Banded manganese ore
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Fact sheet
Description: 

This ore specimen is from the largest manganese deposit in the United Kingdom. It occurs in silicates and carbonates within a hard cherty bed near the base of the Cambrian-age Haffoty Formation in the mountains inland of Harlech, North Wales. It is thought that the ore formed in a shallow marine basin under reducing conditions, where the manganese in the sediment column was extensively remobilised and deposited at the sediment-water interface as manganese carbonate (rhodochrosite). Although the rock was originally of sedimentary origin, the deposit has been modified by diagenesis and greenschist facies metamorphism, crystallising a Mn-rich form of garnet known as spessartine. This specimen can be found in the National Museum Wales.

The thin section illustrates the very fine-grained nature of the rock and the deformation of the fine sedimentary banding around the nodules of calcium-manganese-rich carbonate (calcian rhodochrosite) that grew during metamorphism. The red bands consist of a fine-grained intergrowth of spessartine and rhodochrosite (plus quartz). The pink colour is derived from hematite inclusions in the spessartine. Yellow bands have a similar mineralogy but do not have the hematite inclusions. Bands that appear bluish-black in the hand specimen consist of intergrowths of rhodochrosite and spessartine with grains of a black opaque phase (MnO2). The chocolate brown bands in hand specimen are mainly intergrowths of rhodochrosite and alleghanyite, a hydrated Mn-rich silicate. The black (opaque) mineral with strain fringes is pyrite.

For a detailed review of this mineralisation, see: Cotterell (2013) J. Russell Soc., 16, 39-51.

In Place
In Time
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Location description: 
Moel Ysgyfarnogod, Harlech, North Wales
Accuracy:
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Good
Timescale:
Timescale: 
Cambrian
Ma = Millions of years ago
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metamorphic
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