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Concretions in sandstone are hardened, compact masses of mineral matter that have precipitated within the pore spaces of the host sandstone, effectively cementing the sand grains together. They are typically spherical, ovoid, discoidal, or irregularly shaped, and are often harder and more resistant to weathering than the surrounding sandstone. Iron oxide concretions are characterized by their reddish-brown to black coloration due to the presence of iron oxides (e.g., hematite, goethite) as the cementing agent. They can range in size from a few millimeters to several meters in diameter. The internal structure may show concentric layering or a massive texture, sometimes preserving original sedimentary structures or fossils.
How to Identify
- Color
- Concretions are typically reddish-brown, dark brown, or black due to iron oxides. The surrounding sandstone can be various shades of tan, yellow, white, or grey.
- Luster
- Dull to earthy in the concretion, similar to the host sandstone, but can be slightly more vitreous if silica-cemented or sub-metallic if hematite-rich.
- Texture
- The concretion is generally denser and harder than the surrounding sandstone, often with a finer grain size due to complete cementation. The host sandstone is clastic, granular, and friable to well-cemented.
- Crystal Form
- Concretions do not exhibit external crystal form; they are massive or concentrically layered aggregates of cemented grains. The cementing minerals (e.g., hematite, goethite) are typically cryptocrystalline or microcrystalline.
- Cleavage
- No cleavage in the concretion itself, as it is a cemented aggregate. The constituent minerals (e.g., quartz grains) do not show cleavage in this context.
- Geological Environment
- Sedimentary basins, particularly in fluvial, deltaic, shallow marine, and eolian sandstone deposits. They are common in formations where groundwater flow and chemical conditions favored the precipitation of iron minerals.
Key Facts
- Hardness: Variable, typically 5-6.5 on Mohs scale for the iron oxide cement, but the overall hardness is influenced by the quartz grains (7) and the degree of cementation. Generally harder than the host sandstone.
- Specific Gravity: 2.6-3.8, depending on the density of the cementing minerals (e.g., hematite ~5.2, goethite ~3.3-4.3) and the porosity of the concretion.
- Crystal System: Not applicable for the concretion as a whole. The cementing minerals (e.g., hematite is trigonal, goethite is orthorhombic) have their own crystal systems, but are typically cryptocrystalline within the concretion.
- Color: Concretion: Reddish-brown, dark brown, black. Sandstone: Tan, yellow, white, grey.
- Luster: Dull to earthy.
- Transparency: Opaque.
- Fracture: Conchoidal to irregular, often breaking across sand grains.
- Cleavage: None.
- Composition: Primarily quartz (SiO2) sand grains cemented by iron oxides/hydroxides (e.g., hematite Fe2O3, goethite FeO(OH)). Minor amounts of other minerals may be present depending on the host rock and fluid chemistry.
Quick Check
- Color: Reddish-brown to black (concretion), variable (sandstone)
- Luster: Dull to earthy
- Streak: Reddish-brown (for iron oxide concretions)
Physical Characteristics
- Crystal Habit: Massive, nodular, spherical, ovoid, discoidal, or irregular aggregates. Internal structure can be massive or concentrically layered.
- Cleavage Type: None.
- Fracture Type: Conchoidal to irregular.
- Tenacity: Brittle.
- Luster Type: Dull to earthy.
Formation
Concretions form within sedimentary rocks, typically sandstones, during diagenesis. They grow by precipitation of mineral cement (often iron oxides/hydroxides like hematite or goethite, but also calcite, dolomite, or silica) around a nucleus in the pore spaces of the host sediment. This precipitation is driven by chemical gradients, often related to redox conditions, microbial activity, or fluid flow. Iron oxide concretions specifically form where iron-rich fluids interact with oxygenated zones or where iron-reducing bacteria create localized conditions for iron precipitation.
Usage
Primarily of scientific and educational interest for studying diagenetic processes, fluid flow, and paleoenvironmental conditions. Some aesthetically pleasing or unusually shaped concretions are collected as geological curiosities or for decorative purposes. 'Moqui Marbles' are used in some spiritual practices.
Age Distribution
Found in sedimentary rocks of various ages, from Precambrian to Cenozoic, depending on the host sandstone formation.
Where to Find
Navajo Sandstone, Utah, USA
Famous for 'Moqui Marbles' (iron oxide concretions) in the Jurassic Navajo Sandstone, particularly in Zion National Park and Grand Staircase-Escalante National Monument.
Dakota Sandstone, Kansas, USA
Known for large, spherical to discoidal ironstone concretions within the Cretaceous Dakota Sandstone.
Badlands National Park, South Dakota, USA
Various concretions, including iron-rich types, are found weathering out of the Oligocene White River Group sediments.
Fossil Butte National Monument, Wyoming, USA
While known for fish fossils, concretions are also present in the Eocene Green River Formation, sometimes preserving organic matter.
Various localities worldwide
Concretions are a common diagenetic feature and can be found in sandstone formations on every continent where suitable conditions for their formation existed.
Finding Tips
Look for differential weathering
Concretions are often more resistant to erosion than the surrounding host rock, causing them to weather out and accumulate on slopes or at the base of outcrops. They may appear as rounded or oddly shaped 'rocks' on the surface.
Examine sandstone outcrops
Carefully inspect exposed sandstone layers for spherical, ovoid, or irregular masses that stand out in color or texture from the main rock body. Look for concentric banding or a distinct boundary with the host rock.
Check for iron staining
The reddish-brown to black color of iron oxide concretions is a key indicator. Look for areas where iron-rich groundwater might have flowed, as this often promotes concretion formation.
Consider the geological context
Concretions are common in specific sedimentary environments. Researching the local geology and known occurrences of concretions in an area can increase success.
Similar Rocks
Calcareous Concretion in Shale
Shale with Calcareous Concretion
Also known as: Limestone Concretion, Septarian Nodule
Chert Nodule in Limestone
Limestone with Siliceous Nodule
Also known as: Flint Nodule
Manganese Nodule
Manganese-Iron Oxide Nodule
Also known as: Ferromanganese Nodule
Scientific Classification
- Mineral Class
- Not a single mineral, but a diagenetic feature composed of rock-forming minerals and cementing agents.
- Group
- Sedimentary rock with diagenetic concretions.
- Crystal System
- Not applicable for the concretion as a whole.
- Chemical Formula
- Variable, primarily SiO2 (quartz) + Fe2O3 (hematite) or FeO(OH) (goethite) + minor constituents.
- Composition
- Quartz sand grains cemented by iron oxides/hydroxides. The iron oxides can be hematite (Fe2O3), goethite (FeO(OH)), or limonite (a mixture of hydrated iron oxides). Other minor cementing agents like calcite or silica may also be present in trace amounts.
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