Browse Items (264 total)

Hand Specimen Sedimentary textures, very poorly sorted. Sub-round to angular grains up to 20mm across. Wide variety of clasts: dark purple, cream, rusty orange, grey, several types of lava. Some black, shiny igneous crystals. Brown-grey…

Hand Specimen Greenish black, coarse-grained rock, comprising olivine, clinopyroxene and plagioclase.Grains typically 1-2 mm across.  Thin-section Olivine - Subhedral to euhedral grains making up ~45% of the rock. Alteration to…

Hand Specimen Black, porphyritic rock comprising black clinopyroxene, grey feldspar and greenish olivine phenocrysts, in a fine-grained groundmass. Thin-section Plagioclase feldspar - Phenocrysts 1-4mm long, with a…

Hand Specimen Fine-grained, greenish black rock comprising dark and white minerals. Contains amygdales, mostly white, though some have black cores. Thin-section Fine-grained plagioclase laths and irregular clinopyroxene…

Hand Specimen Very dark grey, coarse-grained rock, typical grain size 1-2 mm. Main mineral phases are black clinopyroxene, greenish olivine and pale grey feldspar. The rock is cross-cut by a small number of hairline white…

Hand Specimen Porphyritic. Large, tabular, pale grey-yellow phenocrysts of plagioclase feldspar, randomly oriented, twin striations are visible on many of them. Pink-brown cryptocrystalline groundmass contains black, shiny…

Hand Specimen Mostly moderately coarse-grained (typically 1-2 mm), some regions finer-grained (typically ~0.5 mm). “Salt and pepper” appearance being speckled black and white. Two black phases, biotite mica and hornblende amphibole. White…

NB, the term ‘obsidian’ refers to a jet-black glassy rock. It usually has a rhyolitic composition, but not always. Hand Specimen Black rock with the characteristic vitreous lustre and conchoidal fracture of glass. The surfaces are…

Hand Specimen Pink-brown, fine-grained rock. Strong planar shear fabric, produced by flow: streaks of dark red-brown and black-brown in the groundmass, and siliceous streaks and lenses. Gold-coloured, metallic pyrite, and a black mineral are…

Hand Specimen Pink-brown, porphyritic rock with a very fine-grained groundmass. Abundant pink macrocrysts (5-10 mm). Phenocrysts of feldspar (2-4 mm) and quartz (2-4 mm). Accessory black mineral. Thin-section Quartz - Rounded phenocrysts (1-4…