Ferricrete
Iron-oxide cemented regolith (Fe2O3, FeO(OH))
Hard surface crust formed when iron oxides cement soil and sediment into a rusty, durable duricrust in tropical and weathered terrains.
- Mohs hardness
- 4-6
- Color
- Red, brown, orange, to dark rust
- Type
- sedimentary
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Overview
Ferricrete is a duricrust formed when iron oxides and hydroxides cement soil, sand, and gravel into a hard, rust-colored rock. The name combines 'ferruginous' and 'concrete,' reflecting both its iron content and its concrete-like induration.
Ferricrete is common in tropical and subtropical landscapes, closely related to laterite, and often caps hills and plateaus where it resists erosion. Its deep red, brown, and orange colors come from hematite and goethite. It forms protective armor over softer regolith and records ancient weathering and groundwater conditions.
Formation & geology
Ferricrete forms through the mobilization and reprecipitation of iron in the weathering zone. In warm, wet climates, intense chemical weathering releases iron from minerals; this iron is transported in groundwater, especially where conditions are reducing, then precipitates as oxides where it meets oxygen, such as at the water table or surface.
Repeated wetting, drying, and fluctuating groundwater concentrate iron, cementing surrounding grains into a hard crust. Ferricrete is often associated with lateritic profiles and long-term landscape stability. It can engulf pre-existing sediments, forming iron-cemented conglomerates and sandstones.
How to identify it
Ferricrete is recognized by its strong rusty red, brown, or orange color and its hard, often vesicular, nodular, or conglomeratic texture. It is heavy for its size due to iron content and gives a yellow-brown to red streak.
Hardness varies (about 4-6) depending on the iron minerals and cementation. It typically occurs as a surface crust or cap rock in weathered terrain rather than as a bedded sequence. It does not fizz in acid.
Look-alikes include laterite, which is its close relative and often grades into it, and bog iron or ironstone. The duricrust setting and iron-oxide cement engulfing other grains help confirm ferricrete.
Uses & significance
Ferricrete and related lateritic ironstones are used locally as building blocks and road-surfacing material, and where iron content is high enough they can be a low-grade iron ore. In parts of Africa and India, lateritic crusts have long been cut into bricks for construction.
Geologically, ferricrete is an important marker of past tropical climates, ancient land surfaces, and groundwater behavior, aiding studies of landscape evolution and mineral exploration.
Frequently asked questions
What is ferricrete made of?
It is soil, sand, or gravel cemented by iron oxides and hydroxides such as hematite and goethite, giving it a rusty color.
How is ferricrete different from laterite?
They are closely related; laterite is an iron- and aluminum-rich weathering product, while ferricrete specifically refers to iron-cemented, hardened duricrust.
Does ferricrete react with acid?
No. It is iron-oxide cemented, not carbonate, so it does not fizz in hydrochloric acid.
Where does ferricrete form?
Mainly in tropical and subtropical climates with intense weathering and fluctuating groundwater, often capping hills and plateaus.
Ferricrete guides
In-depth guides for identifying, valuing, and understanding Ferricrete.
Ferricrete identified by the community
Real specimens identified with Rock Identifier.
