How to identify Stromatolite

To correctly identify Stromatolite (Stromatolite (layered sedimentary rock formed by cyanobacteria)), check each of these features in order. Many rocks and minerals look alike, so cross-reference multiple properties before deciding.

Step-by-step identification

1. 1

   Color

   Overall color and any zoning or banding

   Highly variable, often shades of grey, brown, tan, green, or reddish, depending on the mineral composition and presence of organic matter or iron oxides.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Dull to earthy, sometimes waxy if polished.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Finely laminated, often with wavy, domal, or columnar layers visible on cut or weathered surfaces. Can feel slightly rough or smooth depending on the degree of lithification and weathering.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Not a mineral with a distinct crystal form. It is a rock composed of microcrystalline minerals (e.g., calcite, dolomite, quartz) and organic matter, forming macroscopic layered structures.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Does not exhibit mineral cleavage. May break along bedding planes or exhibit irregular fracture.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Typically forms in shallow marine or lacustrine (lake) environments, particularly in hypersaline or alkaline conditions where grazing organisms are limited, allowing microbial mats to flourish. Modern examples are found in Shark Bay, Australia; Highborne Cay, Bahamas; and various freshwater lakes.

Key Facts

Hardness

Variable, typically 3-4 on Mohs scale if primarily calcite/dolomite, higher if silicified.

Specific Gravity

2.6-2.9 (if primarily calcite/dolomite), varies with composition.

Crystal System

Not applicable (rock, not a single mineral). Constituent minerals are typically trigonal (calcite, dolomite) or amorphous/cryptocrystalline (chert).

Color

Grey, brown, tan, green, reddish, often banded.

Luster

Dull, earthy, sometimes waxy when polished.

Transparency

Opaque.

Fracture

Irregular to conchoidal (if silicified).

Cleavage

None (as a rock). Constituent minerals may have cleavage.

Composition

Primarily calcium carbonate (calcite or dolomite), often with silica (chert), organic matter, and trapped detrital sediments (silt, sand, clay).

Physical characteristics

• Crystal Habit: Not applicable (rock). Forms macroscopic layered structures.
• Cleavage Type: None (as a rock).
• Fracture Type: Irregular to conchoidal.
• Tenacity: Brittle.
• Luster Type: Dull to earthy.

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