How to identify Fossiliferous Limestone

To correctly identify Fossiliferous Limestone (Limestone with fossil inclusions (likely a brachiopod or bivalve impression)), 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

   Typically white, gray, tan, light brown, or yellowish, but can be darker if organic matter or other impurities are present.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Dull to earthy in the matrix; fossil fragments may exhibit a vitreous or pearly luster if well-preserved and crystalline.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Variable, ranging from fine-grained to coarse-grained. The texture is clastic, composed of bioclasts (fossil fragments) cemented together. The presence of visible, often intact, fossil shells or impressions is diagnostic.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   The primary mineral, calcite, forms rhombohedral crystals, but in limestone, it typically occurs as microcrystalline grains (micrite) or as sparry calcite cement. Fossils retain their original skeletal forms.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Calcite, the main component, exhibits perfect rhombohedral cleavage in three directions at 74 and 106 degrees. This is often visible in larger calcite crystals within the rock or in fossil shells.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Shallow marine environments, such as continental shelves, lagoons, and epicontinental seas, where biological activity is high and sedimentation rates allow for the preservation of organic remains. Also found in ancient reef complexes.

Key Facts

Hardness

3 on the Mohs scale (for calcite)

Specific Gravity

2.71 g/cm³ (for pure calcite, variable for rock)

Crystal System

Trigonal (for calcite)

Color

White, gray, tan, light brown, yellowish, or darker shades

Luster

Dull to earthy (matrix), vitreous to pearly (fossils)

Transparency

Opaque to translucent (thin sections)

Fracture

Uneven to conchoidal

Cleavage

Perfect rhombohedral (for calcite)

Composition

Primarily calcium carbonate (CaCO3) in the form of calcite, with varying amounts of fossil material and minor impurities (e.g., clay, quartz, iron oxides).

Physical characteristics

• Crystal Habit: Microcrystalline to macrocrystalline calcite matrix, with embedded macrofossils or microfossils retaining their original skeletal forms.
• Cleavage Type: Perfect rhombohedral cleavage in three directions for the constituent calcite.
• Fracture Type: Uneven to subconchoidal, depending on grain size and cementation.
• Tenacity: Brittle
• Luster Type: Dull to earthy for the rock matrix; individual fossil shells or crystalline calcite may exhibit vitreous or pearly luster.

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