Lesley Andrews

Garnets are not only attractive, but also useful. As residents of the Cape, many of us know about the use of garnets as markers in diamond exploration. Other examples include garnet use in sandpaper, especially for wood sanding, and the production of synthetic garnets for laser generation.

The colours of garnet group minerals and varieties is a complex subject. Not all garnets are red, in fact they are found in all colours except bright blue. Briefly, the most common ions influencing colour are iron, manganese and chromium. The colours produced depend on the valence of the colour ion and therefore on its site in the garnet crystal lattice. Titanium-bearing varieties can be black, and pure grossular may be white. Some examples of the colour/composition of local garnets are provided elsewhere in this issue of the Mineral Chatter.

Garnets can be very variable in composition, and this can influence colour. Many garnets show colour variations within a deposit or even within the same hand specimen, e.g. “Transvaal jade” (hydrogrossular) and the grossular crystals pictured here.


Grossular from Ayrshire in Scotland.
The garnet crystals vary from yellow to reddish brown and are round 400 µm in diameter.
L. Andrews photograph

Zoning (compositional and sometimes coloured) may be found within crystals. This often relates to differences in formation conditions and may also be influenced by crystal symmetry.
 
Zoned andradite crystals “to 5 mm” from N’Chwaning II Mine, Northern Cape.
Photograph by Ludi von Bezing appears in the book by Cairncross & Beukes (see reference list). 

Zoning in garnets is a useful tool for geoscientists, and microanalysis provides clues about host rock formation, as well as the history of stressing and chemical alteration of these rocks. The use of garnet to “age” rocks often produced contradictory results until recently, but now that microanalysis of Sm-Nd and Lu-Hf decay products can be confined to zones, a clearer picture is obtained. The garnet zones contain a record of different ages due to overgrowths and/or alteration over time.

False colour zoning patterns illustrating major and trace element distribution in the garnets of metamorphic rocks (often almandine or andradite) can be acquired like a map on electron microscopes or microprobes. Almandine found in pressure-metamorphosed rocks frequently develop Ca, Mn-rich cores and Fe,Mg-rich rims. There is a continuous variation between (FeO + MgO) vs (CaO + MnO) with increasing metamorphic grade, so microanalysis of the garnets can inform us about formation conditions.  

High resolution false colour mapping has revealed the thin oscillatory zoning that forms when garnets are subjected to hydrothermal and meteoric fluid alteration. This type of research in skarn-based garnets is often supported by oxygen isotope zoning measurements.

Pictured below are two maps of andradite from Outer Mongolia, China. These form part of the research project being run by Professor Geiger of Salzburg University (Austria). The andradite pattern arises from hydrothermal fluid action and the oscillatory zoning map reflects even small changes in fluid composition.

  
Aluminium (left) and iron (right) false colour maps of andradite from Huanggangliang, Outer Mongolia. The garnet is about 1 mm in diameter. Images supplied by Professor C. A. Geiger, University of Salzburg.

Some of the material for this article has been sourced from the special Garnet Edition, Elements vol. 9 no. 6, December 2013. This is a fascinating read, and at least some of the articles are available on the Internet. Elements is published jointly by mineralogical and geochemical societies from all over the world.

Other sources are Deer, Howie and Zussman, The Rock-Forming Minerals; Miller, D. 2008. Glorious garnets. South African Lapidary Magazine 40(3):4-7; and Cairncross & Beukes The Kalahari Manganese Field, 2013.