agates in sedimentary environments

Pseudo-Sedimentary Agates

Let us start by dispelling the myth, there is no such a thing as a sedimentary Agate, they do not exist as such, look-alike agates in a sedimentary environment form by different processes.

The rocks that appear on casual glance to have Agate like features are just look-alikes, certain ...

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features such as banding or nodular concretions can on casual glance look like agates, especially the so called Fairburn Agates with their unusual distinctive banding.

Examples that can look like Agate are sometimes to be found in Sedimentary Rocks, these are however pseudo-agates, the only exceptions are from recent sedimentary deposits that may contain locally derived actual agates that have been eroded by fast flowing streams and end up in glacial outwash deposits.


Sedimentary Geodes

Within rocks that were formed in a depositional environment, hollow Geodes can sometimes be found within the sedimentary rock, these may contain later mineral infillings in the internal cavities.

These hollow geodes will usually contain secondary Carbonate Crystals, and even Quartz but not the Chalcedony of Agates. These geodes can often look very spectacular, one beautiful example from England is found in the Bristol area; here the geodes often contain pale blue to white Celestine crystals a Strontium Sulphate.

There are examples of a brighter blue colour for this mineral and these are found in Madagascar and are Strontium rich with a little Barium or Calcium. The pale colour of Bristol examples is influenced by the lower levels of Strontium and higher levels of Barium within the Celestine crystals; the strontium is used in the manufacture of fireworks.

Dulcote Agates

Another more solid pseudo-agate is often referred to as Mendip Hills Agate; this type of agate was found in Somerset in a clay seam in a Limestone Quarry on Dulcote Hill. These concretions were also known locally as Mendip Potato Stones, they are mostly solid but occasionally a very few will have hollow centres that are filled with secondary minerals.

When Dulcote nodules are cut open, they may display a faint reddish pink banded structure, some of the nodules also contain small Quartz crystals with Goethite needles along with other minerals such as Calcite and even Amethyst. Sadly the Dulcote Quarry is now closed and it has been infilled with rubbish and therefore no further new examples of these lovely potato stones will be found.


Dry head Agates

Another type of pseudo-agate comes from Montana in the USA and they are found in the Big Horn River area in the Prior Mountain range. These are the Dry head Agates that are in fact stringers of Chalcedony with a distinctive looking banded appearance, found running through an altered sedimentary rock. The original marine sedimentary rock has been modified by a later hydrothermal process that has hardened the sedimentary rock as well, the circulating silica rich fluids in this case have impregnate the fine grained sedimentary rock.

These weathered pseudo agates are often called marine agates and they are very old, in fact they are around 250 million years old.
A similar looking variety of pseudo-agate has also been found in the Nepa River Valley area in the far eastern section of Siberia in Russia.


Fairburn Agates

These Agates are again look alike agates, they are similar to the Dry head Agates,
examples of these Fairburn agates are also sometimes known by the alternative name of Black hills Agates. They have been found scattered over the surface area in a glacial outwash formation. These nodules occur sparsely over a large part of South Dakota even into nearby Nabraska, these pseudo agate nodules often have part of the original altered sedimentary rock attached.
Like the Dry head Agates these are also around 250 million years old, the nodules occur along with other silica rich rocks such as Quartz and Jasper. Another similar type of pseudo agate is known as the Teepee Canyon Agate they are from Teepee Canyon, Custer County, South Dakota and are related to the Fairburn agate.

Florida Agate

This is an example of silica replacement of the original organic coral and these are found in the limestone’s of the Florida area of south east America and are formed as a Chalcedony replacement of the original coral. These altered corals are actually found in the Tampa Bay area of Florida in what was once an ancient fossil Coral Reef, these corals even though they are now Chalcedony are obviously a later alteration and are not therefore agates at all, true agates only being found in the definitive type of volcanic environment.

Silicified Material


Another example of a Coral that has been altered by Silica rich infusions occurs in Sumatra, Indonesia and this material has now been extensively used in Jewellery.

Another example of silica replacing original organic material is Silicified wood that was originally a biological material and now has been altered by circulating silica rich fluids derived from nearby volcanic rocks.

Other examples of this type of replacement have been found in many parts of the world, chiefly in Utah and also Wyoming in America where they are known as the Blue forest fossil wood. Another common example of silicified fossil wood is found in the Triassic deposits of Madagascar, these are of silicified fossil Gyinco Trees. Also Silicified organic material is Dinosaur Bone that is found in the volcanic ash deposits in Utah, this is another example of silicified fossilisation.

Further examples can also be found in another replacement of original biological material, this time it comes from the Jurassic Dinosaur bone deposits of the county of Arizona.

Banded minerals

Common banded minerals such as Barites include the Oakstone Barites that is found in Ashover, Derbyshire, similar material is the Ashburnham Agate, another example of this type of deposit from New South Wales, Australia and this is a modified Barite.
There are also the silica rich nodules of Flint and Chert found in the Cretaceous Chalk deposits of Poland, these nodules can have distinct markings in the “banded nodules”.
Also another example of silicification is from Hertfordshire where we have what is called a Puddingstone, this is a bed of Flint and Chert pebbles sometimes with banding in an original sandy matrix that has later been silicified into a comprehensive hard rock that was originally of Glacial outflow origin.

Banded metallic deposit

This is a deposit of a mineral called Wurtzite along with other Sulphides and this is attributed to a fossilised hydrothermal Ophiolite deposit, this mineral often requires hydrothermal generation to form, instead of the more usual zinc sulphide that is Sphalerite. This mineral was first discovered in Bolivia and later in the USA; this particular deposit from Poland is known as Schalenblende and is from the Pomorzany Mine, Olkusz, Malopoliskie, Poland.

A lovely pink banded material found is the lovely mineral Rhodocrosite, this forms as beautiful pink manganese banded deposit in the old Inca Silver Mines in Capillitas, Argentina.

Malachite also forms as a lovely green banded mineral; this also displays a very distinctive banding.

Ironstone from the Pilbara Region of Western Australia is yet another example of mineral banding; here it is the Iron rich deposits of Western Australia, these are ancient, in fact some are over 2 billion years old, other examples of banded ironstone are found in Africa and America.

There are several other examples of banded rocks such as Tiger eye from Africa that is actually a silicified asbestos group mineral deposit.

There are many examples of banding but none of these are actually like the Volcanic derived chalcedony Agate, be aware of imitations that call themselves Agate that are pseudo agates.

Written by
Barry Taylor

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