Garnet lherzolite (field of view ~1.6 cm across). Purplish red = pyrope garnet Bright green = chromian diopside pyroxene Dark green crystals = olivine & orthopyroxene

One of the most incredibly rare rocks in my collection is this garnet lherzolite from South Africa. This is a slice of a xenolith nodule (“xeno” = foreign, and “lith” = rock) from a kimberlite pipe in Kimberley, central South Africa. As magma rises in a pipe during kimberlite formation, it plucks rock from the deep walls of the pipe. Many kimberlite xenoliths turn out to be samples from the deep crust, lithospheric mantle, and even possibly sub-lithospheric mantle. This garnet lherzolite xenolith is such a sample of lithospheric or sub-lithospheric mantle! Rocks such as garnet lherzolite and spinel lherzolite have traditionally been inferred to be primitive mantle materials.

This rock is a variety of peridotite, an ultramafic intrusive igneous rock. It is dominated by two dark-greenish minerals: forsterite olivine (having veinlets of serpentine) and enstatite orthopyroxene. Notice that the rock also contains rounded, purplish-red crystals. These are pyrope garnets. There are also widely scattered, bright emerald green colored crystals of chromian diopside pyroxene. Small amounts of accessory minerals are also present.