Tachylite

Tachylite (also spelled tachylyte) is a basaltic volcanic glass — the mafic, iron-rich counterpart of obsidian. While obsidian forms from high-silica rhyolitic lava, tachylite forms from low-silica basaltic magma chilled too quickly to crystallize.

It is typically black to dark brown and opaque, owing to its high iron and magnesium content and abundant tiny crystallites, whereas obsidian is often translucent on thin edges. Tachylite tends to be denser and duller than obsidian.

It commonly occurs as thin selvages and chilled margins on basaltic lava flows, dikes, and pillow lavas, and is a familiar product of subaqueous and rapidly cooled eruptions.

Tachylite forms by the very rapid quenching of basaltic (mafic) lava, most often where hot lava meets water, air, or cold rock and cools too fast for crystals to grow.

It is typical of the glassy chilled margins of basaltic dikes, sills, and lava flows, and of pillow-lava rims formed during underwater eruptions. Because basaltic magma is iron- and magnesium-rich, the resulting glass is dark and contains abundant minute crystallites (microlites) that make it opaque.

Over time tachylite is unstable and tends to alter to palagonite (a yellow-brown hydrated material). It is found in basaltic provinces worldwide, including Hawaii, Iceland, and the Scottish Hebrides.

Look for a dull to slightly glassy, opaque black or dark brown material occurring as the chilled rim of a basalt flow or dike. It breaks with a conchoidal to splintery fracture and has a hardness around 5-6.

Tachylite is denser and less glassy-bright than obsidian, and its opacity even on thin edges is a key distinction — obsidian usually transmits some light at its edges. Its streak is gray to brown.

Look-alikes: obsidian is silica-rich, often translucent on edges, and brighter glassy; basalt is fully crystalline and granular under a lens; sideromelane is a clearer, less crystallite-laden basaltic glass. Field context (basaltic setting, chilled margins) strongly supports identification.

Tachylite has little commercial or lapidary use because it is opaque, dull, prone to alteration, and not attractive when polished. It is not a gemstone.

Its importance is primarily scientific: as a basaltic glass it preserves the original composition of mafic magma, making it valuable for studying volcanic eruptions, cooling histories, and magma chemistry. Tachylite and its alteration product palagonite are also studied as analogs for volcanic processes on Mars and the ocean floor.

It has no significant metaphysical tradition and is mainly of interest to volcanologists, petrologists, and geology students.