A thin section of quartzite under crossed polarisers is one of the quietest sights in geology. Nothing glitters and nothing fizzes. You get pale greys and whites, dark grain outlines, and — if the rock has been squeezed — the odd fanning shadow of undulose extinction sweeping across a crystal as you turn the stage.
Stay with it for a few minutes and a history starts to emerge. Quartz is common, tough and remarkably good at recording its own deformation. It recrystallises at temperatures that matter to anyone studying mountain belts, shear zones or oil reservoirs, so its textures work as a rough thermometer and strain gauge rolled into one.
Why thin sections show more than a hand lens
A hand specimen gives you colour, lustre and habit. A thin section — rock ground to about 30 micrometres and glued to a glass slide — gives you the inside of the grains. Under plane polarised light you see shape, cleavage (quartz has none), relief and inclusions. Under crossed polars, where the analyser is inserted, you see how the crystal lattice is oriented, and whether it is bent.
A useful routine for any quartz-bearing rock is to check the same handful of things every time:
- Grain size, shape and the style of the boundaries between grains
- What happens to each grain as you rotate the stage — uniform darkening, or a wave?
- Whether subgrains or faint planar lamellae are visible inside the crystals
- Inclusion types, and whether they sit randomly or in trails
- Thin rims of clear quartz grown around older, dustier cores
Undulose extinction: the giveaway of strain
Rotate the stage with crossed polars in place and a clean, unstrained quartz grain goes dark all at once. A strained grain does not. Part of it darkens while the rest stays bright, and the dark patch sweeps across like a cloud shadow as you keep turning. That is undulose, or undulatory, extinction, and it means the lattice is bent rather than broken.
Strong examples can show sweeping extinction across 30 degrees or more of rotation. You will see it in quartz veins near faults, in mylonites, and in quartzite that has been through a mountain belt. It is also common in coarse vein quartz you can find at the surface, which is why a polished slice of a thick quartz vein, viewed between two polarising filters, often shows the effect without any lab work at all.
Subgrains and the chessboard pattern
Look closely at a strongly undulose grain and you may see it divided into smaller domains that extinguish at slightly different angles. These are subgrains — low-angle boundaries formed when dislocations pile up and reorganise during recovery. They are the halfway house between a bent crystal and a fully recrystallised one.
In quartzite from high-grade terrains, you sometimes meet chessboard extinction: a patchy grid of alternating dark and light domains that looks like a chessboard as the stage turns. It points to slip on two different crystallographic systems at once, and in practice it signals hot, deep deformation.
Deformation lamellae and recrystallisation textures
Deformation lamellae are thin planar features, usually only a few degrees out of alignment with the host grain, showing up as faint parallel bands. They are typical of quartz deformed at relatively low temperatures — quartz veins in fault zones are a good place to find them.
The boundaries between grains tell an equally useful story. In broad terms, and remembering that strain rate and water content shift things around:
- Straight boundaries with 120-degree triple junctions suggest annealing, where grains have relaxed into a foam-like texture at high temperature.
- Bulging boundaries, with small lobes nipping into a neighbour, indicate lower-temperature recrystallisation.
- Subgrain rotation produces new small grains roughly the size of the subgrains, typical of mid-crustal conditions.
- Sutured, interlobate boundaries point to grain boundary migration, which needs more heat.
- Long ribbon grains full of subgrains are the signature of a mylonite, a rock sheared in a fault zone at depth.
Treat those as bands rather than fixed rules. A single thin section rarely gives one clean answer; the interest lies in the mix.
Inclusions: the small print inside the crystal
Switch back to plane polarised light and the interiors become interesting. Solid inclusions in quartz include rutile needles — often aligned along crystallographic directions, giving the effect we call rutilated quartz — plus tourmaline, mica flakes, zircon, monazite, apatite and iron oxides. Zircon and monazite grains are sometimes ringed by tiny radiation haloes.
Fluid inclusions are the more revealing lot. Some are single-phase; others show a liquid with a bubble rattling around inside. Isolated inclusions that appear to have been trapped as the crystal grew are primary and can hold a sample of the original fluid. Others are secondary, formed much later.
Healed fractures and fluid inclusion planes
When quartz cracks, later fluids can deposit new quartz along the fracture and trap microscopic droplets in the process. The result is a trail of inclusions cutting straight across a grain, ignoring its internal structure. These healed microcracks, called fluid inclusion planes, are extremely useful: in a region of deformed rock, their orientations tend to cluster, and geologists use those clusters to work out the direction of past stress.
Quartz overgrowths in sandstones
In sandstone, the story is about growth rather than damage. A detrital quartz grain sitting in a pore space often gets a syntaxial overgrowth — new quartz that continues the parent crystal's orientation. The clue is a dust rim of clay, iron oxide or fluid inclusions marking the original grain outline, with clear quartz beyond it.
Under crossed polars the core and its overgrowth go extinct together, because they share the same lattice orientation. That single observation is how you tell an overgrowth from a separate grain, and it matters: overgrowths are a major reason sandstones lose porosity as they are buried.
Practical: how to look at your own quartz
You do not need a research microscope to start. A cheap pair of polarising filter sheets, or an old pair of polarised sunglasses, will get you surprisingly far.
- Set a phone or laptop screen to plain white and hold the sample flat against it. The screen light is already polarised.
- Look through the polarised sunglasses, then rotate the sunglasses while watching the sample. When the filters cross, the field darkens.
- Use coarse material: a slice of thick vein quartz, a slab of quartzite, or crushed grains sprinkled onto sticky tape. Fine dust tells you almost nothing.
- Watch for the sweeping darkening of undulose extinction as you rotate, and for grains that stay uniformly bright until they snap dark all at once.
For anything serious, buy or commission a proper thin section. Commercial geological slide makers, university earth science departments and local geological societies are the usual routes, and many societies run microscopy evenings where you can use a real petrological microscope with a rotating stage. Photograph what you see through the eyepiece with a phone; a scale bar or a note of the magnification will save you later confusion.
Keep notes on the same features each time you sit down: boundaries, extinction, subgrains, inclusions, overgrowths. After a dozen slides you will start reading a quartz grain the way you read a mineral label — and the quiet grey view under crossed polars becomes one of the more talkative things in geology. If you crush your own material, wear eye protection and avoid breathing the dust.
