Fossiliferous Chert

Description

Fossiliferous chert is a variety of chert characterized by the presence of macroscopic or microscopic fossil inclusions. Chert itself is a hard, dense, cryptocrystalline sedimentary rock composed of silicon dioxide (SiO2). The fossils can range from entire organisms (e.g., brachiopods, crinoids, gastropods, corals, sponges, foraminifera, radiolarians, diatoms) to fragments, and their preservation can vary from molds and casts to complete silicification of the original organic material. The presence of fossils provides direct evidence of the biological activity and paleoenvironment at the time of the chert's formation.

How to Identify

Color

Highly variable, often gray, black, brown, tan, white, or reddish, depending on impurities and the original material. The fossils themselves may be a different color or texture.

Luster

Dull to waxy, vitreous on fresh fracture surfaces.

Texture

Very fine-grained (cryptocrystalline), smooth to slightly granular. The texture will be interrupted by the presence of visible fossils, which may have their own distinct textures.

Crystal Form

Cryptocrystalline aggregates of quartz. Individual crystals are too small to be seen without high magnification. Fossils retain their original biological forms.

Cleavage

None, exhibits conchoidal fracture.

Geological Environment

Typically found in marine sedimentary sequences, often interbedded with limestones, shales, or sandstones. It can occur as nodules, concretions, or bedded layers. Common in ancient shallow marine shelf environments, deep-sea environments (where radiolarian oozes accumulate), and areas of volcanic activity providing silica.

Key Facts

• Hardness: 6.5-7 on the Mohs scale (due to quartz composition)
• Specific Gravity: 2.58-2.64
• Crystal System: Trigonal (for the constituent quartz)
• Color: Highly variable, commonly gray, black, brown, tan, white, or reddish, often mottled or banded. The color of the fossils may contrast with the matrix.
• Luster: Dull to waxy, vitreous on fresh fracture surfaces.
• Transparency: Opaque to translucent (thin edges may be translucent).
• Fracture: Conchoidal (shell-like, smooth, curved breaks).
• Cleavage: None
• Composition: Primarily silicon dioxide (SiO2), with varying amounts of impurities (e.g., iron oxides, clay minerals, organic matter) and fossil remains.

Quick Check

• Color: Variable (gray, black, brown, tan, white, red), often with visible fossil outlines or structures.
• Luster: Dull to waxy, vitreous on fresh breaks.
• Streak: White

Physical Characteristics

• Crystal Habit: Cryptocrystalline aggregates of quartz, forming massive, nodular, or bedded structures. Fossils retain their original biological morphology.
• Cleavage Type: Absent
• Fracture Type: Conchoidal, producing sharp edges.
• Tenacity: Brittle
• Luster Type: Dull to waxy, vitreous

Formation

Fossiliferous chert forms primarily through the diagenetic alteration of biogenic silica (e.g., opaline tests of diatoms, radiolarians, and sponge spicules) into microcrystalline quartz. This process often occurs in marine environments where these silica-secreting organisms are abundant. The fossils themselves, or fragments thereof, become entombed within the silica matrix during or shortly after deposition, and are subsequently silicified. It can also form through the replacement of carbonate rocks (like limestone) by silica-rich fluids, where pre-existing fossils within the limestone are preserved through silicification.

Usage

Historically, chert, including fossiliferous varieties, was used for tools (flintknapping) due to its conchoidal fracture and hardness. Today, it is primarily of scientific interest for paleontological and sedimentological studies. Attractive pieces may be used in lapidary work for cabochons or ornamental objects, and it can be a component of aggregate in construction, though less desirable than other chert types due to potential weakness from fossil inclusions.

Age Distribution

Ranges from Precambrian to Cenozoic, depending on the age of the included fossils and the chert formation.

Where to Find

Finding Tips

Similar Rocks

Scientific Classification

Mineral Class

Oxides (specifically, a variety of quartz)

Group

Silica minerals

Crystal System

Trigonal (for quartz)

Chemical Formula

SiO2

Composition

Silicon dioxide

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