The one number

Thirtymicrons

At 0.03 mm most minerals let light through, and each one colours polarised light by an amount set by its thickness and its birefringence. Grind every section to the same thickness and the colours become a way to name minerals. We stop when quartz turns grey.

Final grind · 1000-grit on glass

120µm

  1. 120 µm. Quartz shows second-order blues and yellows. Far too thick: nothing can be named yet.
  2. 80 µm. First-order reds and violets in the quartz. Keep going, lightly.
  3. 50 µm. Quartz is straw-yellow. Feldspar has reached white. Close now: every pass counts.
  4. 30 µm. Quartz grey to white, feldspar grey. Stop. The section is ready for its cover slip.

At the bench

Seven steps, about ten days

  1. 01CutA chip about 30 × 20 × 10 mm is sawn from your sample on a diamond blade, cooled with water (or oil, for salts and swelling clays).0.3 mm diamond rim
  2. 02ImpregnatePorous or crumbly rock is soaked in epoxy under vacuum. We dye it blue if you want to see the pores.vacuum, blue dye optional
  3. 03LapOne face is ground flat on silicon carbide slurry, finer and finer, until it is scratch-free under a lens.220 → 600 → 1000 grit
  4. 04MountThe flat face is glued to a frosted glass slide with a resin whose refractive index matches Canada balsam's.27 × 46 mm · n ≈ 1.54
  5. 05SectionMost of the chip is sawn off, leaving a slice a few tenths of a millimetre thick on the glass.≈ 0.3 mm left
  6. 06Grind to 30 µmBy hand at the end, checking between crossed polars: quartz and feldspar colours are the gauge. The scene above shows this last step.quartz = first-order grey
  7. 07Cover or polishA glass cover slip for microscopy, or a diamond polish for electron microprobe and reflected light.0.17 mm slip · 1 µm diamond

The Michel-Lévy chart

Colour is thickness × birefringence

Between crossed polars a grain shows the colour of its retardation, Γ = thickness × δ. At 30 µm each mineral has a ceiling. This strip is computed from the spectrum, the same way the microscope on each slide page is drawn.

quartz 270K-feldspar 210augite 720olivine 1080muscovite 1260calcite 5160 →

000 nm

inserting the quartz wedge