They occur in the form of intrusive sheets, both within the associated granitic bodies and in the surrounding country rock.

[4] Syenite-aplites consist mainly of alkali feldspar; the diorite-aplites of plagioclase; there are nepheline-bearing aplites, including those containing the elaeolite variety of nepheline.

In this state, the parent intrusion is mechanically coherent enough to support tensile stresses, leading to fractures.

These fractures may then be filled by the remaining uncrystallised parts of the original magma (the residual fluids) to form the aplites.

This has been observed in the Half Dome granodiorite in Yosemite National Park, California, with multiple stages of aplite intrusion recognised, with the earlier aplite sheets becoming deformed by later intrusions into the main granodiorite body, but recognisable from their consistent chemistry.