L340 Mudstone

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.

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.

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.

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.

mudstone