L500 Slate

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.

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

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.

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.