Harzburgite

Harzburgite is an ultramafic rock in the peridotite family. It is characterized by a dominant assemblage of olivine and orthopyroxene, with little or no clinopyroxene. Accessory minerals such as chromian spinel and, in altered rocks, serpentine may occur. In hand sample, harzburgite appears dense and pale green to brownish due to the high olivine content.

In terms of mineral proportions, harzburgite typically contains roughly 70–90 percent olivine and 10–30 percent orthopyroxene, with clinopyroxene often absent or present only in trace amounts. The exact composition can vary with metamorphic history and degree of depletion. Serpentinization can modify the rock, producing serpentinite in altered sections.

Formation and occurrence of harzburgite are linked to mantle processes. It forms mainly as a residue after partial melting of mantle peridotite during the production of basaltic crust. As basaltic magma is extracted at mid-ocean ridges or in supra-subduction-zone settings, the remaining mantle becomes depleted in clinopyroxene, yielding harzburgite. Consequently, harzburgite is a common constituent of ophiolites and of the upper mantle in xenoliths, and it helps characterize mantle regions beneath spreading centers and some subduction zones.

Etymology-wise, the name harzburgite derives from Harzburg, a locality in the Harz mountains of Germany, where the rock was first described. Harzburgite is often discussed alongside lherzolite and dunite as a record of mantle composition and melt extraction processes.