The Alnö Carbonatite Complex, Central Sweden by Peter Kresten & Valentin R. Troll

Author:Peter Kresten & Valentin R. Troll
Language: eng
Format: epub
Publisher: Springer International Publishing, Cham

2.9 Zircon and Dalyite

Zircon was first reported by Holmquist (1893) from pyrochlore-bearing boulders from the small islands north of Alnö. Högbom (1895) describes zircon crystals measuring up to five millimetres from cavities in a nepheline syenite at the shore of Ås Bay, along with calcite and pyrite. He also published an analysis of the mineral (weight%): SiO2 29.68, ZrO2 64.94, FeO 1.15, MnO 0.28, H2O 3.86, total 99.91. By contrast, von Eckermann (1948a) stated that zircon does not occur in Alnö rocks and that Högbom’s sample was either misidentified as a mineral, or else the zircon came from a place other than Alnö. Zircon has, however, been found in several thin sections and in many heavy mineral concentrates and to our knowledge, zircon occurs in fenites, sövites, (kimberlitic) alnöites, alnöite breccias, and in carbonatite dykes.

In the wall-rock, zircon is a common accessory phase (Kresten 1990). In the metagreywackes, it is found as small euhedral or subhedral grains, often embedded in biotite and then surrounded by “radioactive halos”. Notably, during the very initial stages of fenitization, when alkali amphiboles are formed at the expense of biotite, zircon remains stable. In a slightly more advanced stage, when aegirine-rich clinopyroxene is formed, zircon disappears. Biotite, although corroded, is still present but as clear flakes without zircon or halos. In one sample, a remnant grain of zircon was found with lowered Zr/Hf ratio, indicating leaching processes. It is surrounded by a shell of a colourless phase which on the basis of qualitative data may be dalyite (K2ZrSi6O15), which appears to be an intermediate phase in the reaction between zircon and alkaline solutions. The instability of zircon in alkaline solutions is further illustrated by the absence of the mineral in medium to high-grafe fenites, but not in very low grade ones, alkaline plutonic rocks and in alkaline dykes. The only other established relationship with other mineral phases, except dalyite, is to baddeleyite, which is occasionally overgrowing zircon or being overgrown by it.

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