Petrologic Evidence for a Rhodochrosite Precursor of Spessartine in Coticules of the Venn-Stavelot Massif, Belgium

W. Schreyer, H.-J. Bernhardt and O. Medenbach
Institut für Mineralogie, Ruhr-Universität Bochum, Germany

Abstract: Minute CaMg-bearing rhodochrosite inclusions were discovered in spessartine crystals of small coticule globules enclosed within redschists that form the country rock of the famous coticule bands of Belgium. This points to a former metamorphic reaction relationship between the two minerals, such as rhodochrosite + pyrophyllite = spessartine + quartz + CO2 + H2O. The coexistence of spessartine with Mg-chlorite in the coticule globule could also indicate more complex reactions such as rhodochrosite + sudoite + quartz = spessartine + Mg-chlorite + CO2 + H2O. Our finding casts some doubt on the suggestion by Kramm (1976) that spessartine formed from Mn-montmorillonite in previously glassy volcanic tuffs.

Keywords: rhodochrosite • spessartine • coticule • Venn-Stavelot Massif • Belgium

Mineralogical Magazine; December 1992 v. 56; no. 385; p. 527-532; DOI: 10.1180/minmag.1992.056.385.08
© 1992, The Mineralogical Society
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