Perpendicular feldspar rock near the western margin of the Skaergaard intrusion,
Mellemø. The rock is defined by branching crystals of plagioclase
feldspar directed inwards toward the centre of the intrusion. The rock
is commonly taken as a good illustration of heterogeneous crystal nucleation
and growth on a solid surface. The crystallisation is probably closely
related to volatile fluctuations. Photo by Jens C. Andersen.