https://wserv3.esc.cam.ac.uk/p1acollections/items/browse?tags=reduction&output=atom <![CDATA[1A Collections]]> 2024-04-09T21:16:25+01:00 Omeka https://wserv3.esc.cam.ac.uk/p1acollections/items/show/37 <![CDATA[<h2><strong>L500<br /></strong></h2> <h2>Slate</h2>]]>

Hand Specimen

Very fine-grained rock with a prominent, well-developed cleavage. The dark red-purple colour is due to iron-staining. The light green smudge is a reduction spot.

Thin-section

Mud size grains.
Mainly indistinguishable clay minerals.
Some quartz, muscovite, opaques.

Rock History

The protolith of this rock was a pelite (mudstone). Under low-grade, regional metamorphic conditions, crystals within the pelite were aligned, producing the slaty cleavage.

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-purple colour of this rock is probably produced by hematite.

Reduction spot 
The dark red-purple of the majority of this rock is due to the presence of iron in the 3+ oxidation state (Fe3+). The light green colour of the reduction spot is instead due to the presence of iron in the 2+ oxidation state (Fe2+). Hence, reduction of the iron ions from Fe3+ to Fe2+ has taken place in the reduction spot.

Rock Name

slate
]]> 2019-07-04T09:52:13+01:00

Title

L500

Slate

Subject

Age

Lower Cambrian

Location

Llanberis, North Wales

Description

Hand Specimen

Very fine-grained rock with a prominent, well-developed cleavage. The dark red-purple colour is due to iron-staining. The light green smudge is a reduction spot.

Thin-section

Mud size grains.
Mainly indistinguishable clay minerals.
Some quartz, muscovite, opaques.

Rock History

The protolith of this rock was a pelite (mudstone). Under low-grade, regional metamorphic conditions, crystals within the pelite were aligned, producing the slaty cleavage.

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-purple colour of this rock is probably produced by hematite.

Reduction spot 
The dark red-purple of the majority of this rock is due to the presence of iron in the 3+ oxidation state (Fe3+). The light green colour of the reduction spot is instead due to the presence of iron in the 2+ oxidation state (Fe2+). Hence, reduction of the iron ions from Fe3+ to Fe2+ has taken place in the reduction spot.

Rock Name

slate

Date

Lower Cambrian
]]> https://wserv3.esc.cam.ac.uk/p1acollections/items/show/3 <![CDATA[<h2><strong>L340<br /></strong></h2> <h2>Mudstone</h2>]]>

Hand Specimen

Superficially specimens “a” and “b” look very different due to their colours. L340a is pale grey, while L340b is dark purple-red. The colour of L340b indicates that it is ferruginous, containing significant quantities of iron oxide.

These are mudstones, made up of clay and/or silt sized grains. 
Contain some glinting mica flakes. 
Do not react with acid, so do not contain significant carbonate.

Thin-section

L340a 
- Homogeneous texture apart from one hairline quartz vein. Mainly mud sized grains too small to be distinguished, but probably clay minerals. Some silt sized grains of quartz, plagioclase and white mica. Patches of minor red-brown iron staining.

L340b 
- Hold up to light to see discontinuous, wavy laminations. Mud sized grains of quartz, feldspar, white mica and opaque minerals. Red iron-staining.

Rock History

The dark purple-red of L340b indicates an oxidising environment during diagenesis, while the grey colour of L340a indicates a reducing environment.

Non-marine environments are more likely to be oxidising than marine environments where organic matter tends to promote a reducing environment during diagenesis. Reducing environments are associated with grey, black or greenish rocks.

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 / red-brown colours of these rocks is probably produced by hematite.

Rock Name

mudstone
]]> 2019-07-04T09:52:02+01:00

Title

L340

Mudstone

Subject

Age

Cambrian

Location

Solva, South Wales

Description

Hand Specimen

Superficially specimens “a” and “b” look very different due to their colours. L340a is pale grey, while L340b is dark purple-red. The colour of L340b indicates that it is ferruginous, containing significant quantities of iron oxide.

These are mudstones, made up of clay and/or silt sized grains. 
Contain some glinting mica flakes. 
Do not react with acid, so do not contain significant carbonate.

Thin-section

L340a 
- Homogeneous texture apart from one hairline quartz vein. Mainly mud sized grains too small to be distinguished, but probably clay minerals. Some silt sized grains of quartz, plagioclase and white mica. Patches of minor red-brown iron staining.

L340b 
- Hold up to light to see discontinuous, wavy laminations. Mud sized grains of quartz, feldspar, white mica and opaque minerals. Red iron-staining.

Rock History

The dark purple-red of L340b indicates an oxidising environment during diagenesis, while the grey colour of L340a indicates a reducing environment.

Non-marine environments are more likely to be oxidising than marine environments where organic matter tends to promote a reducing environment during diagenesis. Reducing environments are associated with grey, black or greenish rocks.

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 / red-brown colours of these rocks is probably produced by hematite.

Rock Name

mudstone
]]>