Chart 1e - Gabbroic Rocks. IUGS classification of gabbroic rocks based on the relative modal proportions of pyroxene (Px), plagioclase (Pl), and Olivine (Ol). Some of the names depend on the relative modal proportions of orthopyroxene (Opx) and clinopyroxene (Cpx). Use the slider to select the Opx-Cpx proportions for the rock. If the rock has more than 5% modal quartz relative to feldspar, add "quartz" as a prefix to the name. If the rock has feldspathoid, add "foid-bearing" as a prefix to the name. If the rock has more than 5 modal percent hornblende, click here to find the name: IUGS Hbl Gabbroic Rocks