Fossiliferous Chert

Description

Fossiliferous chert is a hard, dense, cryptocrystalline sedimentary rock composed of silicon dioxide (SiO2), characterized by the presence of macroscopic fossils, most commonly crinoid ossicles. These fossils are typically silicified, meaning their original calcium carbonate material has been replaced by silica, preserving their detailed morphology within the chert matrix. The crinoid fragments often appear as distinct, often white or lighter-colored, circular or star-shaped cross-sections against the typically darker chert background. The rock can range in color, but often exhibits shades of gray, brown, black, or reddish-brown. Its hardness and resistance to weathering make it a durable rock, and the embedded fossils provide significant paleontological and geological information.

How to Identify

Color

Variable, commonly gray, black, brown, tan, or reddish-brown. The crinoid fossils often appear as lighter-colored (white, light gray) inclusions.

Luster

Dull to waxy (subvitreous) on fresh fracture surfaces.

Texture

Cryptocrystalline (individual crystals too small to be seen without magnification). Smooth to slightly granular feel. The presence of visible crinoid ossicles provides a distinctive texture.

Crystal Form

Microcrystalline quartz (chalcedony) forming a dense, interlocking mosaic. Crinoid fossils retain their original skeletal forms, often appearing as circular or star-shaped cross-sections.

Cleavage

None. Exhibits conchoidal fracture.

Geological Environment

Marine sedimentary environments, typically associated with ancient shallow seas where both silica-rich organisms (e.g., sponges) and carbonate-producing organisms (e.g., crinoids) thrived. Often found as nodules or beds within limestone or dolostone formations.

Key Facts

• Hardness: 6.5-7 on the Mohs scale.
• Specific Gravity: 2.58-2.64 g/cm³.
• Crystal System: Trigonal (for quartz, the constituent mineral).
• Color: Highly variable, commonly gray, brown, black, tan, red; often with lighter-colored fossil inclusions.
• Luster: Dull to waxy (subvitreous).
• Transparency: Opaque to translucent on thin edges.
• Fracture: Conchoidal (shell-like, smooth, curved breaks with sharp edges).
• Cleavage: None.
• Composition: Primarily silicon dioxide (SiO2), with minor impurities and embedded silicified calcium carbonate (calcite) fossils.

Quick Check

• Color: Gray, brown, black, or tan with lighter (often white) fossil inclusions.
• Luster: Dull to waxy.
• Streak: White.

Physical Characteristics

• Crystal Habit: Cryptocrystalline aggregates of quartz (chalcedony). Fossils retain their original biogenic forms.
• Cleavage Type: Absent.
• Fracture Type: Conchoidal.
• Tenacity: Brittle.
• Luster Type: Dull to waxy.

Formation

Fossiliferous chert forms primarily through the diagenetic replacement of carbonate sediments (limestone or dolostone) by silica (SiO2). The silica is typically derived from the dissolution of biogenic silica, such as sponge spicules or radiolarian tests, in marine environments. This dissolved silica then precipitates as microcrystalline quartz, replacing the original carbonate matrix and often preserving the embedded fossils, such as crinoid ossicles (stem segments, calyx plates). Alternatively, it can form directly from the accumulation of siliceous skeletal remains, though replacement is more common for fossiliferous chert with well-preserved macrofossils. The crinoid fossils themselves are composed of calcium carbonate (calcite) and are replaced by silica during the chertification process, preserving their intricate structures.

Usage

Historically, chert was used by early humans for tool-making (flintknapping) due to its conchoidal fracture and sharp edges. Fossiliferous chert, while less common for industrial applications due to its heterogeneous nature, is highly valued by paleontologists and collectors for its well-preserved fossils, providing insights into ancient marine ecosystems. It is also used as decorative stone or for lapidary purposes when aesthetically pleasing.

Age Distribution

Common in Paleozoic (especially Mississippian and Pennsylvanian) and Mesozoic marine sedimentary sequences, but can occur in any age where chert formation coincided with abundant crinoid life.

Where to Find

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Scientific Classification

Mineral Class

Oxides (specifically, a variety of quartz)

Group

Silica Group

Crystal System

Trigonal (for quartz)

Chemical Formula

SiO2 (with embedded silicified CaCO3 fossils)

Composition

Microcrystalline quartz (chalcedony) with silicified crinoid remains.

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