How to identify Concretion with possible mineral inclusion

To correctly identify Concretion with possible mineral inclusion (Sedimentary concretion with unknown mineral), 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, 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.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   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.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   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).

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   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).

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   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.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   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.

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.

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