Concretion with possible mineral inclusion

Description

A concretion is a hard, compact mass of sedimentary rock, often spherical or ovoid in shape, formed by the localized precipitation of mineral cement within a sedimentary host rock. The 'possible mineral inclusion' refers to a distinct mineral phase or crystal(s) found within the concretion that is different from the primary cementing material. The nature of this inclusion is unknown without specific analysis, but it could be anything from quartz, calcite, pyrite, or even more exotic minerals, depending on the geochemical environment during formation. Concretions are typically harder and more resistant to weathering than the surrounding rock, causing them to stand out.

How to Identify

Color

Highly variable, depending on the cementing mineral and host rock. Common colors include shades of brown, grey, black, red, or yellow. The inclusion's color will depend on its specific mineralogy.

Luster

Variable. The outer concretion typically has a dull to earthy luster, but if broken, the cementing mineral might show vitreous (e.g., quartz) or pearly (e.g., calcite) luster. The inclusion's luster will be characteristic of its mineral.

Texture

Typically fine-grained to microcrystalline, often dense and compact. The surface can be smooth, bumpy, or irregular. The inclusion will have its own characteristic texture (e.g., crystalline, granular).

Crystal Form

Concretions themselves do not typically exhibit external crystal forms, but rather a nodular or spherical habit. The internal mineral inclusion, however, may show distinct crystal forms if it grew in an open space (e.g., euhedral crystals).

Cleavage

Concretions as a whole do not exhibit cleavage. The cementing mineral (e.g., calcite) or the included mineral may exhibit cleavage planes if broken. For example, calcite has perfect rhombohedral cleavage.

Geological Environment

Found within various sedimentary rocks, including shales, siltstones, sandstones, and limestones. They are common in marine and lacustrine (lake) environments where organic matter or other nuclei are present, and diagenetic fluids can circulate.

Key Facts

• Hardness: Variable, typically 3-7 on Mohs scale, depending on the cementing mineral (e.g., calcite 3, quartz 7, iron oxides 5-6.5). The inclusion's hardness will be specific to its mineralogy.
• Specific Gravity: Variable, typically 2.6-3.5, depending on the cementing mineral and density of the inclusion. For example, calcite (2.71), quartz (2.65), pyrite (5.0).
• Crystal System: Concretions are amorphous or cryptocrystalline aggregates. The internal mineral inclusion will have its own specific crystal system (e.g., trigonal for calcite/quartz, isometric for pyrite).
• Color: Highly variable (brown, grey, black, red, yellow, etc.)
• Luster: Dull to earthy (exterior); variable for internal minerals (vitreous, metallic, pearly, etc.)
• Transparency: Opaque to translucent (concretion); inclusion can be transparent to opaque.
• Fracture: Conchoidal, uneven, or splintery, depending on the cementing mineral and texture.
• Cleavage: Absent in the concretion as a whole; present in individual mineral grains within the concretion or the inclusion if they possess it (e.g., calcite has perfect rhombohedral cleavage).
• Composition: Primarily composed of a cementing mineral (e.g., calcite, silica, iron oxides, dolomite) and host rock particles (e.g., clay, silt, sand). The 'inclusion' is an additional, distinct mineral phase.

Quick Check

• Color: Variable (brown, grey, black, red, yellow)
• Luster: Dull to earthy (concretion exterior); variable for internal minerals (vitreous, metallic, pearly)
• Streak: Variable, depending on the dominant cementing mineral and any inclusions. For example, calcite (white), hematite (red-brown), pyrite (greenish-black).

Physical Characteristics

• Crystal Habit: Nodular, spherical, ovoid, discoidal, irregular (for the concretion). The inclusion may exhibit euhedral, subhedral, or anhedral crystal forms.
• Cleavage Type: None for the concretion as a whole. Individual mineral components or inclusions may exhibit perfect, good, or poor cleavage (e.g., rhombohedral for calcite, prismatic for some silicates).
• Fracture Type: Conchoidal, uneven, or splintery, depending on the mineralogy and grain size.
• Tenacity: Brittle to tough, depending on the cementing agent and degree of lithification.
• Luster Type: Dull, earthy, waxy (for the concretion). The inclusion can have metallic, vitreous, pearly, or other lusters.

Formation

Concretions form through the precipitation of mineral cement around a nucleus (e.g., a fossil fragment, shell, or organic matter) within porous sedimentary rock. This process, known as diagenesis, occurs after deposition but before full lithification. The mineral inclusion, if present, would have either served as the nucleus or precipitated later within a void inside the concretion.

Usage

Primarily of scientific interest for understanding diagenetic processes, paleoenvironments, and as sources of fossils. Some aesthetically pleasing concretions are collected as specimens. If the inclusion is a valuable mineral, it could have commercial value.

Age Distribution

Can form in sedimentary rocks of all ages, from Precambrian to Cenozoic, depending on the host rock and diagenetic conditions.

Where to Find

Finding Tips

Similar Rocks

Scientific Classification

Mineral Class

Not a single mineral; it's a rock structure. The cementing mineral and inclusion would belong to specific mineral classes (e.g., Carbonates, Silicates, Sulfides, Oxides).

Group

Sedimentary structure (diagenetic).

Crystal System

Amorphous or cryptocrystalline for the concretion matrix. The inclusion will have its own specific crystal system.

Chemical Formula

Variable, depending on the cementing mineral and inclusion. E.g., CaCO3 (calcite), SiO2 (silica), Fe2O3 (hematite), FeS2 (pyrite).

Composition

A mixture of detrital sedimentary particles cemented by a precipitated mineral (e.g., calcite, quartz, iron oxides, dolomite). The 'inclusion' is a distinct mineral phase within this matrix.

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