aplite - hand specimen of aplite from an aplite-pegmatite dike that intruded anorthosite

Aplite is essentially a fine-grained granite. With most aplites, the composition is the same as granite, but the crystal grains are a millimeter or smaller in size, so it has a sugary texture. Aplites occur as narrow dikes and veins, and because the injected magma loses heat rapidly, crystals do not have much time to grow.

Aplites are often associated with pegmatites, forming in the late stages in the crystallization of the pegmatite, when volatile elements have migrated higher in the crust. The aplite then solidifies as a granite, but because of rapid heat loss, is finer grained than a granite would be.

This aplite was collected from an aplite-pegmatite dike intruded into the Poe Mountain Anorthosite of the Laramie Anorthosite Complex in Sybille Canyon, Wyoming. The pegmatitic portion of the dike contains garnet and muscovite, indicating that it was derived from crustal material that lay below or adjacent to the anorthosite.

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