Petrology, Economic Geology and Tectonics
The Hard Rock Group, Department of Earth Sciences, University of Oxford
Hard Rocks Grouped - the "Rock of the Week" gallery ...
Click on an image to show the full-size picture with its caption.
Actinolite schist from the Canisp Shear Zone, Achmelvich, NW Scotland. Who would think a rock made of just one mineral could have such a strikingly coloured and picturesque microstructure? DJW's specimen.
Garnet-kyanite-schist from the Danba region of eastern Tibet. A sea of colourful mica provides excellent contrast with the isotropic syn-tectonic garnet and the extensively recrystallised quartz. Submitted by Owen.
The blue domains are cordierite-rich pseudomorphs after staurolite, which contain corundum, sillimanite, spinel, magnetite and ilmenite in the cores, along with relict staurolite. The pseudomorphs are surrounded by partial melts in a biotite-rich matrix. The sample is from the Basong Tso region of SE Tibet.
A Warhol-esque major cation map of one of the Hoosac garnets, from the upper staurolite zone, western Mass. A high-alumina schist similar to the famous Gassets schist, and the garnets evidence polymetamorphism as defined by mineral chemistry and inclusion patterns. Courtesy of Nick Gardiner.
Ca map of a garnet porphyroclast from a kyanite-grade mylonitic schist. The map reveals a euhedral garnet core (that includes sillimanite) and a zoned rim, indicating the rock's two-stage history, prior to post-peak mylonitisation. Courtesy of Owen, from his Tibet collection.
Texture of pyroxene, olivine and Fe-Ti oxide from the Upper Zone of the Rustenburg Layered Suite in the northern limb of the Bushveld Complex, South Africa. Victoria Honour's project, 2013.
Grain-boundary migration in a high-grade Himalayan schist, by Brendan. This quartz grain has been caught at a stage of the coarsening process when it has succeeded in enclosing the small mica flakes in its core, but its boundary is still pinned by the larger flakes around its margins.
Manganese nodule in UHP mica-schist from the Zermatt-Saas ophiolitic unit at Lago di Cignana, Western Alps. Made of spessartine-rich garnet, Mn oxide and piemontite, in a matrix of quartz, phengite and more piemontite. Crop from a whole-slide scan.
Bushveld anorthosite T20 from the teaching collections, scanned on DJW's Nikon film scanner between polaroid sheets. This was the first 'cover picture' for the Hard Rock Group's internal web pages.
Chrome-rich jade rock from Myanmar (Burma): a corroded chromite grain surrounded by sprays of blue-green pyroxene of the rare variety kosmochlor (NaCrSi2O6). Field of view 2.5 mm.
Hand specimen of granulite from the upper part of the Jijal metagabbros, at the base of the Kohistan arc, northern Pakistan. Garnet forms coronas around clinopyroxene in the upper left of the photo. Across the reaction front (below and right) the rock is a two-pyroxene granulite. Field of view 4 cm across.
An oval porphyroclastic enclave of garnet-clinopyroxene granulite, about 1 cm across, in high-temperature amphibolite from the metamorphic sole beneath the Oman ophiolite. These enclaves demonstrate that the uppermost part of the sole was metamorphosed at high-pressure granulite facies before being hydrated and sheared during ophiolite obduction.