Geode

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Geode

Geodes (Greek

Greek language

Greek is an Indo-European languages native to the southern Balkan peninsula, the language of the Greek people. It forms an independent branch within Indo-European....
geoides, "earthlike") are geological

Geology

Geology is the science and study of the solid and liquid matter that constitute the Earth. The field of geology encompasses the study of the composition, structural geology, physical properties, dynamics, and History of the Earth of Earth materials, and the processes by which they are formed, moved, and changed....
rock

Rock (geology)

In geology, rock is a naturally occurring solid aggregate of minerals and/or mineraloids.The Earth's outer solid layer, the lithosphere, is made of rock....
formations which occur in sedimentary and certain volcanic rocks.

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Encyclopedia

Geodes (Greek

Greek language

Geology

Rock (geology)

Vug

Vugs are small to medium-sized cavities inside Rock that may be formed through a variety of processes. Most commonly cracks and fissures opened by tectonic activity are partially filled by quartz, calcite, and other secondary minerals....
s with internal crystal

Crystal

A crystal or crystalline solid is a solid material whose constituent atoms, molecules, or ions are arranged in an orderly repeating pattern extending in all three spatial dimensions....
formations or concentric banding. The exterior of the most common geodes is generally limestone

Limestone

File:Limestone Formation In Waitomo.jpgLimestone is a sedimentary rock composed largely of the mineral calcite . The deposition of limestone strata is often a by-product and indicator of biological activity in the geology record....
or a related rock, while the interior contains quartz

Quartz

Quartz is the most abundant mineral in the Earth's continental crust . It is made up of a Crystal structure of silica tetrahedra. Quartz has a hardness of 7 on the Mohs scale and a density of 2.65 g/cm?....
crystals and/or chalcedony

Chalcedony

Chalcedony is a cryptocrystalline form of silica, composed of very fine intergrowths of the minerals quartz and moganite. These are both silica minerals, but they differ in that quartz has a trigonal crystal structure, whilst moganite is monoclinic....
deposits. Other geodes are completely filled with crystal, being solid all the way through. These types of geodes are called nodules

Nodule (geology)

A nodule in petrology or mineralogy is a secondary structure, generally spherical or irregularly rounded in shape. Nodules are typically solid replacement bodies of chert or iron oxides formed during diagenesis of a sedimentary rock....
.

Formation

Geodes can form in any cavity, but the term is usually reserved for more or less rounded formations in igneous and sedimentary rocks, while the more general term "vug" is applied to cavities in fissures and veins. They can form in gas bubbles in igneous rock

Igneous rock

Igneous rock is one of the three main Rock types . Igneous rock is formed by magma being cooled and becoming solid . They may form with or without crystallization, either below the surface as Intrusion rocks or on the surface as extrusive rocks....
s, such as vesicles in basaltic lavas, or as in the American Midwest, rounded cavities in sedimentary formations. After rock surrounding the cavity hardens, dissolved silicate

Silicate

A silicate is a compound containing an anion in which one or more central silicon atoms are surrounded by electronegative ligands. This definition is broad enough to include species such as hexafluorosilicate , [SiF6]2-, but the silicate species that are encountered most often consist of silicon with oxygen as the ligand...
s and/or carbonate

Carbonate

In chemistry, a carbonate is a salt or ester of carbonic acid....
s are deposited on the inside surface. Over time, this slow feed of mineral constituents from groundwater

Groundwater

Groundwater is water located beneath the ground surface in soil porosity spaces and in the fractures of lithologic formations. A unit of rock or an unconsolidated deposit is called an aquifer when it can yield a usable quantity of water....
or hydrothermal solutions allows crystals to form inside the hollow chamber. Bedrock containing geodes eventually weathers and decomposes, leaving them present at the surface if they are composed of resistant material such as quartz.

Coloration

The size of the crystals, including their form and shade of color, vary—making each geode unique. Some contain clear, pure quartz crystals, and others have rich purple amethyst

Amethyst

Amethyst is a violet variety of quartz often used as an ornamental stone in jewelry. The name comes from the Ancient Greek a- and methustos , a reference to the belief that the stone protected its owner from drunkenness; the Ancient Greece and Ancient Rome wore amethyst and made drinking vessels of it in the belief that it would prev...
crystals. Still others can have agate

Agate

Agate is a microcrystalline variety of quartz , chiefly chalcedony, characterised by its fineness of grain and brightness of color. Although agates may be found in various kinds of rock, they are classically associated with volcanic rocks but can be common in certain metamorphic rocks....
, chalcedony, or jasper

Jasper

Jasper is an Opacity , impure variety of silica, usually red, yellow or brown in color. This mineral breaks with a smooth surface, and is used for ornamentation or as a gemstone....
or minerals such as calcite

Calcite

Calcite is a Carbonate minerals and the most stable Polymorphism of calcium carbonate . The other polymorphs are the minerals aragonite and vaterite....
, dolomite

Dolomite

Dolomite is the name of a sedimentary carbonate rock and a mineral, both composed of calcium magnesium carbonate calciummagnesium2 found in crystals....
, celestite

Celestite

Celestine or celestite is a mineral consisting of strontium sulfate. The mineral is named for its occasional delicate blue color.Celestine occurs as crystals, and also in compact massive and fibrous forms....
, etc. There is no easy way of telling what the inside of a geode holds until it is cut open or broken apart. However, geodes from any one locality usually have a more restricted variety of interior mineralization.

Geodes and geode slices are sometimes dyed with artificial colors. Samples of geodes with unusual colors or highly unlikely formations have usually been synthetically altered.

Prevalence

Geodes are common in some formations in the United States

United States

The United States of America is a Federal government constitutional republic comprising U.S. state and a federal district. The country is situated mostly in central North America, where its Contiguous United States and Washington, D.C., the Capital districts and territories, lie between the Pacific Ocean and Atlantic Oceans, Borders of the U...
(mainly in Indiana

Indiana

The State of Indiana was the 19th U.S. state admitted into the union. It is located in the Midwestern United States of the United States of America....
, Iowa

Iowa

The State of Iowa is a U.S. state in the Midwestern region of the United States of America, an area often referred to as the "American Heartland." It is bordered by Minnesota to the north, Wisconsin and Illinois to the east, Nebraska and South Dakota to the west, and Missouri to the south....
, Missouri

Missouri

Missouri is a U.S. state in the Midwestern United States of the United States bordered by Iowa, Illinois, Kentucky, Tennessee, Arkansas, Oklahoma, Kansas and Nebraska....
, Kentucky

Kentucky

The Commonwealth of Kentucky is a U.S. state located in the East Central United States of America. Kentucky is normally included in the group of Southern United States , but it is uncommonly included, geographically and culturally, in the Midwestern United States....
and Utah

Utah

The State of Utah is a western United States U.S. state of the United States. It was the List of U.S. states by date of statehood admitted to the United States on January 4, 1896....
). They also are common in Brazil

Brazil

Brazil , officially the Federative Republic of Brazil , is a country in South America. It is the List of countries and outlying territories by total area country by geographical area, occupying nearly half of South America, the List of countries by population country, and the fourth most populous democracy in the world....
, Namibia

Namibia

Namibia, officially the Republic of Namibia, is a country in southern Africa on the Atlantic Ocean coast. It shares borders with Angola and Zambia to the north, Botswana to the east, and South Africa to the south....
and Mexico

Mexico

The United Mexican States , commonly known as Mexico , is a federalism constitutionalism republic in North America. It is bordered on the north by the United States; on the south and west by the Pacific Ocean; on the southeast by Guatemala, Belize, and the Caribbean Sea; and on the east by the Gulf of Mexico....
. A large geode was discovered in Put-in-Bay, Ohio

Put-in-Bay, Ohio

Put-in-Bay is a village #Ohio located on South Bass Island, Ottawa County, Ohio, Ohio, United States. The population was 128 at the United States Census 2000....
in the early 20th century. It is known as Crystal Cave

Crystal Cave (Ohio)

The Crystal Cave is a limestone cave located in Put-in-Bay, Ohio, a small town on South Bass Island in Lake Erie, with walls that display large crystals....
, and tours are possible during the summer. In 1967, Iowa designated the geode as the official state rock, and it has a Geode State Park

List of Iowa state parks

This is a list of Iowa state parks.These state parks of the U.S. state of Iowa can be split into two groups based on who manages them.The first group are those state parks managed by the Iowa Department of Natural Resources....
.

Sources

Rocks and Minerals, written by Fredrick H. Pough, ISBN 0-395-91096-X

Geode

Formation

Coloration

Prevalence

See also

Sources

External links