Optical features in thin section

What these features look like under the microscope, and which minerals show them.

Polysynthetic (albite) twinning

Repeated twin domains appear as parallel stripes that go dark alternately as you rotate the stage in cross-polarized light; typical of plagioclase. In albite twinning the domains are related by reflection across a near-vertical plane.

**Plagioclase twinning.** Wikimedia Commons (author and license on the file page). Source

**Albite twinning.** Wikimedia Commons (author and license on the file page). Source

See plagioclase

Source: Perkins et al., Mineralogy 4.4.6: Crystal Twinning

Carlsbad (simple) twinning

Two domains (two halves) related by a rotation about a near-vertical axis, so one half is dark while the other is bright under crossed polars; common in orthoclase and sanidine.

See orthoclase

Source: Perkins et al., Mineralogy 4.4.6: Crystal Twinning

Tartan (cross-hatched) twinning

A crosshatch pattern under cross-polarized light: the identifying feature of microcline, which otherwise has low relief and is colorless.

**Tartan twinning in microcline.** Wikimedia Commons (author and license on the file page). Source

See microcline

Source: Athabasca University: Microcline (thin section)

Undulose extinction

The grain does not go dark all at once; extinction sweeps across it as you rotate the stage, because plastic deformation by dislocations (without recovery) bent the crystal lattice. Common in deformed quartz.

**Quartz going to extinction as the stage rotates.** Wikimedia Commons (author and license on the file page). Source

See quartz

Source: Wikipedia: Undulose extinction

Pleochroism

A change in color or darkness as you rotate the stage in plane-polarized light; strong in biotite, hornblende and tourmaline.

**Biotite and muscovite in thin section.** Wikimedia Commons (author and license on the file page). Source

See biotite

Interference colors

Under crossed polars, anisotropic minerals show colors set by their birefringence and the section thickness (retardation = birefringence × thickness), read on the Michel-Lévy chart. Olivine shows bright 2nd–3rd order colors; quartz and feldspar stay in 1st-order grays.

**Olivine in basalt, thin section.** Wikimedia Commons (author and license on the file page). Source

See olivine

Photos are Wikimedia Commons files. Click one for its author and license.

Michel-Lévy chart All minerals