Dolomite overgrowths suggest a primary origin of cone-in-cone

Journal article

A long-debated aspect of cone-in-cone structures is whether the mineral aggregates composing the structure precipitated with their conical form (primary cone-in-cone), or whether the cones formed after precipitation (secondary cone-in-cone). A calcite deposit from the Cretaceous of Jordan bears all the defining characteristics of the structure. Trace dolomite within the sample supports the primary cone-in-cone hypothesis. The host sediment is a biosiliceous mudstone containing abundant rhombohedral dolomite grains. Dolomite rhombohedra are also distributed throughout the calcite of the cone-in-cone. The rhombohedra within the calcite locally have dolomite overgrowths that are aligned with calcite fibres. Evidence that dolomite co-precipitated with calcite, and did not replace calcite, includes (i) preferential downward extension of dolomite overgrowths, in the presumed growth-direction of the cone-in-cone, from the dolomite grains on which they nucleate, and (ii) planar, vertical borders between dolomite crystals and calcite fibres. Because dolomite overgrows host-sediment rhombohedra and forms part of the cones, it follows that the host-sediment was incorporated into the growing cone-in-cone as the calcite precipitated, and not afterward. The host-sediment was not injected into the cone-in-cone along fractures, as the secondary-origin theory suggests. This finding implies that cone-in-cone in general does not form over multiple stages, and thus has greater potential to preserve the chemical signature of its original precipitation.

Authors: Hooker, J; Cartwright, J

• Publication status: Published
• Peer review status: Peer reviewed
• Publisher: Cambridge University Press
• Journal: Geological Magazine
• Volume: 155
• Issue: 3
• Pages: 568-585
• Publication date: 2016-09-22
• Acceptance date: 2016-07-18
• DOI: 10.1017/S0016756816000807
• EISSN: 1469-5081
• ISSN: 0016-7568
• Keywords: cone-in-cone; diagenesis; SEM; calcite; dolomite