DuniteEdit

Dunite is an ultramafic igneous rock dominated by olivine, a magnesium-iron silicate that gives the rock its characteristic greenish hue. Most dunite is rich in olivine to the point of near-purity, with pyroxene and chromite occurring only as minor accessories in many samples. Because of its simple, high-magnesium composition and high density, dunite forms in the mantle and in magmatic cumulates where olivine crystallizes early from silica-poor magmas. In the field, dunite is often associated with larger ultramafic complexes and can occur as isolated blocks within ophiolites, where slices of the oceanic mantle are preserved. The rock has long been of interest to geologists because it records mantle composition and the processes that drive magmatic differentiation and tectonic evolution. olivine pyroxene chromite ultramafic peridotite ophiolite

Dunite’s name traces to the Dun Mountain in northern New Zealand, where the rock type was first described and studied in the context of early mantle geology. The name has since become a standard term for nearly monospecific olivine-rich ultramafic rock. It is closely related to, but distinct from, other rocks of the peridotite family, such as harzburgite and lherzolite, which together document the composition of the upper mantle and its heterogeneous layering. Dun Mountain peridotite ultramafic rock

Etymology

• The term dunite is traditionally linked to the Dun Mountain area, where early examples helped define the rock type. The nomenclature reflects the historical practice of naming igneous rocks after prominent localities and the geological context of their discovery. Dun Mountain igneous rock

Mineralogy and petrology

• Dunite is characterized by a dominant olivine phase, with forsterite-rich olivine typically constituting most of the rock. The olivine content in dunite commonly exceeds 90%, giving a texture that is coarse-grained and massive in many exposures. Minor pyroxene and chromite may be present, but the rock remains simple in mineralogy compared with more siliceous assemblages. The consistency of composition makes dunite a useful natural laboratory for studying mantle-derived materials and high-temperature crystallization processes. olivine forsterite pyroxene chromite

• In texture terms, dunite often preserves cumulate textures when it crystallized from a molten reservoir in the mantle or in the upper crust as part of a larger magmatic chamber. Where it forms as a cumulate, the rock records the sequence of olivine crystallization and the early stages of mantle differentiation. :