12" Smoky Keokuk Geode with Calcite & Filiform Pyrite - Missouri

This is a massive, sparkling smoky quartz geode with calcite and filiform pyrite crystals, collected from St. Francisville, Missouri. It's been cracked open to reveal an interior lined with glimmering smoky quartz crystals. Both halves are included.

This specimen is a part of an old collection that we recently acquired. Some of these Keokuk geodes come from locations that are no longer collectable.

Unlike most geodes that form in volcanic rock, Keokuk geodes are found in sedimentary rock. They started out as concretions of mud that formed around organic material about 340 million years ago. The outer shells of these concretions were subsequently replaced by chalcedony and the interiors of the concretions were dissolved, leaving a hollow space into which quartz crystals could grow. Most geodes are 2 to 5 inches wide, though specimens as large as two feet across have been found.

Keokuk geodes contain a variety of minerals, but quartz is dominant in most. Many geodes are filled with clear to white quartz crystals. Micro-crystalline quartz, or chalcedony, whose component crystals are too small to be seen with the naked eye, forms the outer shell. Chalcedony layers also encrust the interior walls of many geode cavities, covering the surfaces of the earlier-generation quartz crystals in a variety of colors, including white, gray, blue, yellow and orange. Calcite is also a common mineral in many geodes, though seventeen other minerals have been identified in Keokuk Geodes, including pyrite and sphalerite.

The area around Keokuk, Iowa is sometimes referred to as “the geode capital of the world". In 1967, these geodes were even named the official state rock of Iowa. Geodes have been collected from the Lower Warsaw Formation within about 100 miles of the city for over 150 years.

Smoky quartz is a grey-brown to black variety of quartz. This common name is derived from the appearance of smoke within the quartz crystal. Depending on the location and the chemicals present during formation, smoky quartz can appear opaque black, but it is typically translucent to some extent. It is believed that the quartz gains this color from a combination of natural irradiation and aluminum impurities.

Quartz is the name given to silicon dioxide (SiO2) and is the second most abundant mineral in the Earth's crust. Quartz crystals generally grow in silica-rich environments--usually igneous rocks or hydrothermal environments like geothermal waters--at temperatures between 100°C and 450°C, and usually under very high pressure. In either case, crystals will precipitate as temperatures cool, just as ice gradually forms when water freezes. Quartz veins are formed when open fissures are filled with hot water during the closing stages of mountain formation: these veins can be hundreds of millions of years old.

The mineral pyrite, also known as iron pyrite, is commonly referred to as Fool's Gold because its metallic luster and pale brass-yellow hue give it a superficial resemblance to gold. In the old mining days, pyrite was sometimes mistaken for gold.

It is the most common of the sulfide minerals with the chemical formula FeS2. Pyrite crystals occur in many shapes and habits, including cubes of all sizes, penetration twin cubes, pyritohedral clusters and as small druzy crystals that can exhibit a beautiful glistening effect.

Calcite, CaCO3, is a carbonate mineral and the most stable polymorph of calcium carbonate. The other polymorphs are the minerals aragonite and vaterite. Calcite crystals are trigonal-rhombohedral, though actual calcite rhombohedra are rare as natural crystals. However, they show a remarkable variety of habits including acute to obtuse rhombohedra, tabular forms, and prisms. Calcite exhibits several twinning types adding to the variety of observed forms. It may occur as fibrous, granular, lamellar, or compact. Cleavage is usually in three directions parallel to the rhombohedron form.
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DETAILS
SPECIES
Quartz var. Smoky, Calcite & Pyrite
LOCATION
St. Francisville, Missouri
SIZE
12" wide
CATEGORY
SUB CATEGORY
ITEM
#144812