Looking at rocks through a microscope

Thin section photomicrograph, crossed polarizers, banded particles are plagioclase feldspar that is twinned. Citronelle Field, Rodessa Formation, Cretaceous, South Alabama, more than 2 miles below the surface.

Yet more sandstone

Wavy muscovite (upper right) and large twinned plagioclase feldspar (lower half). Sandstone, Rodessa Formation, Citronelle field, South Alabama, about 2 miles below the surface.

Something rotten

A close-up of sandstone, showing a grain of plagioclase feldspar (blue-gray) having been partially dissolved, and the holes filled in with calcite (pink)