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PPL: Thin section of granite from Shap Fell, Cumbria. The photo shows the characteristic brown & occasionally green colours of biotite mica. |
PPL: The same section has been rotated 90deg. clockwise to show the strong pleochroism of biotite. Pleochroism is the term used when some coloured minerals change colour (under plane polarised light) when the microscope stage is rotated. Not to be confused with interference colours as shown in the photo below. |
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XPL: Under crossed polars (left) the the rock is seen to be composed of biotite (light & dark brown). The cloudy white minerals in the w.n.w. section as well as in the southern and eastern peripheries are alkali feldspars. The rest of the section consists mainly of quartz. The rock is known as an adamellite, being composed of quartz, biotite, plagioclase and alkali (potassic) feldspars, although no plagioclase is visible in this section. The type locality for this rock is around Adamello in the Tyrol from whence it gets its name.
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