Banded manganese ore
Collection:
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Fact sheet

Banded manganese ore

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.

Additional images
  • banded manganese ore - width 14 cm
Map
52.891153, -3.996173
Description:
Moel Ysgyfarnogod, Harlech, North Wales
Precision:
Good
About this collection

The United Kingdom Virtual Microscope (UKVM) collection consists of igneous, sedimentary and metamorphic rocks from around the UK.

It is intended as a teaching resource, helping to tell the story of the common rock types and how they form, and reflecting the history of the UK at the margins of the continent of Europe. The collection is a series of teaching sets, for example igneous rocks from the North Atlantic Igneous Province and SW England; high-temperature metamorphic rocks from Scotland and low-temperature metamorphic rocks from Wales; and sedimentary rocks, including English limestones and sandstones.

Sample details

Type
metamorphic
Category
metasediment
Rock-forming mineral
spessartine
quartz
Accessory minerals
alleghanyite
sonolite
tephroite
Category guide  
Category Guide
Title
Refers to any word or phrase that appears in the individual rock names. Names are generally descriptive; they allow users to search for broad terms like ‘granite’ as well as more specific names such as ‘breccia’. However, the adjacent descriptions of the specimens captures a wider range of general words and phrases and is a more powerful search tool.
Description
Refers to any word or phrase that appears anywhere in the descriptions of the specimens
Accessory minerals
Minerals that occur in very low abundance in a rock. They are usually not visible with the naked eye and contribute perhapssver, they often dominate the rare elements such as platinum group metals.
Rock-forming minerals
Minerals that make up the bulk of all rock samples and are also the ones used in rock classi?cation.
Timescale
Selecting one or more period, for example 'Jurassic'.
Theme
A term used to group together related samples that are not already gathered into a single Collection. For instance, there is a ‘SW England granites’ theme that includes such rock types as granite, hydrothermal breccia, skarn and vein samples.
Category
A general term used to label a rock sample. It is a useful way of grouping similar samples throughout a collection. Category names are often, but not exclusively, common rock names (e.g. granite, basalt, dolerite, gabbro, greisen, skarn, gneiss, amphibolite, limestone, sandstone).
Owner
The owner of the sample that appears in the collection. For example, NASA owns all the samples that appear in the Moon Rocks collection
We would like to thank the following for the use of this sample: