The name of this blog is “Life in Plane Light” and to live up to the name, we need more thin section pictures 🙂
Last week’s mineral of the week was quartz. The optical properties for quartz are fairly simple and, unfortunately, overlap with a number of other minerals.
In plane light, quartz is clear and has low relief.
In crossed polars, quartz has 1st order whites to greys.
Ok, so what could we mistake it for and how do we avoid that?
- plagioclase: in igneous rocks, plagioclase will tend to have lamellar twins in crossed polars; in sedimentary & metamorphic rocks, things become more complicated since twinning is rare; metamorphic plagioclase may have deformation twins or core-rim zoning (I’m looking for a photomicrograph), but will not have the undulatory extinction or subgrains that can occur in quartz; when thin sections are slightly too thick, quartz will be 1st order yellow in XPL, but plagioclase will remain whites to greys; plagioclase in a water-rich environment may result in sericite (a fine-grained white mica), which doesn’t happen in quartz
- K-feldspars: in igneous rocks, sanidine / orthoclase usually have simple twins and microcline has tartan twins; in metamorphic rocks, we start having similar issues as with plagioclase; K-feldspar can also alter to become sericite like plagioclase and deformation twins may also form in K-feldspar, but this is also a problematic mineral for metamorphic petrologist
- cordierite: found in relatively few rocks (low pressure, moderate to high temperatures, Al-rich metamorphics) it may also have deformation twins, but the great salvation is that cordierite will form pleochroic haloes around U/Th-rich minerals (e.g. monazite, zircon), which won’t happen in either of the feldspars or quartz
What are you’re options if you happen to be a metamorphic petrologist?
- Find an SEM
- Technically, we could use the fact that quartz is uniaxial and the others are biaxial to distinguish between them using an optic axis figure, but finding grains with vertical optic axes tends to be difficult in metamorphic schists
The next mineral of the week is hematite / magnetite, so the next photomicrograph post is going to be “interesting.”
Oh, and if anyone has a good link to a core-rim metamorphic plagioclase zoning picture, please leave me a comment!