Websterite

Websterite is a type of pyroxenite, an ultramafic igneous rock made up of both orthopyroxene and clinopyroxene in significant amounts, with little or no olivine. It is named after Webster, North Carolina, where it was first described.

As a pyroxene-dominated rock, websterite is dark, dense, and rich in magnesium and iron. It commonly occurs as cumulate layers and veins in mafic-ultramafic complexes and as bands within mantle peridotite.

Websterite is uncommon and primarily of interest to geologists studying layered intrusions and mantle processes.

Websterite forms largely by the accumulation of orthopyroxene and clinopyroxene crystals from cooling mafic magma, producing cumulate layers within layered intrusions alongside pyroxenite, peridotite, and gabbro. It also occurs as pyroxenitic veins and layers within mantle peridotite, where it may form by melt-rock reaction or by crystallization of melts in mantle conduits.

It is found in ophiolites, layered complexes, and as mantle xenoliths, recording crystal accumulation and melt evolution at depth.

Websterite is a heavy, coarse-grained, dark green-to-greenish-black or brownish rock dominated by two pyroxenes, with little or no green olivine. Pyroxene cleavage faces (meeting near 90 degrees) and a dull-to-glassy luster are common.

Hardness is about 5-6. Distinguish websterite from other pyroxenites by the presence of both orthopyroxene and clinopyroxene, from peridotites such as lherzolite and wehrlite by its scarcity of olivine, and from gabbro by its near-absence of plagioclase feldspar. Thin-section study usually confirms the two pyroxenes.

Websterite has limited direct commercial use but can be crushed for aggregate. Websterite-bearing intrusions and ophiolites are sometimes associated with chromite, nickel, and platinum-group element mineralization.

It has no gemstone or metaphysical tradition. Its principal value is scientific, as a record of pyroxene accumulation and mantle melt processes within ultramafic systems.