Concretions                Type of Rock: SEDIMENTARY

Concretions are rounded rocks embedded in layers of stone in sedimentary rocks. They are often harder than the surrounding rock and weather out of the host rock. They are often spherical or nodules. Nodules are often shaped like potatoes.

Some concretions have unique appearances and are given special names. Some are:



Rockwood, Michigan, USA

Cannonball Concretion
Yukon Territory, CANADA
Concretion with pyrite nucleus

Ovoid shaped Concretion with Pyrite Nucleus
18 Mile Creek, Eden NY USA

Concretion with leaf fossil

Ovoid Concretion split along plane of weakness
Leaf Fossil at core

Iron Sulfide Concretions

Iron Sulfide Concretion - aka "Pyrite Nodules"
Sir James Mine, Wawa, Ontario CANADA

Note how the iron sulfide is starting to disintegrate. This is an example of "pyrite disease" - aka Bynesian decay.


Cannon Balls

Cannon Balls - Kettle Point, Ontario, CANADA
This photo was taken at the Indian Reservation at Kettle Point, Ontario. The shale at the edge of Lake Huron is cracking and eroding away from the cannonball. The Point was named because the cannonballs appear like upside down cooking kettles coming through the shale. Many of the local people have dug up cannonballs from the lake and displayed them as ornaments on the front lawns of their homes.

Photos & description courtesy B. Daniels May 2003

Photos were taken in Central Australia, north of Alice Springs. Photos show the seashore in southeast New Zealand (South Island). Many cannonballs are emerging and rolling into the sea from the low bluffs to the left of the picture. Note that not all concretions are cannonball shaped.

For more photos of cannon ball concretions, check out the following sites:

Septarian Nodules

Septarian nodules are common decorative stones that have been used making objects like bookends, clock faces, spheres, eggs or carvings. In North America, the most recognized septarian nodules come from Utah.

Turtle Stones

Turtle stones are concretions like septarian nodules that look like the backs of turtles. The concretions are more oval in shape - like a turtle, rather than spherical - like a ball. The yellow calcite has "oozed out" between the grey rock material and looks like the markings on the back of a box turtle.



Septarian Nodule with Calcite   from  MOROCCO
Fairy Stone Calcium Carbonate Concretion from Abitibi, Quebec, Canada

Concretions and Geodes – Surprise Packages
© Bert Ellison 1999 - 2002

Some of our collectors can spot the difference immediately. Can you? Here is what to look for.

First, concretions. These are aggregates in sediments of inorganic matter in many but usually spherical or discoidal shapes. Often there is a nucleus of some sort – a bit of pyrite. A tiny fossil remnant, a shark’s tooth – but it may be absent or microscopic.

The origin of concretions is controversial; see what you think after you’ve read the rest of this story.

They are commonly composed of one material but others may be present as impurities. These are some likely contenders; calcite (the most common), silica, hematite, siderite and other iron compounds; gypsum, barite, aragonite (cousin to calcite), manganese oxide, calcium phosphate, fluorite and bauxite (the ore of aluminum).

Calcite is the chief material in claystone and calcareous concretions so common in shale and sandstones.

Siliceous concretions usually are nearly pure silica, as in chert and flint. Commonly scattered throughout chalk beds these often contain the tiny remains of radiolarians (siliceous foram inifera) and sponge spicules (skeletons).

Siderite concretions (iron carbonate) may be so abundant as to form (low-grade) iron ores. Pyrite and marcasite concretions (iron sulfides) are widespread in shale, limestone and especially dark marine shales. Barite "roses" are usually products of desert sands. Calcium phosphate nodules are valuable sources of that mineral.

Other concretionary shapes are due to other processes – oolites – like fish-eggs – is the common form for bauxite, the ore of aluminum. Then there are larger pea-sized forms – pisolites, rolled about in shallow warm seas.

Sizes? How about log-shaped forms to 10 m? These usually form in sandstone.

Origins? What causes minerals to migrate to a center and become harder i.e. a strange foreign shape, in a sedimentary bed? Some are revealed only when they weather out of an outcrop. How come the fine bedding planes of the host rock continue right through the concretion?

Septarian nodules (from Latin septum = partition) are marked by a network of cracks usually filled with calcite.

The prizes for collectors of concretions are the hollow types called geodes. These globular types vary from centimetres to about 30 centimetres and usually have a rough surface of dense chalcedony. They are typical of some limestone beds but seldom in shales.

What intrigues collectors is the possibility of a geode to contain spectacular crystal growths. But because geodes, once released from their enclosing beds tend to look like ordinary boulders they must be broken open to reveal any treasure. And guessing which "boulder" to smash open – and possibly destroying the interior – is a problem facing the finder. Estimating weight may be of some use. Random sawing is time consuming.

It’s difficult to imagine the lowly concretion being involved in deep intrigue – literally. In the dark days of the Cold War, the Hughes Company was commissioned to design, build and operate an immense floating "mining machine" to allegedly recover from the deep Pacific sea-floor the millions of manganese nodules known to form there. They did.

Only years later did the public learn that this was in fact a CIA secret operation to recover a sunken Russian submarine. Nearly at the surface, the apparatus failed and the hulk sank again among the nodules.

The cause of geodes and concretions and various modular objects make interesting but lengthy and erudite reading for those interested.

But this isn’t necessary to enjoy the wonder and often the beauty of these strange objects.




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