Browse Items (10 total)

Formula Mg3[Si2O5](OH)4 Structure Sheet silicate Crystal System Crystal system: monoclinic or orthorhombic (-) Optical Properties Serpentine is commonly very fine-grained. It can be recognised by its textural association with olivine…

Formula (Mg,Fe2+,Fe3+,Mn,Al)12[(Si,Al)8O20](OH)16 Structure Sheet silicate Crystal System Crystal system: monoclinic (+) (-) Physical Properties Colour in hand specimen: green Cleavage: one, good Hardness: 2-3 Optical…

Hand Specimen Pale buff coloured, crystalline rock. Numerous cavities, thought to have been formed by shrinkage during alteration of primary calcite (CaCO3) to secondary dolomite (CaMg[CO3]2). There is a weak reaction to dilute acid, which…

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 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 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…