A single interesting crystal of alkali feldspar, mostly orthoclase, dominates the field of view. From 00:08 to 00:20, a carlsbad twin is evident by difference of extinction when the stage is rotated in XPL. At times during this rotation, "wispy" exsolution lamallae of albite are brightly illuminated while the k-spar host is partially extinct, making the perthitic character of this feldspar obvious. (The term "wispy" is used by Winter, 2010, in the caption for his Figure 3.18.) It seems that the k-spar host is only partially transformed from orthoclase (higher T) to microcline (lower T), as indicated by patchy tartan twinning in the host (00:30-00:40, 40x magnification).

This video was produced during the spring of 2020, at the heart of COVID, as part of my effort to teach petrology online.

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