Winford Ochre & Iron Oxides

We met at Red House Farm where Melanie and Lionel Patch had kindly allowed us to park our cars while we went off to inspect the quarry.

The idea of the trip was two fold. One was to look at the industrial archaeology and secondly to look at and understand why the ochre was there and how it related to the surrounding geology.

After a short briefing on the area’s geology we formed up in single file, because of the narrow roads with blind bends and set off to find the quarry. We got as far as the end of the farm track when someone spotted a mill stone so we had to have a quick look and discussion before moving on with promises to see more mill stones in the quarry.Unlike the lead and zinc ores, many of the iron ore deposits are secondary deposits. Intense weathering of the iron pyrite-rich Coal Measures, and other iron bearing rocks during Permian and Triassic times released the iron into the groundwater. The iron was subsequently redeposited as many thin discontinuous veins of haematite or pyrite, within the Carboniferous Limestone and the Dolomitic Conglomerate, ( MMG /MMMF ) and especially along the unconformity between the two. Many of these pyrite veins have now been altered to form limonite or ochre. Ochre also occurs infilling cavities in the Carboniferous Limestone and Dolomitic Conglomerate, or as a replacement ore-body, where metal-rich ground-waters have chemically replaced the host rock with iron ore.

The colour of ochre depends on the type of iron oxide and the impurities in the clay.

Yellow ochre is normally Limonite, ( rust ) which is a hydrated form of Goethite. As the percentage of Haemetite increases the colour changes from yellow to orange, red, purple and finally black.

Iron56 is the most common isotope of iron. About 91.754% of all iron is iron-56. … This means that as the Universe ages, more matter is converted into extremely tightly bound nuclei, such as 56Fe.

Iron is a “special” element because of its nuclear binding energy. The idea is that when you fuse two light elements together, you get a heavier element plus energy. You can do this up to iron. Similarly, if you have a heavy element that undergoes fission and splits into two lighter elements, you also release energy. Down to iron. The physical reason for this has to do with the balance between nuclear forcesand the electromagnetic force.

Due to the way these energies work, and because iron is thus thought of as the most stable, if you want to get energy from fusion or fission, your best bet is to use atoms that are farthest away from iron — very light (like hydrogen) or very heavy (like uranium).

As a side note, this is also why Type 2 supernovae happen — the star can no longer gain energy from fusion because it can’t fuse past iron, so the outward pressure from energy generation stops and the star collapses. 

This will happen to the sun soon – as the sun runs out of hydrogen and starts producing heavier elements until it gets to iron and then fusion will stop and the sun will collapse – in about 5 billion years time…

Glossary – Minerals

Botryoidal / Reniform

Texture or mineral habit is one in which the mineral has a globular external form resembling a bunch of grapes as derived from the Greek botruoeidēs. This is a common form for many minerals, particularly haematite, the classically recognised shape.

Boxwork

Honeycomb pattern of limonite (a mixture of hydrous iron and manganese oxide minerals) that remains in the cavity after a sulfide mineral grain has dissolved. The boxwork may be spongelike, triangular, pyramidal, diamond – like, or irregular in shape and may be coloured various shades of ochre and orange through dark brown. The colour and shape of the boxwork can sometimes be used to identify the dissolved sulfide minerals

Druse

Refers to a coating of fine crystals on a rock fracture surface, vein or within a vug or geode.

Limonite – FeO(OH) – nH2O

This a hydrated version of Goethite. It is a major component of rust and is yellow to brown in colour. Mined as yellow ochre. eg Winford quarries.

Pyrite – FeS2

Iron sulfide – fool’s gold. Often found in anoxic, shallow seas. Easily oxidised so specimens often decay.

Siderite – FeCO3

Iron carbonate.  48% iron so a valuable iron ore

Haematite – Fe2O3

This mineral is one of the most important ores of iron. It can vary in colour from metallic grey to bright red. It is a form of ferric oxide Fe2O3.. It  is the oldest oxide of iron ever to have formed on the earth. Its occurrence is widespread in rocks and soils. It is harder than pure iron. It has been used throughout history as a pigment.

It occurs in several forms. – Botryoidal or kidney ore, magnetite, iron rose and specularite

Goethite  – (FeO(OH) 

Is a hydroxide of iron that has also been used as a pigment – brown ochre. Its chemical formula is (FeO(OH). It contains iron of ferric form.Its main use is as iron ore and is also the source mineral for yellow ochre. Colour is yellowish to dark brown and black.

Most often in botryoidal, reniform, or stalactitic aggregates of radiating crystals or ball-like crystals. Also grainy, in veins, concretionary, oolitic, and in earthy masses. It often assumes the shape of other minerals forming a pseudomorph in place of the original mineral or as a coating above it. It is the main component of rust and bog iron ore. It forms prismatic needle-like crystals ( Needle iron ore ) acicular.

Geode

Are geological secondary formations within sedimentary and volcanic rocks. Geodes are hollow, vaguely circular rocks, in which masses of mineral matter (which may include crystals) are secluded. The crystals are formed by the filling of vesicles in volcanic and sub-volcanic rocks by minerals deposited from hydrothermal fluids; or by the dissolution of syn-genetic concretions and partial filling by the same, or other minerals precipitated from water, groundwateror hydrothermal fluids.  Sometimes known as “Bristol Diamonds” in the Bristol area.

Gossan

Rust-coloured oxide and hydroxide minerals of iron and manganesethat cap an ore deposit. Gossans form by the oxidation of the sulfide minerals in an ore deposit and they thus may be used as clues to the existence of subsurface ore deposits. especially if distinctive boxworks are present.

In addition to hydrous oxides of iron and manganese, gold and silver in the native (natural, nearly pure) state and various sulfate, carbonate, and silicate minerals can occur in gossans. The hydrous oxide minerals occur as the residuum when sulfide minerals are dissolved from the outcrops; they are either indigenous (i.e., fixed at the site of the original sulfide mineral) or transported. Indigenous hydrous oxides indicate the presence of copper, whereas transported hydrous oxides indicate its absence or its presence in very low proportion to iron and manganese. 

Harptree Beds.

Found on Felton Common and behind Leighdown Farm ( quarry ). Also in large beds on the Mendip Plateau where across much of the central Mendips, outcrops of the Jurassic Lower Liassic and Inferior Oolite limestones have been replaced by chert. These cherts are known collectively as the ‘Harptree Beds’. These cherts are very hard and have been quarried from a small outcrop behind Leighdown Farm – near the “Crown” pub. – use unknown. These cherts have been formed by the process of Metasomatism. ie they have been metamorphosed but by hot gas or fluids rather than the more usual heat and pressure. This happened during the Missisippi Valley Mineralization when the mineralisation of the Mendips took place.

Vug,  

A small to medium-sized cavity inside rock. It may be formed through a variety of processes. Most commonly, cracks and fissures opened by tectonic activity (folding and faulting) are partially filled by quartzcalcite, and other secondary minerals. Open spaces within ancient collapse breccias are another important source of vugs. Vugs may also form when mineral crystals or fossils inside a rock matrix are later removed through erosion or dissolution processes, leaving behind irregular voids. The inner surfaces of such vugs are often coated with a crystal druse. Fine crystals are often found in vugs where the open space allows the free development of external crystal form. The term vug is not applied to veins and fissures that have become completely filled, but may be applied to any small cavities within such veins. Geodes are a common vug-formed rock, although that term is usually reserved for more rounded crystal-lined cavities in sedimentary rocks and ancient lavas.

Processes

Levigation using an edge runner mill

  is the process of grinding an insoluble substance to a fine powder, while wet. The material is introduced into the mill together with water, in which the powdered substance remains suspended, and flows from the mill as a turbid liquid or thin paste, according to the amount of water employed. The amount of grinding depends on the particle size required. The slurry is then fed to settling tanks where the water is drawn off.

Calcining

is also used to mean a thermal treatment process in the absence or limited supply of air or oxygen applied to ores and other solid materials to bring about a thermal decomposition.

There are examples of the mill stones used for levigation, near the entrance to the quarry. They are granite, which is unusual. Most mill stones are made from Carboniferous Sandstone from the Eponymous Millstone Grit strata from Derbyshire and Cumbria.

This was an edge runner type of mill where the stones are mounted vertically and run around a deep dish. The stones were often fitted with iron tyres to extend their life. It is difficult to carve this size of stones from granite so they are “frenched” – this means that parts are connected by iron staples – it does not mean that the stones came from France. Their origin is unknown but as there are large feldspar crystals in the granite and Darmoor is not too far away, it seems likely that they were made from the Giant Granite on Dartmoor, which is the biggest emplacement of granite in Britain.

There were also four water powered mills along the Winford Brook. These were used for processing the ochre from the many pits and mines in the area. It is worth noting that some mills were only used for yellow ochre as yellow was so easily contaminated by the darker shades.

One of the mills at Upper Littleton was used to produce gunpowder, using the Saltpetre that was shipped into Bristol from India. This mill was built to replace the city centre mill near the present site of Temple Meads Railway station. ( Knight’s Templars meadows )  This was in the medieval building Tower Harratz. This is now under the foundations of the old Bristol and West building. It was thought too dangerous to keep a gunpowder factory in the centre of Bristol so all ships carrying explosives, including saltpetre, had to unload their cargoes at the Powder House at Pill before sailing up the Avon to the Bristol Docks. It may be that the encircling mines and quarries near Winford gave a ready market for some of the gunpowder. Stocks were kept in the explosives store near the spoil tip which can still be seen.R

References

BIAS JOURNAL No 26 1993 Winford Ochre and Oxide Peter Addison

Earth Colours Marie Clarke, Neville Gregory & Alan Grey

Mendip and Bristol Ochre Mining.  – Available for ordering from the Mendip Cave Registry and Archive ( MCRA )

https://www.mcra.org.uk/wiki/doku.php?id=for_sale– £9.00

BGS Bristol Geology  Map S & D Sheet 264

BGS Bristol and Gloucester region geology – memoir,

OS Map. Bristol West and Portishead. Explorer No. 154
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© Richard Kefford 2020 Eorðdraca

My books are for sale here:  Richard

Author: Richard

I live in Somerset UK. I am a writer and geologist and blogger.

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