Plagioclase feldspar is very common in a great variety of rocks, including most volcanic and plutonic igneous rocks and basic and pelitic metamorphic rocks.  In A, plagioclase is the colourless mineral with low relief, relative to coexisting grey clinopyroxene.  The characteristic lamellar twinning (albite law) is the dominant feature of the plagioclase grains in B, with one grain near the centre with twins developed according to both the albite and pericline laws.  Several grains also show simple Carlsbad twinning.  The maximum interference colours here of first order white are typical of most plagioclase compositions, though Ca-rich varieties can range to first order yellow interference colours.  In C, polysynthetic twinning according to the albite and pericline laws is almost equally well-developed.  Note that compared to similar twins in microcline, in which the lamellae “pinch and swell” along their length, the lamellae in plagioclase remain relatively constant along their length, or taper somewhat.  D shows a complexly zoned plagioclase grain, in which compositional zonation (variation in Na/Ca) is seen as concentric bands varying in shades of grey and blue/grey, representing different extinction angles and thus different compositions.  Some very fine-scale zonation is present.  The white/pale yellow colour of the zone mantling the inclusion-rich core indicates a high Ca content .  Zoned plagioclases are common in volcanic and hypabyssal rocks. A and B from a troctolite from an unknown locality, C is from a granulite xenolith from the Moses Rock diatreme, Utah and D is from an andesite from Mt. St. Helens, Washington.  Fields of view in All fields of view are 5.5 mm across. A ppl, B, C and D x-nicols.