Geodes and septarian concretions have boring exteriors and fascinating interiors.
Geodes and septarian concretions are both rocks formed in unique geologic settings that contain mineral crystals in the interior of the structure. From the outside, neither geodes nor septarian concretions look special, but the inside reveals a unique history of the rocks that surround them. Both geodes and septarian concretions are interesting specimens for geologists and collectors.
Host Rock
Both geodes and septarian concretions form in a host rock. Septarian concretions form in sedimentary rocks such as sandstone, limestone or chalk. Geodes sometimes form in sedimentary rocks but most often form in igneous rocks such as basalt.
Septarian Concretion Formation
Septarian concretions, also known as septarian nodules, are mineral deposits that have a different composition than the host rock. Often the concretion forms by accretion when silica or another mineral begins to form a nodule within the host rock. The nodule may grow around grain of sand or a plant or animal particle. The concretions often elongate parallel to the direction of the bedding plane of the rock. Over time, the concretions shrink and form cracks in the interior of the nodule. The cracks are typically largest in the center of the concretions and taper toward the edges. The cracks fill with a mineral-rich liquid that crystallizes in the void spaces of the septarian concretion.
Geode Formation
Geodes form in a pre-existing hole in a rock, often from an air bubble trapped in lava as it cools. The hole fills with a silica-rich liquid that forms mineral bands in concentric circles around the interior of the hole. Geodes that are not completely filled with banded minerals will then develop a crystal druze in the interior of the rock.
Mineral Types
Geodes typically fill with a silica-rich liquid that forms concentric bands of agate, which is microcrystalline quartz. The agate may vary in color creating a unique pattern within the geode. The crystals that form are typically quartz, amethyst or citrine and often cover every surface of the agate. Sometimes, calcite-rich liquid will enter the geode and create calcite crystals on the interior of the geode as well. Silica minerals such as quartz or chert, pyrite or various metals often form septarian concretions. Either a calcite-rich or silica-rich liquid fills the interior cracks and crystallizes into calcite or quartz.
Fossil Evidence
Septarian concretions can form around plant or animal remains, which fossilize within the concretions. The fossils help geologists determine the age of the concretion and the geologic environment of the formation. Geodes typically do not contain fossils. Fossils are not present in igneous rocks and geodes that form in sedimentary rocks typically do not contain fossils since the geode forms from a void space.
Tags: septarian concretions, concretions form, form sedimentary, form sedimentary rocks, host rock, liquid that, sedimentary rocks
