Ultramafic rocks are especially poor in SiO2 and have high Mg:Fe ratios. Pyroxene (either orthopyroxene or clinopyroxene), olivine, and (less commonly) plagioclase are the dominant minerals in these rocks. The IUGS system, shown in Figure 6.117, names ultramafic rocks based on their relative amounts of olivine, orthopyroxene, and clinopyroxene. We call ultramafic rocks rich in olivine, peridotite. Those that are less than half olivine are pyroxenites. Other names, shown in the figure, divide peridotites and pyroxenites into smaller classes.
Many specimens of ultramafic rock are carried up from the mantle as xenoliths in magmas. The left and middle photos below (Figure 6.118 and 6.119) show examples. The photo on the left contains two different composition xenoliths. See the opening photo of this chapter for a larger view of this photo. The brown xenolith is mostly orthopyroxene and olivine. The green one contains light-green olivine and darker-green clinopyroxene, but they are hard to tell apart. The specimen also contains a few dark colored grains of orthopyroxene. Figure 6.119 shows a sample of garnet lherzolite that was carried to the surface in a South African kimberlite pipe. The garnets are red, olivine and orthopyroxene are green, and the less common emerald green grains are clinopyroxene. The photo on the right (Figure 6.120) shows a sample of pyroxenite, composed of large crystals of orthopyroxene and clinopyroxene. It is from the layered ultramafic Stillwater Complex of Montana; two other photos of Stillwater rocks appeared earlier in this chapter (Figures 6.20 and 6.21).
Ultramafic minerals are most stable at high temperature and they alter by reaction with water or carbon dioxide when exposed to normal Earth surface conditions. Consequently, finding fresh, unaltered ultramafic rock is difficult. Variable amounts of secondary serpentine, chlorite, talc, brucite, or calcite are nearly always present. We often find rocks called serpentinites, in which serpentine has replaced all or nearly all of the mafic minerals. Many ultramafic rocks – including those in kimberlite nodules – have been altered so much that we cannot determine their original mineralogy.bla