1A Collections

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

L162 Basalt, porphyritic

Fri, 14 Jun 2019 12:22:41 +0100

Title

L162

Basalt, porphyritic

Subject

Specimen Age

Carboniferous

Location

Volcanic neck, Lion's Haunch, Arthur's Seat, Edinburgh

Description

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 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.

Rock History

Two stage cooling
1. Phenocrysts grow slowly in a magma chamber.
2. Fine-grained groundmass grew rapidly → extrusive.
Hydrothermal alteration, possibly as part of the volcanic pile.

Advanced Notes

Cummingtonite is an amphibole. It is colourless to pale green in thin-section, has moderate relief and two cleavages which intersect at 56°.

Rock Name

porphyritic basalt

L172 Ultramafic cumulate

Fri, 14 Jun 2019 17:08:51 +0100

Title

L172

Ultramafic cumulate

Subject

Age

Precambrian

Location

Lewisian Complex, Dyke
Assynt, Sutherland

Description

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 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.

Rock History

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.

Rock Name

ultramafic cumulate

L200 Volcanic Breccia

Fri, 14 Jun 2019 17:24:37 +0100

Title

L200

Volcanic Breccia

Subject

Age

Carboniferous

Location

Melrose, Scotland

Description

Hand Specimen

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.

Thin section

Wide range of clasts including
- Basalt
- Slate or phyllite
- Schist
- Chert
- Sandstone
- Carbonate
- Quartz

Matrix of fine volcanic ash.

Rock History

This is an example of the coarser type of material deposited by explosive volcanic action.

NB,
This is not a typical volcanic breccia, as one usually sees a greater abundance of igneous clasts; other clasts could be wall-rock. Another name for this rock could be “lithic wacke”.

Rock Name

volcanic breccia
lithic wacke

L201 Tuff

Fri, 14 Jun 2019 17:27:47 +0100

Title

L201

Tuff

Description

Hand Specimen

Fine-grained, blue-green, sedimentary texture.
Slaty cleavage.
Some patches of lighter and finer-grained material.

Thin-section

Slaty cleavage due to the alignment of chlorite and muscovite.
Cross-cut by the blobs of the fine-grained material. These are orange-brown in thin-section. Could be water-lain ash, ripped-up to form clasts.
Very fine-grained, brown matrix, probably volcanic ash.
Carbonate, quartz and opaque clasts slightly larger than the matrix.

Rock History

Fine-grained volcanic ash deposited by an explosive volcanic eruption has undergone low grade metamorphism to produce a slaty cleavage.

Rock Name

tuff

L300 Granulometer

Fri, 14 Jun 2019 17:44:06 +0100

Title

L300

Granulometer

Description

Hand Specimen

Six vials containing unlithified terrigenous sediments of different grain sizes. These are:

	grain size (mm)	grain size (phi)
Very coarse sand	1-2	-1-0
Coarse sand	0.5-1	0-1
Medium sand	0.25-0.5	1-2
Fine sand	0.125-0.25	2-3
Very fine sand	0.063-0.125	3-4
Mud	<0.063	>4

Thin-section

These are not sections cut from a rock. They are cut from a block of resin containing sedimentary grains. Use these sections to familiarise yourself with grain sizes, as seen in thin-section.

Rock Name

granulometer

L310 Breccia

Fri, 14 Jun 2019 17:48:12 +0100

Title

L310

Breccia

Subject

Specimen Age

Permian

Location

Brockram, Cumbria

Description

Hand Specimen

Very poorly sorted.Angular to sub-angular clasts up to 30 mm long. Randomly orientated.
Clasts mainly pale grey, some white, some red.
Red-brown colour of the fine-grained matrix indicates iron staining.
Matrix and some clasts fizz with acid, indicating that they are made of carbonate. This is a limestone.

Thin-section

Variety of clasts: sedimentary rock fragments, carbonate and feldspar crystals, fine-grained quartz and carbonate, some bioclasts. Clasts are quite angular.
Matrix of very fine-grained clays containing fragments of quartz and carbonate.

Rock History

Clast-supported. Clasts are angular, so sediments cannot have been transported far. (NB, if this rock were matrix supported the clasts could have been transported a long way without losing their angularity.)
Probably scree accumulation during desert conditions, from approximately the same horizon as L326.

Advanced Notes

Iron-staining
Hematite (Fe₂O₃) → red (blood-red, cherry-red, brown-red) iron-staining
Limonite (FeO(OH)•nH₂O) → yellow or brown iron-staining

The red-brown colour of this rock is probably produced by hematite.

Rock Name

breccia

L311 Conglomerate

Mon, 17 Jun 2019 09:06:20 +0100

Title

L311

Conglomerate

Subject

Age

Eocene

Location

Hertfordshire

Description

Hand Specimen

Poorly sorted. Rounded clasts up to ~15 mm long.
Pale yellow matrix of quartz grains.
Cement fizzes with acid, indicating that it contains carbonate.
This conglomerate is just clast-supported.

Thin-section

Rounded clasts of very dark red brown amorphous silica (chert) and polycrystalline quartz.

Matrix of quartz grains (typically 0.25 mm) and fine-grained carbonate cement (sparite). Dark red-brown iron staining.

Rock History

Deposited in a high energy environment.

Advanced Notes

Iron staining
Hematite (Fe₂O₃) → red (blood-red, cherry-red, brown-red) iron-staining
Limonite (FeO(OH)•nH₂O) → yellow or brown iron-staining

The red-brown colour of this rock is probably produced by hematite.

Rock Name

conglomerate

L323 Sandstone, arenite

Mon, 17 Jun 2019 09:08:52 +0100

Title

L323

Sandstone, arenite

Subject

Age

Jurassic

Location

Estuarine Series, North Yorkshire

Description

Hand Specimen

Pale yellow grey rock, almost entirely made up of medium sand quartz grains. These are sub-rounded, very well sorted, and are held together by a matrix of siliceous silt. Contains a significant amount of clay, but <15 vol%, so it is an arenite.
No fizzing with acid, so does not contain significant carbonate.
Weakly laminated.

Thin-section

Sub-round grains of quartz, some opaque grains and a few flakes of white mica.
Siliceous, silty matrix.

Rock History

This rock was deposited in a moderate energy, aqueous environment. The grains are not well rounded enough to be aeolian sediments.

Rock Name

sandstone
arenite

L324 Sandstone, feldspathic arenite

Mon, 17 Jun 2019 09:12:32 +0100

Title

L324

Sandstone, feldspathic arenite

Subject

Age

Precambrian

Location

Torridonian Group, North West Scotland

Description

Hand Specimen

Reddish brown rock made up of pinkish feldspar, colourless quartz, and some dark grey grains. Medium to coarse sand. Moderately rounded. Fairly well sorted.
Contains <15 vol% clay, so this is an arenite.
No fizzing with acid, so does not contain significant carbonate.

Thin-section

Quartz (~65%)
- Includes fresh quartz grains with undulose extinction, quartzite grains with internal sutures, and quartz vein material. Some quartz grains have opaque rims.

Feldspar (~35%)
- Mainly microcline, some plagioclase. Some microperthite texture and decomposition to clays.

All grains are moderately rounded and fairly well sorted. The rock is well compacted. A thin film of cement mainly hematite and clay coats the grains.

Rock History

This rock contains too much feldspar, and the grains are not sufficiently well rounded nor well sorted to have been deposited in an aeolian environment. The sediments were deposited in an aqueous environment.

Rock Name

sandstone
feldspathic arenite