Micrite Identification Guide
Identify micrite, a fine-grained microcrystalline limestone, and distinguish it from chalk, lithographic limestone, mudstone, and chert.
Read the full Micrite encyclopedia entry →
What Micrite Looks Like
Micrite (microcrystalline calcite, lime mud) is a very fine-grained limestone made of carbonate mud — calcite crystals too small to see without magnification. It is dense, dull, and homogeneous, breaking with a smooth to subconchoidal fracture. Colors range from white and cream to grey, tan, and dark grey (when organic-rich). It can contain scattered fossils, but the matrix itself is uniform and muddy in texture.
- Color: white, cream, grey, tan, brown to dark grey
- Luster: dull, earthy
- Transparency: opaque
- Texture: extremely fine-grained, homogeneous; smooth, sometimes conchoidal fracture; possible fossils/burrows
Step-by-Step Field-ID Checklist
- Apply the acid test: micrite fizzes vigorously in dilute HCl — it is limestone (calcite).
- Note the fine, uniform texture: no visible grains; looks muddy and dense.
- Check hardness: soft (3) — a steel knife or nail scratches it easily.
- Examine fracture: smooth to subconchoidal, sometimes near-porcelain-like.
- Look for fossils: scattered shells or burrows in an otherwise structureless mud matrix.
Key Diagnostic Tests
- Hardness: 3 (calcite); scratched by a knife and even a copper coin.
- Acid: strong effervescence with cold dilute HCl — the decisive carbonate test.
- Streak: white.
- Cleavage: calcite has rhombohedral cleavage, but in micrite grains are too fine to see it; the rock breaks conchoidally.
- Density: ~2.7.
Common Look-Alikes and How to Tell Them Apart
- Chalk: also fine carbonate, but chalk is porous, soft, friable, and writes/rubs off as powder; micrite is denser and more compact. Both fizz in acid.
- Lithographic limestone: essentially an extremely pure, even-textured micrite; the terms overlap — lithographic limestone is a very smooth, homogeneous micrite (e.g., Solnhofen).
- Mudstone/claystone (siliciclastic): looks fine-grained too but does not fizz in acid and is often softer/clay-smelling when breathed on. Acid test separates them instantly.
- Chert: fine-grained but hard (7), waxy, and acid-inert; micrite is soft (3) and effervesces.
- Dolostone: similar appearance but reacts only weakly to acid (or only when powdered); micrite (calcite) fizzes readily.
Where It Is Typically Found
Micrite forms in low-energy carbonate environments where lime mud settles and is preserved — lagoons, deep shelves, tidal flats, and quiet basins. It is widespread in marine limestone successions worldwide, and exceptionally pure micrites (lithographic limestones) occur in restricted basins such as Solnhofen, Germany.
Frequently asked questions
What is micrite?
Micrite is a fine-grained limestone made of microcrystalline calcite, essentially lithified lime mud. It is dense, dull, and homogeneous, with calcite crystals too small to see without magnification.
How can you tell if a rock is micrite?
Micrite is a soft (Mohs 3), very fine-grained, homogeneous rock that fizzes strongly in dilute hydrochloric acid because it is calcite. The combination of vigorous effervescence, softness, and muddy fine texture identifies it.
Micrite vs mudstone — how do I tell them apart?
Carbonate micrite fizzes vigorously in dilute acid, while a siliciclastic mudstone does not react. The acid test is the quickest way to separate the two fine-grained rocks.
Is micrite the same as lithographic limestone?
They overlap. Lithographic limestone is an extremely pure, very smooth and even-textured micrite, such as the famous Solnhofen limestone, prized for its fine grain.
Micrite identified by the community
Recent Micrite specimens identified with Rock Identifier.
