Rocks and Microscopy > Harrisite (olivine crescumulate) Isle of Rum, Scotland by Ian Butler

151 images of a Harrisite thin section taken on a Nikon D60 using a modified Meiji microscope and direct objective projection. Stitched in PTGUI. Images taken under crossed polars

The type area for Harrisite is Harris Bay, Isle of Rum, Scotland. At the bay are exposed rocks of the Western Layered Series and the Central Series of the Rum Igneous Complex. Harrisite is an olivine crescumulate (i.e. where cumulate crystals have grown in place normal to the cumulate layering direction), and is the type example of this igneous texture. The elongate olivine textures in harrisite bear more than a passing resemblance to spinifex olivine from komatiites, but in the case of harrisite the rock is intrusive rather than extrusive. The texture can be reproduced experimentally and, like spinifex olivine, the skeletal crystal forms are attributed to crystal growth from a melt at high olivine supersaturation developed as a consequence of supercooling of the magma.