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All About Geodes.
Welcome to MoPop’s Geodes. This website is dedicated to all things geode and is a work in progress. More information and topics of interest will be added from time to time. I hope you enjoy.
What is a geode?
The word “Geode” is derived from the ancient Greek word “geodes”, meaning earthlike. They are roughly spherical structures found within a host rock that is usually of sedimentary origin. They contain an outer shell that is most commonly composed of the mineral chalcedony, with an interior possessing various degrees of hollowness. The insides of geodes are lined with minerals that have precipitated, or grown inward toward the center. If the mineral inside a geode continues growing until it fills the entire cavity it is referred to as a nodule, although the term “solid geode” is commonly used. The majority of the time, the main mineral found inside a geode is one of the many varieties of quartz. This type of crystal is referred to as euhedral, although depending on the variety of quartz, other crystal forms can be found. Many times various secondary minerals and crystal combinations can be found adorning the quartz making them very beautiful. It is these types of geodes that discerning rockhounds and collectors seek and they can be very valuable. One can never tell what may be found inside a geode until it is opened. Each one is unique.
Where are geodes found?
Geodes can be found in many places around the world. In this section we will be discussing Keokuk Geodes which are unique to a certain area in the midwestern United States. Keokuk Geodes can be found where the states of Missouri, Illinois and Iowa meet. This tri-state junction is known as the “Keokuk Region” to geode collectors. Geodes may be found in the rock strata, streams, and fields within a 60 mile radius of the city of Keokuk, Iowa which is where they derive their name. Geodes formed in a rock layer known as the Warsaw Formation which formed during the geologic time period known as the Mississippian Period. This rock layer is approximately 340 million years in age. Wherever this rock layer is exposed on the Earth’s surface geodes can be found. They appear as rounded rocks with cauliflower-like textures that stand out in the host rock. The outer geode shell is harder and more resistant to weathering than the host rock which allows them to become separated from the rock intact. When a river or stream cuts across this rock layer the geodes are exposed and wash out into the stream. In these cases geodes can literally pave the stream bed. In other instances this where this rock layer has been exposed due to highway construction or rock quarry operations, the geode beds are exposed. Property owners who are fortunate enough to have this rock layer close to the earth’s surface have located the geode beds underground and conduct mining operations to reach them.
Minerals found in Geodes.
Many different minerals are found in Keokuk geodes, some far more common than others. In this section we will discuss the more commonly found minerals.
Quartz.
By far the most abundant and common mineral found in Keokuk geodes is quartz. It makes up the outer shell in almost every geode in the quartz variety known as chalcedony. It also fills the insides of geodes as quartz crystals growing toward the center. The two most common crystal habits include the euhedral variety. This is nothing more than a mineralogical term for well-formed crystals with sharp and distinct faces that can easily be seen with the naked eye. When you crack open a geode and see the sparking crystals, this is what you have. Another commonly found type of quartz is the cryptocrystalline variety known as chalcedony. Cryptocrystalline is just the scientific word for crystals too small to be seen without a microscope. Both varieties come in different colors, sizes and combinations, and in many instances they compliment each other in the same geode making spectacular specimens. Quartz can frequently be found in colors such as smokey, white, reddish-orange hues and clear. The citrine colored hues in Keokuk geodes are the result of two processes, iron inclusions and iron staining. Sometimes geodes form in iron-rich clay and incorporate the iron while they are forming. This will “include” the iron content of the surrounding clay and groundwater while the crystals are being precipitated causing various shades of orange tinted crystals. Other times the crystals have formed prior to the presence of the iron and end up with only an orange coating. See the photo gallery below for photos of the different colors of quartz that may be found in Keokuk geodes.
Chalcedony has many lovely color varieties as well and will be discussed in the chalcedony section below.
Chalcedony.
As discussed earlier, chalcedony (pronounced Kal-seh-duh-nee) is the cryptocrystalline variety of quartz. It has the same chemical composition as regular quartz but differs in crystal size having crystals which are too small to be seen with the naked eye. Chalcedony is unique in Keokuk geodes in that it makes up the outer shell of almost every geode. Chalcedony is a durable mineral (7 on the Mohs hardness scale) and resistant to weathering. Because of this, a geodes’ outer shell is harder and more durable than the host rock it formed in. This allows geodes to remain intact, separate from the rock layer, and be relatively easy to collect. Chalcedony can be found coating other minerals in geodes or as a single mineral. When not coating other minerals, chalcedony can form in the crystal habit referred to as botryoidal (bot-ree-oy-del) which is derived from the ancient Greek meaning a “bunch of grapes”. In this case it appears as bubbly, grape-like crystals. Chalcedony can be found in any color of the spectrum depending upon the mineral content of the groundwater during formation. Other factors such as mineral weathering and exposure to the elements can cause chalcedony to have more than one color. The photos below are just a sample of the wide variety of color and beauty found in chalcedony geodes.
Calcite.
Calcite (Calcium Carbonate) is another common mineral found in Keokuk Geodes. It can be found in many colors and crystal combinations. Belonging to the Trigonal crystal system calcite forms beautiful rhombohedrons, scalenohedrons (dogtooth crystals), and barrel shaped crystals. Calcite twins, phantoms, and double terminated crystals are also possible. At times calcite can be found filling the entire geode. Translucent and clear crystals, sometimes side by side, can be found adorning the quartz lined interiors of geodes. Shades of pink, brown, orange, yellow, white, and green can be found lending even more beauty to the crystals.
Dolomite.
Another carbonate mineral frequently found in Keokuk Geodes is the mineral Dolomite. It forms as rhombohedral crystals with curved faces that resemble a saddle shape which mineralogists and geologists refer to as “saddleback” crystals. Dolomite crystals can be white, pink, and colorless, but are more commonly found in Keokuk Geodes in shades of orange, tan, and brown. This is due to the presence of iron and iron minerals that the geodes were exposed to during formation. Sometimes a mineral high in iron content such as Hematite, Goethite, or Limonite completely invades the dolomite and changes its mineral content in a process known as replacement. If replacement occurs and the dolomite’s saddle shaped crystal structure remains it is referred to as a pseudomorph, a word meaning false form. Dolomite geodes are beautiful and easily identifiable and will soon become one of your favorites.
Aragonite.
Aragonite is another carbonate mineral found in geodes from this region. It shares the same chemical composition as that of Calcite, but has a different crystal structure. This is referred to by geologists as a polymorph. Aragonite is usually found in Keokuk geodes as a white lacy mineral coating quartz and other minerals, but sometimes forms as globular or tiny ball-like crystals. Aragonite can fluoresce brilliantly in beautiful colors, so before you decide to get that ugly white coating off your quartz crystals, break out a UV light and look closely. You might have a beautiful fluorescent specimen on your hands.
Pyrite.
Pyrite, also known as fools gold, is an iron sulfide mineral frequently found in Keokuk geodes. It appears as brassy yellow crystals peppering the quartz and other secondary minerals. It also occurs as mineral inclusions as well. Beautiful cubic, filiform, and elongated crystals can be found. If you crack open a geode and see tiny golden sparkles among the quartz crystals, you are likely observing pyrite. If your geode contains quartz that has a rusty red appearance, look closely. It is likely that pyrite crystals have begun to decay and break down, giving up their iron molecules to the quartz in a process known as oxidation. Geodes containing beautiful shades of orange and red can result from this process. But let’s not put pyrite on a pedestal, as other iron minerals are also not stingy with their iron and could be contributing to these fall colors. Hematite, limonite, and goethite are among these iron minerals. Many times a geode has a beautiful citrine tint to the quartz crystals. Whereas this can make for a beautiful geode specimen, it should not be confused with the actual Citrine variety of quartz. It is possible to be fortunate enough to collect a geode containing the semiprecious gemstone variety Citrine, but it is more than likely iron stained quartz.
Sphalerite.
Sphalerite is a black mineral with a brilliant adamantine (diamond-like) luster. It can be found nestled in geodes among other minerals such as Quartz, Kaolinite, and Dolomite to name a few. Sometimes it completely fills the entire geode cavity. It has perfect dodecahedral cleavage meaning the Sphalerite crystals will cleave perfectly in six different directions. This is what causes Sphalerite crystals to reflect light so distinctly when they are turned. You didn’t expect a mineralogy lesson when you came here did you? It is a semi-rare treat to find a Sphalerite geode so if you locate one in a certain area, keep looking. Geodes tend to form similar types of minerals, crystals, and combinations in certain locations to their being subjected to the same environmental conditions during formation.
Kaolinite.
Kaolinite is a clay-like silicate mineral that is most often found as a brilliant white powder resembling flour. Plate-like hexagonal crystals are possible but rarely found in Keokuk geodes. If you open a geode and find this powder like substance inside, be on the lookout for other interesting features that may be present.
There is a possibility that another mineral frequently associated with Kaolinite may be present. Rinsing the powder from the inside may reveal a black mineral with brilliant, diamond-like luster. This semi-rare mineral is known as Sphalerite, and is frequently found alongside Kaolinite in Keokuk geodes.
Sometimes the Kaolinite interacts with other minerals such as Calcite that formed with it. A unique interaction between minerals can occur where the Kaolinite is absorbed by the Calcite replacing its chemical composition while retaining the crystal structure of the Calcite crystal. This new mineral is referred to as a “Kaolinite pseudomorph after Calcite”. The same interaction can happen with Aragonite or many other minerals as well.
With similar colors, pseudomorphs, and crystal types in the same geode, it may seem difficult to distinguish between Kaolinite and other minerals such as Calcite and Aragonite. One way of distinguishing between them is to expose them to ultraviolet light. Calcite and Aragonite will fluoresce while Aragonite will not.
Barite.
Barite is a barium sulfate mineral and comes in many colors. Blue, yellow, white, and clear are common. The crystals can be tabular or bladed shaped and have a pearly or vitreous (glass-like) luster, such as in the photos below and to the left. Barite does not fizz when exposed to HCl which can assist collectors in distinguishing it from calcite.
Barite is one of the more rare minerals found in Keokuk geodes. It may seem more rare than it actually is due to it being overlooked by many collectors. This may be due several factors. In many geodes, the crystals are tiny and simply overlooked. At some collecting locations, Barite can be found in flattened pancake-looking and collapsed geodes that are less desirable than the more rounded ones collectors tend to prefer. Geode collectors in their haste to clean the “junk” out of their geode, may scrub or dissolve this beautiful, although tiny, mineral away. Always remember to take a close look at the inside of your geode before you start cleaning it. An ugly geode may have a tiny treasure staring you in the face.
There is much more to be learned about geodes. More will be added as this website is updated so be sure to check back frequently.