between the blob and surrounding andesite suggests that it may have been a drop of more mafic magma injected into the magma chamber just prior to eruption (or it would have assimilated). This injection may have triggered the eruption that ejected this andesite!
This sample has been handled by too many dirty hands: the gray is turning brownish and greasy. Yum. Nevertheless, you can still see the black hornblende crystals set against the fine-grained gray groundmass.
This is another andesite like the one above: gray groundmass with black hornblendes.

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