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From Chang and others (2009): "SG06-3 is the only potassic trachybasalt (Le Bas and others, 1986) recovered from St. George. Both texturally and chemically, SG06-3 is different from all other rocks collected. Texturally, sections of the rock are relatively fine-grained, whereas other regions are coarse-grained. The rock contains olivine, titanaugite, plagioclase, K-feldspar, opaque minerals (ilmenite), and apatite. Natrolite is present in some vesicles. Olivine occurs as large, acicular phenocrysts with the lowest MgO contents of all rocks, with average core compositions of Fo40 and average rim compositions of Fo30. The olivine is also enriched in Ca relative to other rocks analyzed, with average cores and rims have 0.70 wt % and 0.91 wt % Ca, respectively. Clinopyroxene phenocrysts are large and abundant. They are pink plane-polarized light and are commonly zoned with darker pink cores. Many grains have sieved cores with opaque inclusions. Grains are euhedral to subhedral and have rims with corroded edges or a green alteration product along the edges. Clinopyroxene cores have average TiO2 contents of 2.83 wt % and Al2O3 contents of 4.68 wt %. Rims have average TiO2 contents of 2.16 wt % and Al2O3 contents of 3.70 wt %. Plagioclase compositions are more sodic than other rocks, with average core compositions of An44 and rims of An 22. Large, euhedral, needle-shaped apatite inclusions are abundant as inclusions in plagioclase grains." |