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An introduction to minerals and rocks under the microscope

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Figure 59 (a) Plagioclase feldspar, showing lamellar twinning, although this is rarely visible in hand specimen (crystal 4 cm long). (b) Plane-polarised light image of plagioclase in a gabbro. The plagioclase has very low relief and is hard to see, although it lies between the higher relief pyroxene crystals. (c) The same field of view as in (b) between crossed polars, with distinctive zebra-striped lamellar twinning now obvious.

 3.6.2 Feldspars