sedimentary
Ironstone Nodules / Siderite Concretion
Siderite (FeCO3) / Ferruginous Septarian Concretion
Hardness: 3.5-4.5 (Mohs scale); Color: Dark brown, rusty orange, or dull grey; Luster: Earthy to sub-metallic; Crystal structure: Trigonal (often massive/concretionary); Specific Gravity: 3.8
- Hardness
- 3
- Color
- Dark brown, rusty orange, or dull grey
- Luster
- Earthy to sub-metallic
Identified More sedimentary →
Identify your own rocks.
Get a report just like this from any photo, free.
Physical properties
Hardness: 3.5-4.5 (Mohs scale); Color: Dark brown, rusty orange, or dull grey; Luster: Earthy to sub-metallic; Crystal structure: Trigonal (often massive/concretionary); Specific Gravity: 3.8
Formation & geological history
Formed through the precipitation of iron minerals within sediment layers, often around a nucleus like a leaf, shell, or bone. These typically form in diagenetic environments within marine or swampy sedimentary basins across various geological ages.
Uses & applications
Historically used as a low-grade iron ore. Today, they are primarily of interest to amateur geologists and fossil hunters, as they often contain well-preserved fossils inside.
Geological facts
Many of the world's most famous fossils, such as those from the Mazon Creek formation, are found inside these types of iron-rich nodules. They are sometimes called 'cannonball stones' due to their rounded shapes.
Field identification & locations
Identify in the field by their heavy weight relative to size (high iron content) and distinct rounded or oblong shapes that stand out from surrounding shale or sandstone. Common in coal-bearing regions or coastal cliffs.
More like this
Other sedimentary specimens
Sandstone or Siltstone
Arenite (if sandstone)
Sedimentary
Sandstone
Arenite (primarily SiO2)
sedimentary
Shale or Slaty Mudstone
Argillaceous sedimentary rock
sedimentary
Chert Breccia
Brecciated Chert (Microcrystalline Silica)
sedimentary
Sandstone (Quartz-rich)
Arenite (SiO2)
sedimentary
Sandstone (with man-made markings)
Arenite (composed primarily of Quartz, SiO2)
sedimentary