Concretionary zoning (oolite, pisolite, framboid)

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Characteristics:

• Concentric or spherical shaped zoning;

• A grain that served as substratum for mineral growth is generally observed in the core of the concentrically-zoned mineral;

• Generally described in a sedimentary context.

Associated terminology:

• Ooid / oolite

• Pisolite (correspond to oolite larger than 2 mm)

• Framboid texture in pyrite

Not to mistake for:

• Orbicular rock – concentric pattern developed in a magmatic rock (rare texture with an unclear origin);

• Variolite – light-grey spheroid generally developed in basalt (origin unclear, may correspond to devitrification figures).

Origin:

• Point to development in a liquid (marine generally) environment;

• Point to direct deposition from solution due to changing physical-chemical conditions – these changes may be due to the activity of bacteria, algae, etc.

• Framboidal pyrite: has been attributed to bacterial activity, but can result from H2S supersaturated fluid (Butler and Richard 2000).