Geodes     ~ Type of Rock
RocksForKids.com

 


Kentucky, USA
This geode half is lined with clear quartz crystal tips, or terminations. They sparkle. The geode “skin” is a smooth, light gray rock.
   
The Beauty of Geodes ~ Geodes & Fractals

Amethyst lined geode
Mexico
 

To see how this image of geode has been used in a video about fractals, check out http://youtu.be/93akxnQ1xxw
 

Quartz Geode, Grays Cr, Indiana USA
 
   


Keokuk, Iowa  USA

 

 

 

 


Calcite Geode
Ohio USA


Blue Celestite Geode
MADAGASCAR


Dolomite "geode", Dolomite, Strontianite, Marcasite
Manitoulin Island, Ontario, CANADA


Quartz Geode
MEXICO

 


Botryoidal Quartz Geodes
Kentucky, USA

Quartz Geode
Kentucky, USA

Botryoidal Chalcedony Geode
Kentucky, USA

The hollow spherical geode section is partially filled in on the inside with smooth, beige, small botryoidal chalcedony. It looks like rounded gray peas.

Quartz Geode  from Hall's Gap, Lincoln County, Kentucky  USA
 

Locations & types

  • Botryoidal Chalcedony Quartz Geode from South Fork Creek, Hall's Gap, Lincoln County, Kentucky USA
  • Quartz Crystal Geode from Hall's Gap, Lincoln County, Kentucky USA
  • "Keokuk" Geode from Scheffler's Geode Mine, Alexandria, Missouri USA

 

Formation theory #1 - Brazil

  • found in ancient volcanic rock filled with cavities produced by steam, later filled when solutions of seeping water found there way in & formed quartz crystals or agate

  • quartz is the most common material found in geodes

  • colour depends on what impurities were picked up by the solutions

  • if the solution is deposited slowly & has relatively few impurities, it results in crystal lined cavities

  • if solution is deposited rapidly, cavity is filled with bands of fine-grained quartz - namely agate

Formation theory #2

  • they were originally nodules of limestone or anhydrite

  • formed within a soft sediment by concentric outward growth around small nucleus or core

  • the interior of the concretion washed out, causing a hollow

  • groundwater solutions then left behind a replacement of the geode walls & the crystals inside

  • takes 240 million years to form a geode

 

 

Geodes: Some Locations & Types

USA

  • Indiana, Harrodsburg
  • quartz, calcite, barite
  • Indiana, Washington Co. near Salem Indiana
  • clear calcite crystals on pink-orange dolomite
  • thin walled
  • clear calcite crystals on white quartz
  • Iowa, Keokuk – Warsaw Strata
  • Keokuk geodes are from a 45 mile radius
  • borders Iowa, Illinois & Missouri
  • botryoidal chalcedony with calcite cubes
  • blue & green chalcedony
  • pink calcite crystals
  • sphalerite
  • quartz iron – cheery orange, quartz oxide – cherry red
  • dew drop diamond
  • pink dog-tooth
  •  iridescent brown calcite
  • industrial (garden bed) geodes – usually solid all the way through
  • interior filled with milky coloured quartz
  • found in people’s garden’s
  • Kentucky, Dannville
  • banded agate lining with crystals
  • Kentucky, Hall’s Gap
  • world-famous locality noted for its millerite in small geodes
  • light pink amethyst
  • Kentucky, South Fork Creek
  • bumpy, brown round geodes in river bed
  • stepped calcite, spaghetti, smoky quartz, botryoidal chalcedony
  • orange-yellow solids, clear, white
  • Missouri, Alexandria
  • Sheffler Geode Mine – Keokuk Geodes
  • gray, smoth skinned round
  • clear quartz crystals
  • Mississippi River Valley
  • in limestone
  • Ohio River Valley
  • in limestone

Brazil  Rio Grande do Sul

  • amethyst
  • most fantastic geode known – 33 ft. in length, 16 ˝ ft. wide, 10 ft. high
  • estimated weight of seventy thousand pounds
  • lined with purple amethyst
  • a piece weighing 400 lbs. is in the Smithsonian Institute

Mexico, Chihuahua

  • amethyst quartz, smoky, gray, white, lavender and brown quartz
  • along with contrasting crystals of calcite, goethite

Uruguay

  • adjoining Brazil, Rio Grande do Sol
  • amethyst

Canada

  • High Falls, Manitoulin Island, Ontario
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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