Browse Items (264 total)

Hand Specimen This rock has a porphyritic texture. Phenocrysts of white feldspar up to 30mm long are surrounded by a coarse groundmass of quartz, biotite and white feldspar. Thin-section Alkali feldspar (orthoclase) - Grains typically 2-4mm…

Hand Specimen Coarse-grained, porphyritic texture. Phenocrysts of pink feldspar, 10-30 mm long, tabular, display clear cleavage planes. Groundmass of coarse-grained white feldspar, pink feldspar, quartz and biotite. Quartz grains…

Hand Specimen Superficially specimens “a” and “b” look very different due to their colours. L340a is pale grey, while L340b is dark purple-red. The colour of L340b indicates that it is ferruginous, containing significant quantities of iron…

Hand Specimen Medium to coarse-grained rock with a granular texture. White feldspar laths 2-4 mm long are clearly visible. Other phases: pink feldspar, black biotite mica, black hornblende amphibole and pale grey quartz. These grains 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…

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…

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 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…

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 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…