Petrography

Under the optical microscope cones can be seen to originate from layers of dark ferruginous material including unidentified shell fragments and thin layers of detrital quartz (Figs. 10 and 11). In most studied fragments, cones show a uniform growth direction, but in a few layers several millimeter-sized cones have grown in opposite directions from two different substrate laminae (Fig. 11). The conical layering is marked by argillaceous and carbonaceous material including minute, disseminated pyrite framboids, which more commonly occur around cone apices (Fig. 10-13). Alteration of pyrite to form iron oxide and/or hydroxides appears to be responsible for the dark color of the rock. Annular argillite-filled depressions of a scaly appearance on the cone cups are well developed and form angular patches as wide as 1.25 mm (Fig. 13 and 14).

Sections of cone-in-cone structures cut perpendicular to cone axes show nested circular shapes formed by interference and intersection of smaller arc segments separated by argillaceous films (Fig. 15). The bases of cones

Quartz Cones

Fig. 14. Photomicrograph of cone-in-cone structures showing composite angular argillaceous fillings of annular rings (dark). The shapes of the former calcite fibers now replaced by quartz are still visible. Note that the "stepped" profile is only on one side of the cone surface (right), whereas its counterpart is smooth (left). Cone-in-cone structures commonly break apart along these surfaces producing cone cups with annular rings (right) and striated cones (left). Section cut parallel to cone axes. Plane-polarized light; scale bar length: 0.5 mm.

Fig. 15. Photomicrograph of cone-in-cone structures cut perpendicular to cone axes. Note that nested circular shapes are formed by interference of smaller arc segments. Plane-polarized light; scale bar length: 0.5 mm.

Fig. 15. Photomicrograph of cone-in-cone structures cut perpendicular to cone axes. Note that nested circular shapes are formed by interference of smaller arc segments. Plane-polarized light; scale bar length: 0.5 mm.

are commonly composed of microcrystalline quartz, whereas zones around cone apices contain a mosaic of larger quartz crystals up to 0.1 mm in size. A few samples contain minute relicts of granular calcite crystals partly replaced by quartz, which occur commonly away from cone apices. Although the carbonate relicts are not composed of fibrous calcite, some degree of preferred orientation can be observed in thin section. The c-axes of the calcite crystals tend to be oriented parallel to the cone axes or parallel to the conical layering. These relic fibrous structures indicate that silicification of the cone-in-cone structure postdates the recrystallization of original fibrous calcite to form a microgranular carbonate fabric. Unfortunately, no indications on the timing of these processes are available, as no observations on the general stratigraphy and mesoscopic cross-cutting relationships of the cone-in-cone structure-bearing layers could be made in the field.

Although silicification affected the studied samples to various degrees, the internal structure typical of cone-in-cone structures is well preserved. This suggests that the only example of siliceous cone-in-cone structures described in detail in the literature (from the Lower Ordovician of the Montagne Noir, France) is not necessarily a primary structure, as suggested by Becq-Giraudon (1990). The observation that no carbonate relicts are present in the siliceous structure can not be considered evidence that cone-in-cone structures formed by precipitation of silica, but rather that the original carbonate may have been completely silicified.

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  • Ada
    What is the shape formed when to cone is added?
    2 years ago

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