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 matrix of volcanic ash.
Carboniferous
Melrose, Scotland
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 matrix of volcanic ash.
Olivine
- Subhedral to euhedral grains making up ~45% of the rock. Alteration to magnetite and serpentine, particularly along the curving cracks. Cracks across the thin-section, could be due to expansion of the rock as the olivine was altered.
Clinopyroxene
- Interstitial phase making up ~35% of the rock. Moderate relief, inclined extinction and first order pink to second order orange interference colours.
Plagioclase feldspar
- Interstitial phase making up ~20% of the rock. Low first order interference colours and lamellar twinning.
Accessory minerals - Biotite, small, interstitial grains, altered.
This is a cumulate rock. It has undergone late-stage shear banding.
Olivine phenocrysts crystallised first.
Clinopyroxene grew around the olivine phenocrysts.
Interstitial feldspar filled in the remaining gaps.
Shearing of the rock produced bands of deformation.
Hydrothermal alteration.
Olivine
- Subhedral to euhedral grains making up ~45% of the rock. Alteration to magnetite and serpentine, particularly along the curving cracks. Cracks across the thin-section, could be due to expansion of the rock as the olivine was altered.
Clinopyroxene
- Interstitial phase making up ~35% of the rock. Moderate relief, inclined extinction and first order pink to second order orange interference colours.
Plagioclase feldspar
- Interstitial phase making up ~20% of the rock. Low first order interference colours and lamellar twinning.
Accessory minerals - Biotite, small, interstitial grains, altered.
This is a cumulate rock. It has undergone late-stage shear banding.
Olivine phenocrysts crystallised first.
Clinopyroxene grew around the olivine phenocrysts.
Interstitial feldspar filled in the remaining gaps.
Shearing of the rock produced bands of deformation.
Hydrothermal alteration.
Black, porphyritic rock comprising black clinopyroxene, grey feldspar and greenish olivine phenocrysts, in a fine-grained groundmass.
Plagioclase feldspar
- Phenocrysts 1-4mm long, with a euhedral, tabular form. Randomly orientated. Characteristic lamellar twinning. Some are zoned. Inclusions of magnetite, clinopyroxene and other minerals. Extensive alteration along abundant cracks.
Clinopyroxene (augite)
- Euhedral phenocrysts, 1-2 mm across. Moderate relief, inclined extinction and first order pink to second order orange interference colours. Pale pink in thin-section. Some are zoned.
Olivine
- Very few phenocrysts are fresh enough to be easily identifiable. These are colourless in plane polarised light, have moderate relief, and up to second order pink interference colours. No cleavage.
- 1st phase of alteration along cracks: The majority of the phenocrysts have been completely altered to greenish serpentine.
- 2nd phase of alteration: orange brown mineral – probably the amphibole cummingtonite. Characteristic curving cracks and the prismatic form of olivine are still visible.
Groundmass
- Fine-grained. Plagioclase feldspar needles, clinopyroxene, some olivine (mostly altered), and opaques, probably magnetite. Occasional calcite grain.
Carboniferous
Volcanic neck, Lion's Haunch, Arthur's Seat, Edinburgh
Black, porphyritic rock comprising black clinopyroxene, grey feldspar and greenish olivine phenocrysts, in a fine-grained groundmass.
Plagioclase feldspar
- Phenocrysts 1-4mm long, with a euhedral, tabular form. Randomly orientated. Characteristic lamellar twinning. Some are zoned. Inclusions of magnetite, clinopyroxene and other minerals. Extensive alteration along abundant cracks.
Clinopyroxene (augite)
- Euhedral phenocrysts, 1-2 mm across. Moderate relief, inclined extinction and first order pink to second order orange interference colours. Pale pink in thin-section. Some are zoned.
Olivine
- Very few phenocrysts are fresh enough to be easily identifiable. These are colourless in plane polarised light, have moderate relief, and up to second order pink interference colours. No cleavage.
- 1st phase of alteration along cracks: The majority of the phenocrysts have been completely altered to greenish serpentine.
- 2nd phase of alteration: orange brown mineral – probably the amphibole cummingtonite. Characteristic curving cracks and the prismatic form of olivine are still visible.
Groundmass
- Fine-grained. Plagioclase feldspar needles, clinopyroxene, some olivine (mostly altered), and opaques, probably magnetite. Occasional calcite grain.
Fine-grained, greenish black rock comprising dark and white minerals.
Contains amygdales, mostly white, though some have black cores.
Fine-grained plagioclase laths and irregular clinopyroxene (augite) grains, replaced with calcite. Many of the plagioclase laths have been almost entirely altered.
The amygdales are filled with zeolite, growing in spherulitic and radiate patterns. Many have rims of calcite and fine-grained chlorite.
Some vein material - granular quartz with sutured contacts.
Zeolites are produced at low temperatures. They are commonly precipitated in basaltic volcanic piles.
Cenozoic
Lava, Skye
Fine-grained, greenish black rock comprising dark and white minerals.
Contains amygdales, mostly white, though some have black cores.
Fine-grained plagioclase laths and irregular clinopyroxene (augite) grains, replaced with calcite. Many of the plagioclase laths have been almost entirely altered.
The amygdales are filled with zeolite, growing in spherulitic and radiate patterns. Many have rims of calcite and fine-grained chlorite.
Some vein material - granular quartz with sutured contacts.
Zeolites are produced at low temperatures. They are commonly precipitated in basaltic volcanic piles.
Plagioclase feldspar
- Tabular phenocrysts making-up ~50% of the rock. Characteristic lamellar twinning. Some are zoned.
Olivine
- Phenocrysts have been extensively altered, rendering them almost unrecognisable. The olivine has decomposed to magnetite and serpentine. Relict olivine grains display characteristic curving cracks and euhedral form.
Clinopyroxene (augite)
- Interstitial grains making-up ~35% of the rock. Poikilitic texture. Inclined extinction. Interference colours from first order pink to second order orange. Pale grey under plane polarised light. Some simple twinning.
Accessory minerals
- Opaque, interstitial, probably magnetite.
Coarse grained → slowly cooled, intrusive.
Hydrothermal alteration of olivine after crystallisation of the rock.
Plagioclase feldspar
- Tabular phenocrysts making-up ~50% of the rock. Characteristic lamellar twinning. Some are zoned.
Olivine
- Phenocrysts have been extensively altered, rendering them almost unrecognisable. The olivine has decomposed to magnetite and serpentine. Relict olivine grains display characteristic curving cracks and euhedral form.
Clinopyroxene (augite)
- Interstitial grains making-up ~35% of the rock. Poikilitic texture. Inclined extinction. Interference colours from first order pink to second order orange. Pale grey under plane polarised light. Some simple twinning.
Accessory minerals
- Opaque, interstitial, probably magnetite.
Coarse grained → slowly cooled, intrusive.
Hydrothermal alteration of olivine after crystallisation of the rock.
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 flakes of biotite.
Plagioclase feldspar
- Euhedral phenocrysts, 2-20 mm long, displaying the characteristic lamellar twinning. Some alteration to white mica. In places they have been replaced by chlorite and calcite.
Biotite
- Phenocrysts, most <1mm long, with characteristic brown to pale straw pleochroism and straight extinction.
Accessory minerals
- Quartz
- Apatite
Groundmass
- Mainly cryptocrystalline. A small number of feldspar grains can be identified. Small ragged vesicles.
Two-stage cooling: phenocrysts followed by groundmass.
Phenocrysts are euhedral indicating that they crystallised free-floating in a magma chamber.
The fine-grained groundmass must have crystallised rapidly due to cooling at the Earth’s surface.
Alteration occurred after crystallisation.
Early Paleozoic
Cansip Porphyry Sills
Sutherland
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 flakes of biotite.
Plagioclase feldspar
- Euhedral phenocrysts, 2-20 mm long, displaying the characteristic lamellar twinning. Some alteration to white mica. In places they have been replaced by chlorite and calcite.
Biotite
- Phenocrysts, most <1mm long, with characteristic brown to pale straw pleochroism and straight extinction.
Accessory minerals
- Quartz
- Apatite
Groundmass
- Mainly cryptocrystalline. A small number of feldspar grains can be identified. Small ragged vesicles.
Two-stage cooling: phenocrysts followed by groundmass.
Phenocrysts are euhedral indicating that they crystallised free-floating in a magma chamber.
The fine-grained groundmass must have crystallised rapidly due to cooling at the Earth’s surface.
Alteration occurred after crystallisation.
Hornblende amphibole
- Pleochroic in greens and cream. Some of the grains display simple twinning. Two cleavages intersect at 56°.
Biotite
- Subhedral to anhedral grains, pleochroic in brown and straw. Straight, speckled extinction. Chlorite partially replaces biotite – late stage alteration.
Quartz
- Anhedral, interstitial grains. They display undulose extinction and contain abundant tiny inclusions, but are characteristically fresh and unaltered.
Feldspar, mainly plagioclase
- Mostly anhedral grains, though some are lath-shaped. Alteration. Zonation.
Accessory minerals:
- Sphene
- Magnetite
- Apatite
Rock history Hornblende is euhedral. It is, therefore a primary igneous phase, and must have crystallised from a wet melt.
Magma cooled relatively slowly, allowing large crystals to grow.
Hornblende amphibole
- Pleochroic in greens and cream. Some of the grains display simple twinning. Two cleavages intersect at 56°.
Biotite
- Subhedral to anhedral grains, pleochroic in brown and straw. Straight, speckled extinction. Chlorite partially replaces biotite – late stage alteration.
Quartz
- Anhedral, interstitial grains. They display undulose extinction and contain abundant tiny inclusions, but are characteristically fresh and unaltered.
Feldspar, mainly plagioclase
- Mostly anhedral grains, though some are lath-shaped. Alteration. Zonation.
Accessory minerals:
- Sphene
- Magnetite
- Apatite
Rock history Hornblende is euhedral. It is, therefore a primary igneous phase, and must have crystallised from a wet melt.
Magma cooled relatively slowly, allowing large crystals to grow.
NB, the term ‘obsidian’ refers to a jet-black glassy rock. It usually has a rhyolitic composition, but not always.
Black rock with the characteristic vitreous lustre and conchoidal fracture of glass.
The surfaces are scattered with tiny spherical holes. These are vesicles.
Holding the thin-section up to the light, the spherical vesicles within the brown glass are clearly visible.
Under the microscope the pale brown glass is scattered with tiny opaque speckles in a weak flow texture.
Spherical vesicles are mostly filled in with mounting medium; notice its distinctive appearance under cross-polarised light. There are a small number of opaque infilled circles. These could be artefacts or black clumps of iron oxide.
Tiny crystals are scattered throughout the glass, indicating that some devitrification has occurred. Circles of devitrification, probably nucleated on oxide crystals.
Glass → Very rapidly cooled magma. Probably extrusive.
Vesicles: perfectly spherical, indicating that they were not being sheared by magma flow, i.e. the magma was not flowing.
glassy rhyolite
glass
obsidian
Cenozoic
Yellowstone, U.S.A.
NB, the term ‘obsidian’ refers to a jet-black glassy rock. It usually has a rhyolitic composition, but not always.
Black rock with the characteristic vitreous lustre and conchoidal fracture of glass.
The surfaces are scattered with tiny spherical holes. These are vesicles.
Holding the thin-section up to the light, the spherical vesicles within the brown glass are clearly visible.
Under the microscope the pale brown glass is scattered with tiny opaque speckles in a weak flow texture.
Spherical vesicles are mostly filled in with mounting medium; notice its distinctive appearance under cross-polarised light. There are a small number of opaque infilled circles. These could be artefacts or black clumps of iron oxide.
Tiny crystals are scattered throughout the glass, indicating that some devitrification has occurred. Circles of devitrification, probably nucleated on oxide crystals.
Glass → Very rapidly cooled magma. Probably extrusive.
Vesicles: perfectly spherical, indicating that they were not being sheared by magma flow, i.e. the magma was not flowing.
glassy rhyolite
glass
obsidian
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 associated with the quartz-filled lenses.
Plagioclase feldspar
- Phenocrysts, fairly fresh, tabular, typically 1-2 mm long, with characteristic lamellar twinning.
Opaques
- Grains associated with the quartz-filled lenses. Range of sizes from groundmass up to ~0.5 mm. These are a mixture of pyrite, FeS2, (tending to have a cubic form) and the unidentified black mineral, both seen in hand specimen.
Groundmass
- Cryptocrystalline, almost glassy, speckled with tiny, irregular quartz and feldspar crystals. Probably devitrified glass. Plagioclase altered to white mica.
Secondary quartz
- Narrow veins and lenses of quartz, considerably coarser than the groundmass. The grains of quartz have sutured contacts, and may have been injected after consolidation of the lava. Some of these clumps are rimmed with inward pointing needles of feldspar.
Phenocrysts crystallised in a magma chamber. The magma erupted and cooled very rapidly to a fine-grained, glassy groundmass, preserving flow banding texture. Injection of quartz-rich fluid, which precipitated quartz in lenses and veins.
Very high temperature hydrothermal alteration caused devitrification of the glassy groundmass.
banded rhyolite
rhyloite flow
Glacial boulder, North Wales
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 associated with the quartz-filled lenses.
Plagioclase feldspar
- Phenocrysts, fairly fresh, tabular, typically 1-2 mm long, with characteristic lamellar twinning.
Opaques
- Grains associated with the quartz-filled lenses. Range of sizes from groundmass up to ~0.5 mm. These are a mixture of pyrite, FeS2, (tending to have a cubic form) and the unidentified black mineral, both seen in hand specimen.
Groundmass
- Cryptocrystalline, almost glassy, speckled with tiny, irregular quartz and feldspar crystals. Probably devitrified glass. Plagioclase altered to white mica.
Secondary quartz
- Narrow veins and lenses of quartz, considerably coarser than the groundmass. The grains of quartz have sutured contacts, and may have been injected after consolidation of the lava. Some of these clumps are rimmed with inward pointing needles of feldspar.
Phenocrysts crystallised in a magma chamber. The magma erupted and cooled very rapidly to a fine-grained, glassy groundmass, preserving flow banding texture. Injection of quartz-rich fluid, which precipitated quartz in lenses and veins.
Very high temperature hydrothermal alteration caused devitrification of the glassy groundmass.
banded rhyolite
rhyloite flow
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.
Quartz
- Rounded phenocrysts (1-4 mm) with embayments/cavities and tiny inclusions.
Feldspar
- Phenocrysts ~2mm, subhedral. Mainly alkali feldspar, some plagioclase. Alteration to white mica.
Garnet
- Very rare, euhedral phenocrysts (0.5-1 mm). Isotropic, high relief, colourless to pale pink.
Biotite
- Very rare fragments with chlorite alteration.
Groundmass
- Microcrystalline quartz and feldspar, with some muscovite and biotite.
Calcite
- Some of the feldspars phenocrysts and most of the garnet phenocrysts have been replaced by calcite. These are the pink macrocrysts visible in hand specimen.
Ordovician
Armboth Dyke
Thirlmere, Cumbria
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.
Quartz
- Rounded phenocrysts (1-4 mm) with embayments/cavities and tiny inclusions.
Feldspar
- Phenocrysts ~2mm, subhedral. Mainly alkali feldspar, some plagioclase. Alteration to white mica.
Garnet
- Very rare, euhedral phenocrysts (0.5-1 mm). Isotropic, high relief, colourless to pale pink.
Biotite
- Very rare fragments with chlorite alteration.
Groundmass
- Microcrystalline quartz and feldspar, with some muscovite and biotite.
Calcite
- Some of the feldspars phenocrysts and most of the garnet phenocrysts have been replaced by calcite. These are the pink macrocrysts visible in hand specimen.