Figure 58 (a) Alkali feldspar, illustrating penetrative Carlsbad twinning (the reddened crystal is 2 cm long). The pinkish colour is common in alkali feldspar but not diagnostic. (b) Plane-polarised light image of alkali feldspar in a silica-rich lava (field of view 6 mm across). Although the crystal has low relief, the slightly brown, turbid appearance due to alteration is distinctive. (c) The same field of view as in (b) between crossed polars. The alkali feldspar has first-order grey interference colours. The crystal is a simple twin, its twin plane is defined by the slight difference in interference colours (light and dark grey).