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Variolite

Variolites ( Latinvarius, "speckled, variegated") are a group of dark green basic igneous rocks that, especially on weathered surfaces, exhibit pale colored spots, or spherules, that give them a pockmarked appearance. In some conditions these spots weather out prominently; they are grey, pale green, violet or yellowish, while the matrix of the rock is usually dark green.

The variolites are related most closely to the basalts or diabases. They are nearly always much decomposed, and, since they are also fine-grained rocks, their original composition may be much obscured by secondary changes. The variolitic spots are rounded in outline and are often about a quarter-inch in diameter, but may exceed this size. They have a radiate structure and are sometimes, though not generally, zoned with concentric circles of different appearance and composition. Many authors have compared them with the spherules of the acid rocks ( obsidians and rhyolites), and undoubtedly some kinds of variolite are merely glassy spherulitic varieties of basalt. The tachylyte selvages of the dolerite dikes of the west of Scotland, for example, often contain large brown spherules that are easily visible in hand specimens. These spherulites consist of very thin divergent fibers, and their nature is often difficult to determine on account of the indefiniteness of the optical characteristics of minerals in this state. It seems probable, however, that they are mostly feldspar embedded in dark brown glass. Small phenocrysts or skeleton crystals of olivine, augite and plagioclase feldspar may occur in these tachylytes.

The Collaborative International Dictionary

Variolite

Variolite \Va"ri*o*lite\, n. [L. varius various + -lite: cf. F. variolite.] (Geol.) A kind of diorite or diabase containing imbedded whitish spherules, which give the rock a spotted appearance.

Wiktionary

variolite

n. (context geology English) A kind of diorite or diabase with embedded whitish spherules, which give the rock a spotted appearance.