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 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…

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…