Harzburgite

Harzburgite is a type of peridotite consisting mainly of olivine and orthopyroxene, with very little or no clinopyroxene. It represents "depleted" mantle, the residue that remains after partial melt (basaltic magma) has been drawn out of more fertile lherzolite.

Named after the Harz Mountains of Germany, harzburgite is a key component of the upper mantle and of the mantle section of ophiolites, slices of oceanic lithosphere thrust onto continents.

Accessory chromite (chrome spinel) is common, and economically important chromite deposits, called podiform chromite, occur within harzburgite.

Harzburgite forms in the upper mantle when fertile lherzolite undergoes partial melting and the resulting basaltic melt escapes, leaving behind a residue enriched in refractory olivine and orthopyroxene and depleted in clinopyroxene and aluminum. This process is central to seafloor spreading, where melt extraction beneath mid-ocean ridges produces harzburgitic mantle.

Harzburgite reaches the surface within ophiolite complexes and as mantle xenoliths in volcanic rocks. With further melt extraction it grades toward dunite, an almost pure olivine residue.

Harzburgite is a dense, coarse-grained, green-to-yellow-green rock that commonly weathers to a reddish-brown or rusty surface as olivine alters. Fresh surfaces show granular olivine with bronzy orthopyroxene and scattered black chromite grains, but little or no green clinopyroxene.

Hardness is about 6-7. Distinguish harzburgite from lherzolite by its scarcity of clinopyroxene, from dunite by its visible orthopyroxene content, and from pyroxenite by its olivine dominance. It often serpentinizes, taking on a greenish, soapy, lower-hardness alteration.

Harzburgite hosts podiform chromite deposits, an important source of chromium, and serpentinized harzburgite can yield magnesite, talc, and nickel. The rock is otherwise of limited direct commercial use.

Scientifically it is highly valued as a record of mantle melting and seafloor spreading. It has no gemstone or metaphysical tradition, though associated olivine can occasionally be gem-quality peridot.