From the Siskiyu Mountains of southern Oregon, near the town of Ashland, we collected this consolidated tuff of volcanic origin, which encloses a dramatic range of millimeter sized feldspar crystals. While the colors under simple crossed polarizers are a far cry from the plain polarized view, adding a 550nm wave plate simply opens a window on a completely new impression of feldspar. When it comes to common minerals that play up the phenomena of polarized light intereacting with birefringent crystals, this is hard to beat.
Your thin section of Ashland Feldspar has numerous large crystals, easily viewed with the ~15X magnifier.
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