In plain light the garnet is profoundly color-zoned, with a yellow core grading out to orange, then a sharp discontinuity against a thinner pink overgrowth. The garnet's large core is relatively inclusion-free and slightly birefringent with a four-way sector zonation. The rim is heterogeneous in its birefringence, with a fine-scale structure paralleling the mapped Ca distribution. The rim contains quartz inclusions. Other minerals in the quartz vein assemblage include Ba-Ti rich biotite, Ba-rich muscovite, celsian, barite, hyalophane, Mn-rich cummingtonite, rutile and chlorite.
The garnet was mapped for Al, Mg, Mn and Ca at two different scales. The four images shown above show Mn and Ca at these two scales. The third and fourth images are detail maps of part of the overgrowth in the field of view of the first two maps. Al was mapped with the expectation that some of the optical peculiarities of this garnet would be the result of a changing andradite content, revealed by regions of lower Al. It is to be emphasised that neither quantitative WDS analysis along traverses nor spatial mapping of Al found measureable presence or change in andradite component. Mn and Ca follow two very different patterns of spatial distribution. Mn proceeds from a core (ca. spessartine-52), out across a profound discontinuity into a zone of gentle concentric fluctuations in the overgrown rim. Ca has a somewhat patchy distribution in the core, and in the overgrowth has a 'flake' texture, approximately radial, interpreted as indicating simultaneous growth of two distinct compositions. Other examples from this site, which may well appear on this page in the near future, show layers with high-Ca and low-Ca zones organized on adjacent faces, interspersed with layers showing this 'flake' texture. Further postings may also include quantitative data.
For more information about this image contact: Michael
L. Williams