Pisolite
Calcium carbonate (CaCO3) (pisolitic)
A sedimentary rock built from pea-sized concentric spheres called pisoids, often carbonate but sometimes iron or aluminum-rich.
- Mohs hardness
- 3-4
- Color
- Cream, tan, gray, brown, reddish when iron-rich
- Type
- sedimentary
Got a rock like this?
Identify any rock from a photo, free.
Overview
Pisolite is a sedimentary rock made of pisoids, rounded concentrically layered grains roughly two millimeters or larger across, about the size of a pea. The name comes from the Greek for pea, and the rock is essentially a coarse cousin of oolite, which is built from smaller ooids.
Most pisolites are carbonate, with calcite or aragonite shells grown in concentric layers around a nucleus. Cementation binds the pisoids into solid rock. However, pisolitic textures also occur in bauxite (aluminum ore) and ironstones.
The concentric internal structure is the signature feature, visible on broken or polished surfaces as onion-like rings within each grain.
Formation & geology
Carbonate pisoids form by the accretion of calcium carbonate in successive concentric layers around a nucleus such as a sand grain or shell fragment. This commonly happens in agitated warm shallow seas, around hot springs, and in caves where they grow as cave pearls.
Unlike ooids, which roll constantly in wave-agitated water, some large pisoids form in quieter settings such as soils, springs, and caves, growing in place. Pisolitic bauxite and ironstone form in deeply weathered tropical soils where aluminum or iron oxides accrete into spheres.
The pisoids are later cemented together and lithified into pisolite rock.
How to identify it
Pisolite is identified by its rounded, pea-sized grains packed together, each showing concentric internal layering when broken or cut. The grains are larger than the sand-sized ooids of oolite.
Carbonate pisolite fizzes in dilute hydrochloric acid, is moderately soft (around Mohs 3 to 4), and is usually cream, tan, or gray. Iron-rich pisolite is reddish brown and may be magnetic or heavy, while bauxitic pisolite is earthy and light.
Look-alikes include conglomerate (whose pebbles lack concentric layering) and oolite (smaller grains). Cutting a grain to reveal the onion-ring structure confirms a true pisoid.
Uses & significance
Pisolitic limestone is quarried as building stone, decorative stone, and aggregate, and its distinctive pea-grain texture makes it attractive for ornamental facing and countertops.
Pisolitic bauxite is the principal ore of aluminum, mined in vast quantities in tropical regions, while pisolitic ironstone has historically been smelted for iron. Carbonate pisolites are also useful indicators of ancient shallow marine, spring, and soil environments.
Cave pearls, a free-rolling form of pisoid, are collected as natural curiosities. Overall the rock's main value is industrial and geological rather than as a gemstone.
Frequently asked questions
What is the difference between pisolite and oolite?
Both are made of concentric layered grains, but pisolite grains (pisoids) are pea-sized, about two millimeters or larger, while oolite grains (ooids) are smaller, sand-sized.
Are all pisolites limestone?
No. Most are carbonate, but pisolitic textures also occur in bauxite, the main aluminum ore, and in some iron-rich ironstones.
How do pisoids form?
They grow by adding concentric layers of mineral around a nucleus, in agitated shallow seas, around hot springs, in caves as cave pearls, or in tropical soils.
How can I recognize pisolite?
Look for packed pea-sized round grains; breaking or cutting one reveals onion-like concentric rings, and carbonate types fizz in dilute acid.
Pisolite guides
In-depth guides for identifying, valuing, and understanding Pisolite.
