Sample of an apophysis of limburgite from a layer of semicrystalline, porphyritic hyalobasanite about 3 feet thick. It may be a flow, but is likely a sill. In the apophyses the rock becomes very dense and black, but still remains slightly porphyritic. Thin sections reveal that it corresponds to limburgite containing olivine, augite, and magnetite, but practically no feldspar, as primary phases. Olivine is often distinctly zoned. Augite is reddish in color, shows hourglass structure and is highly zoned. Positive 2V is around 60 degrees with strong dispersion r>v; the directions of the extinctions are also highly dispersed, ??c=45 degrees. It is a titaniferous augite. Zeolites are seen in cavities.
Geology and petrology of the Pribilof Islands, Alaska