Aplite

Aplite

Aplite (fine-grained center)
Composition
Quartz, Feldspar

Aplite (pronounced /ˈæplaɪt/) is an intrusive igneous rock in which the mineral composition is the same as granite, but in which the grains are much finer, under 1 mm across.Quartz and feldspar are the dominant minerals. The term 'aplite' or 'aplitic' is often used as a textural term to describe veins of quartz and feldspar with a fine to medium-grain "sugary" texture. Aplites are usually very fine-grained, white, grey or pinkish, and their constituents are visible only with the help of a magnifying lens. Dykes and veins of aplite are commonly observed traversing granitic bodies; they occur also, though less frequently, in syenites, diorites, quartz-diabases and gabbros.

Aplites usually have a genetic affinity to the rocks they intrude. The aplites of granite areas, for example, are the last part of the magma to crystallize, and correspond in composition to the quartzo-feldspathic aggregates that fill up the spaces between the early-formed minerals in the main body of the rock. They bear a considerable resemblance to the eutectic mixtures which are formed on the cooling of solutions of mineral salts, and remain liquid till the excess of either of the components has separated out, finally solidifying en masse when the proper proportions of the constituents and a suitable temperature are reached.

The essential components of aplites are quartz and alkali feldspar (the latter usually orthoclase or microperthite), microcline and albite. Crystallization has been apparently rapid (as the rocks are so fine-grained), and the ingredients have solidified almost at the same time. Hence their crystals are rather imperfect and fit closely to one another in a sort of fine mosaic of nearly equi-dimensional grains. Phenocrysts of feldspar occur occasionally and of quartz more seldom; but the relation of aplite to quartz-porphyry, granophyre and felsite is very close, as all these rocks have nearly the same chemical composition.