guide to thin section microscopy - Mineralogical Society of America

Guide to Thin Section Microscopy
Extinction
Optical character of the extinction directions
If a birefringent crystal section is in extinction position, the vibration directions Z' and X‘ are
parallel to the N-S and E-W directions of the crosshairs (cf. Fig. 4-23). For a variety of
applications it may be important to know which direction corresponds to the higher,
respectively lower, refractive index:
• Determination of mineral colour in a specific vibration direction (cf. Ch. 4.2.1).
• Determination of optical sign of optically uniaxial minerals which are elongate in c
direction or platy having the basal plane ({001}, {0001}) as the dominant crystal face.
• Determination of sign of elongation (l) of acicular to columnar, platy or flaky minerals in
elongate crystal sections.
For a distinction between vibration directions Z' and X‘, compensator plates are used. These
are anisotropic crystal plates of constant or variable retardation with known orientation of the
X and Z wave vibration directions (α = n x and γ = n z ). The vibration direction of the Z wave
(γ = n z ) is engraved in the metal casing of the compensators. Commonly used compensators
(Fig. 4-42):
The first-order red plate (lambda plate, λ plate, sensitive tint plate) consists of a quartz or
gypsum plate that is cut parallel to the optic axis, about 62 µm thick, which shows a firstorder
red interference colour in diagonal position (Γ = 551 nm).
The lambda/4 (lambda quarter) plate consists of a thin quartz or mica plate which shows a
grey interference colour in diagonal position (Γ = 130-150 nm, depending on manufacturer).
The quartz wedge consists of a wedge-shaped quartz plate with the long axis parallel to the
optic axis. In diagonal position, this quartz wedge shows interference colours ranging across
four orders (Γ = 0 to 2200 nm).
Most commonly, the compensators are inserted diagonally into the microscope tube below the
analyzer. The wave corresponding to n z (γ or Z’) vibrates in NE-SW direction, the wave
corresponding to n x (α or X’) in NW-SE direction.
Raith, Raase & Reinhardt – February 2012
Figure 4-42. Compensators– first-order red plate (λ plate), lambda/4 plate and quartz wedge
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