1A Collections

L100 Alkali granite

Wed, 12 Jun 2019 12:05:16 +0100

Title

L100

Alkali granite

Subject

Specimen Age

Variscan , Permo-Carboniferous

Description

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 long, subhedral to anhedral, dirty looking due to alteration. Many display a microperthite texture.

Plagioclase feldspar
- Grains typically 1-4mm long, subhedral, characteristic lamellar twinning. Many of the grains look “dirty” under plain polarised light, and have high birefringence speckles under cross-polarised light. They have been partially altered to white mica. Some grains are zoned.

Quartz
- Grains typically 1-4mm across, anhedral. They are characteristically clean and unaltered when compared with feldspar grains, but do contain many tiny inclusions.

Biotite
- Grains similar size as the rest of the groundmass. Strongly pleochroic in pale straw and dark reddish brown. Speckled extinction. Some sections show the one good cleavage of biotite. Small black blobs and circles are dotted all over the crystals. These are pleochroic haloes, produced by zircons. Partially decomposed to chlorite.

Accessory minerals
- Apatite
- Small irregular grains of muscovite mica

Rock History

This is a coarse-grained rock, which must have cooled slowly as part of an intrusion. Its granitic composition (essential quartz and feldspar) indicates that it was produced in a continental setting.

Rock Name

alkali granite

L101 Granite

Wed, 12 Jun 2019 12:09:47 +0100

Title

L101

Granite

Subject

Age

Caledonian, Devonian

Location

Shap Fell, Cumbria

Description

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 are anhedral and display their characteristic conchoidal fracture and vitreous lustre.

Thin-section

Alkali feldspar (orthoclase)
- Pink phenocrysts up to ~10 mm long, relatively fresh (unaltered).

Groundmass
- Feldspar (both plagioclase and alkali) grains 1-4mm long, mainly subhedral, though some are quite lath shaped. Partially altered. The plagioclase grains display their characteristic lamellar twinning.
- Biotite grains (0.5-2 mm) with subhedral to anhedral form. Grains have ragged edges and are partially altered to chlorite, mainly along cleavage planes.
- Quartz grains (typically 2-5 mm) contain lots of tiny inclusions and display undulose extinction.

Accessory minerals
- Sphene
- Apatite
- Opaque mineral

Rock History

Porphyritic texture means this rock underwent two stages of cooling; phenocrysts crystallised first followed by the groundmass. The groundmass is coarse-grained, and must have crystallised slowly as part of an intrusion.

Plagioclase and alkali feldspar both occur in this rock, indicating that it formed at high pressures.

Granitic composition (essential quartz and feldspar) indicates that it was produced in a continental setting. Late-stage alteration.

Advanced Notes

This rock is classified as an adamellite because it contains roughly equal proportions of alkali feldspar and plagioclase feldspar, as well as biotite and quartz.

Rock Name

granite
adamellite

L102 Hornblende biotite granite

Wed, 12 Jun 2019 15:23:37 +0100

Title

L102

Hornblende biotite granite

Subject

Age

Caledonian, Devonian

Location

Glencoe

Description

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 more equant, typically 1-2 mm across.
Biotite may be distinguished from hornblende by its flaky habit and one good cleavage. It is shinier than hornblende.

Thin-section

Plagioclase feldspar
- The larger laths (2-4 mm long) are strongly zoned. The cores of these laths, which display clear lamellar twinning, have been overgrown by rims of more feldspar. Some alteration to white mica.

Biotite mica
- Grains typically 1 mm across. Strongly pleochroic in reddish brown and pale straw. Often associated with small grains of an opaque accessory mineral, probably magnetite.

Quartz
- Grains are 1-2 mm across, anhedral, and display undulose extinction.

Alkali feldspar
- Anhedral grains up to 5mm long. Some alteration to white mica. Some grains display a microperthite texture.

Hornblende amphibole
- Pale green, anhedral. Grains appear to have been “eaten away”, reducing their prismatic form (diamond) to a ragged anhedral form. Amphiboles have two cleavage planes which intersect at 56°.

Rock History

This coarse-grained rock must have cooled slowly as part of an intrusion.

Its granitic composition (essential quartz and feldspar) indicates that it was produced in a continental setting.

Strongly zoned plagioclase crystals and “eaten away” appearance of the hornblende could be due to a change in the composition of the liquid from which the phases crystallised; perhaps due to magma mixing or degassing of volatiles from the magma.

Presence of hornblende and biotite indicate that this is a wet granite, probably from a subduction zone.

Hydrothermal alteration of feldspars to white mica.

Advanced Notes

This rock is classified as an adamellite because it contains roughly equal proportions of alkali feldspar and plagioclase feldspar, with additional quartz and biotite.

Rock Name

hornblende biotite granite
adamellite

L110 Rhyolite, porphyritic (quartz porphyry)

Thu, 13 Jun 2019 14:25:01 +0100

Title

L110

Rhyolite, porphyritic (quartz porphyry)

Subject

Age

Ordovician

Location

Armboth Dyke
Thirlmere, Cumbria

Description

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 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.

Rock History

Porphyritic texture indicates two-stage cooling. The phenocrysts grew slowly in a magma chamber; the fine-grained groundmass formed during rapid cooling at the Earth’s surface.

Late stage hydrothermal alteration to white mica and carbonate.

Rock Name

porphyritic rhyolite
quartz porphyry

L112 Rhyolite, banded

Thu, 13 Jun 2019 14:31:24 +0100

Title

L112

Rhyolite, banded

Subject

Location

Glacial boulder, North Wales

Description

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 associated with the quartz-filled lenses.

Thin-section

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.

Rock History

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.

Rock Name

banded rhyolite
rhyloite flow

L113 Rhyolite, glassy (obsidian)

Thu, 13 Jun 2019 14:38:27 +0100

Title

L113

Rhyolite, glassy (obsidian)

Subject

Age

Cenozoic

Location

Yellowstone, U.S.A.

Description

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 scattered with tiny spherical holes. These are vesicles.

Thin-section

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.

Rock History

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.

Rock Name

glassy rhyolite
glass
obsidian

L120 Granodiorite

Fri, 14 Jun 2019 09:43:53 +0100

Title

L120

Granodiorite

Subject

Age

Caledonian, Devonian

Location

Boulder, Moor of Rannoch

Description

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 regions comprise quartz and white feldspar.

Thin-section

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

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.

Rock Name

granodiorite

L130 Andesite, porphyritic (porphyrite)

Fri, 14 Jun 2019 10:00:45 +0100

Title

L130

Andesite, porphyritic (porphyrite)

Subject

Age

Early Paleozoic

Location

Cansip Porphyry Sills
Sutherland

Description

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 flakes of biotite.

Thin-section

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.

Rock History

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.

Rock Name

porphyritic andesite
porphyrite

L141 Gabbro

Fri, 14 Jun 2019 10:03:39 +0100

Title

L141

Gabbro

Description

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 veins.

Thin-section

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.

Rock History

Coarse grained → slowly cooled, intrusive.
Hydrothermal alteration of olivine after crystallisation of the rock.

Rock Name

gabbro

L161 Basalt, amygdaloidal

Fri, 14 Jun 2019 10:38:25 +0100

Title

L161

Basalt, amygdaloidal

Subject

Specimen Age

Cenozoic

Location

Lava, Skye

Description

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 (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.

Rock History

Fine-grained → rapid, extrusive crystallisation.
Gas bubbles trapped within lava produced vesicles.
Circulation of fluids → precipitation of zeolite within the vesicles.

Advanced Notes

Zeolites are produced at low temperatures. They are commonly precipitated in basaltic volcanic piles.

Rock Name

amygdaloidal basalt