Extinction Angle
Definition
The angle, measured in degrees on the rotating stage of a petrographic microscope, between a crystallographic reference direction (cleavage trace or crystal edge) and the position of extinction between crossed polars. Diagnostic for distinguishing mineral families, particularly pyroxenes from amphiboles.
- Measured on
- Petrographic microscope rotating stage, in degrees
- Reference from
- Crystallographic direction such as cleavage trace or crystal edge
- Common use case
- Distinguishing pyroxenes from amphiboles in mineral identification
Common questions
What is extinction angle and how is it measured?+
Extinction angle is the angle at which an anisotropic mineral appears darkest (extinguishes) when viewed between crossed polarizers on a rotating microscope stage. It is measured in degrees from a crystallographic reference direction such as a cleavage trace or crystal edge. This angle is diagnostic for mineral and crystal identification.
Why is extinction angle useful in forensic analysis?+
Extinction angle is diagnostic for telling apart different mineral families, especially pyroxenes from amphiboles. In trace evidence and material analysis, being able to identify minerals precisely helps forensic scientists classify geological evidence and compare samples.
What does it mean when a specimen appears dark between crossed polars?+
When light passes through an anisotropic mineral between crossed polarizers, the mineral transmits light in most rotational positions. When the mineral is rotated to its extinction position, its vibration directions align with those of the polarizer and analyzer, so almost no light passes through and the mineral appears dark.
Related terms
- Birefringence
- The difference between the highest and lowest refractive indices of an anisotropic mineral, seen as a characteristic interference colour between crossed polars....
- Autofluorescence
- Native fluorescence of endogenous biological molecules (collagen, elastin, NADH, flavins) that produces background signal in fluorescence microscopy, potentially mimicking specific probe signals....
- Conoscopic Illumination
- A convergent polarised beam mode in which a range of angles is passed through the specimen simultaneously, producing an interference figure (optical...
- Crossed Nicols
- The configuration in which the polariser (below the condenser) and the analyser (above the objective) are oriented at 90 degrees to each...
- DAPI
- 4',6-diamidino-2-phenylindole; a fluorescent dye that binds double-stranded DNA and emits blue (~460 nm) under UV excitation (~360 nm). Standard nuclear counterstain; used...
- Dichroic Mirror
- A spectrally selective beam splitter that reflects shorter (excitation) wavelengths at 45 degrees and transmits longer (emission) wavelengths, forming the core element...
- Epi-Fluorescence Illumination
- Fluorescence excitation through the objective lens from above the specimen, with the same objective collecting the emission. A dichroic mirror directs excitation...
- Grain Mount
- A microscope slide prepared by dispersing sand-grade grains in a mounting medium of known refractive index. Standard forensic grain mounts use Meltmount...
- Michel-Lévy Chart
- A birefringence-thickness diagram correlating retardation values with the interference colour seen at that retardation under white-light crossed polars. Standard reference for identifying...
- Photobleaching
- The irreversible photochemical destruction of a fluorophore by repeated excitation. Limits the observation time for low-copy forensic fluorescence samples. Mitigated by antifade...
- Pleochroism
- The property of some minerals (hornblende, tourmaline, biotite) to show different colours when the plane of polarised light rotates relative to the...
- QEMSCAN / MLA
- Automated mineralogy systems (QEMSCAN is from FEI/Thermo Fisher; MLA is the Mineral Liberation Analyser) that combine a scanning electron microscope with automated...
Explained in these topics
- Optical Microscopy and Mineral IdentificationThe angle, measured in degrees on the rotating stage of a petrographic microscope, between a crystallographic reference direction (cleavage trace or crystal ed...
- Polarising and Fluorescence MicroscopesThe angular rotation of the stage at which a specific anisotropic specimen appears darkest (extinguishes) between crossed polars. Measured from a crystallograp...