This diagram is for the fraction of plutonic rocks that don't qualify for the QAP classification diagram because they have less than 10 percent in total of quartz (Q), alkali feldspar (A) and plagioclase feldspar (P). They are 90 percent or more mafic (iron- and magnesium-rich) or "dark minerals," which in most cases means some combination of olivine, pyroxene minerals and amphibole minerals (conventionally called hornblende, by far the most common of this group). Biotite (black mica), if present in significant amounts, is customarily added to the hornblende content.
Peridotite includes a wide range of rock compositions that are dominated by olivine. Peridot, the jeweler's name for gem-quality olivine, gives rise to the rock name. Dunite is almost entirely olivine. The other two rock names, pyroxenite and hornblendite, are self-explanatory.
Pyroxenite and peridotite may be further divided according to the particular pyroxene minerals making them up. That diagram is on the next page. You can picture that diagram as taking the left edge of this diagram and fanning it out into a new triangle or ternary diagram. What that means for professional purposes is that "pyroxenite" and "peridotite" are considered field namesnames you can assign just by inspecting the rock by eyerather than precise rock names, but often that kind of name is very useful.
Ultramafic rocks are commonly altered to serpentinite.