Gneiss

Why the bands separate

At high temperature the atoms in a rock become mobile enough for minerals to segregate. Quartz and feldspar, which are light and relatively rigid, recrystallise into continuous light bands; biotite, hornblende and other dark minerals concentrate into their own layers. The process is called metamorphic differentiation, and it produces a rock with strong compositional layering even if the protolith was uniform. The protolith could have been almost anything — granite, shale, volcanic ash, even an older gneiss — which is why geologists name gneisses by their mineral content and prefix: biotite gneiss, hornblende gneiss, augen gneiss for the variety with large eye-shaped feldspars.

Gneiss versus granite, the practical problem

This is the identification that trips up most field workers, because a coarse pink and grey banded rock feels like granite in the hand.

• Look from a distance. Granite shows speckling; gneiss shows bands. If you can see layering at two metres, it is gneiss.
• Trace a single band. Gneiss bands continue across the outcrop; granite's mineral patches do not.
• Check the mica. In granite, biotite flakes sit randomly between grains. In gneiss they lie in connected layers.
• Look for folding. Folded bands mean the fabric predates or accompanied deformation, which only happens in a metamorphic rock.

The oldest rocks on Earth are gneisses

Because gneiss forms at high grade and is relatively resistant to later alteration, it is what survives from the earliest chapters of Earth history. The Acasta Gneiss in the Northwest Territories contains zircon crystals dated to 4.03 billion years, the oldest known terrestrial material. The Lewisian complex of northwest Scotland includes gneisses around 3 billion years old with later intrusions cutting them. In China, the Archaean Taishan gneiss suite of Shandong is the standard reference for the oldest basement in the North China Craton. When you pick up a gneiss you may be holding rock that has been recycled through at least one full mountain-building cycle.

Practical use and field hazards

Gneiss is a decent building stone because it is blocky rather than fissile, and it is widely quarried for aggregate and dimension stone. Its banding is a structural weakness if the bands happen to be aligned so that they dip toward a road cut or a foundation excavation, so engineers treat gneiss as a rock whose engineering properties depend on orientation, much as schist does. In the field, gneiss terrain tends to form rounded, cleared outcrops with a smoother profile than schist — and because it is ancient and deeply weathered in places, saprolite zones can be tens of metres thick and unstable.

Recording a gneiss specimen

Photograph the outcrop before collecting, then record band thickness in centimetres, the mineralogy of both light and dark bands, the orientation of the banding, and whether any folds are present, with their axis direction. If the gneiss contains augen — large eye-shaped feldspars — note their size, because augen gneiss is usually derived from a deformed granite and the eyes preserve evidence of how much strain the rock took.

Sources

Hardness, streak, cleavage and specific-gravity values quoted on this sheet are checked against the references below.

• Geology.com Rocks — rock-type reference — descriptions and photographs of igneous, sedimentary and metamorphic rocks
• National Geographic Society — established educational publisher — encyclopaedic entry on the rock cycle
• British Geological Survey — UK national geological survey — the three rock types, the rock cycle and mineral definitions