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{{Short description|A pattern of birefringence colors}}
A '''conoscopic interference pattern''' or '''interference figure''' is a pattern of [[Birefringence|birefringent]] colours crossed by dark bands (or ''isogyres''), which can be produced using a [[Geology|geological]] [[petrographic microscope]] for the purposes of [[mineral]] identification and investigation of [[Optical mineralogy|mineral optical and chemical properties]]. The figures are produced by [[optical interference]] when diverging light rays travel through
▲A '''conoscopic interference pattern''' or '''interference figure''' is a pattern of [[Birefringence|birefringent]] colours crossed by dark bands (or ''isogyres''), which can be produced using a [[Geology|geological]] [[petrographic microscope]] for the purposes of mineral identification and investigation of [[Optical mineralogy|mineral optical and chemical properties]]. The figures are produced by optical interference when diverging light rays travel through a optically non-isotropic substance - that is, one in which the substance's [[refractive index]] varies in different directions within it. The figure can be thought of as a "map" of how the birefringence of a mineral would vary with viewing angle away from perpendicular to the slide, where the central colour is the birefringence seen looking straight down, and the colours further from the centre equivalent to viewing the mineral at ever increasing angles from perpendicular. The dark bands correspond to positions where optical extinction (apparent isotropy) would be seen. In other words, the interference figure presents all possible birefringence colours for the mineral at once.
Viewing the interference figure is a foolproof way to determine if a [[mineral]] is optically uniaxial or biaxial. If the figure is aligned correctly, use of a [[sensitive tint plate]] in conjunction with the microscope allows the user to determine mineral ''optic sign'' and ''optic angle''.
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To view the figure, the light rays leaving the microscope must emerge more or less in parallel. This is typically achieved either by pulling out the eyepiece altogether (if possible), or by placing a [[Bertrand lens]] (Emile Bertrand, 1878) between the objective lens and the eyepiece.
Any crystal section can in principle produce an interference pattern. However, in practice, only a few different crystallographic orientations are both 1. convenient to identify, to allow a figure to be produced, and 2. able to produce reliable information about crystal properties. Typically, the most useful and easily obtainable orientation is one looking down the [[Optic axis of a crystal|optic axis]] of a crystal section, which yields a figure referred to as an '''optic axis figure''' (see below). Such crystal orientations are findable in thin section by looking for slices through minerals which are not isotropic but that nevertheless appear uniformly black or very dark grey under normal cross-polarised light at all stage angles (i.e., are "[[Extinction (optical mineralogy)|extinct]]"). If you are far from looking down an optic axis, a ''flash figure'' may be seen
== Characteristic figures of uniaxial and biaxial minerals ==
[[File:Uniaxial interference figures.jpg|thumb|center|upright=2.5|Sketches of uniaxial interference figures, viewed along the optic axis of each mineral. The colours approximate [[birefringence]] colours which might be seen if this were a mineral with second order maximum birefringence. The dark "maltese cross" pattern is characteristic of uniaxial minerals. Also shown are schematics of the shape of a cross section through the mineral's optical [[index ellipsoid|indicatrix]] (recording its refractive index in 3D) that would be seen at each position. The elongated direction could be distinguished by adding a [[sensitive tint plate]] to the microscope, letting the user discriminate between "uniaxial positive" (left) and "uniaxial negative" (right) minerals.]]
An interference figure produced looking straight down or close to the optic axis of a uniaxial mineral will show a characteristic
[[File:Biaxial interference figures.jpg|thumb|center|upright=2.5|Possible interference figures for a biaxial mineral with a large 2V, viewed along one of its two optic axes. The curved shape of the isogyre is characteristic of biaxial minerals
The optic axis figure of a biaxial mineral is more complex. One or two curved isogyres (sometimes called "brushes") will be visible, one of which will have its point of maximum curvature perfectly centred. (The figure shows an example with a single isogyre visible.) If two isogyres are visible, they will be positioned back-to-back. Rotating the stage will cause the isogyres to move and change shape strikingly
The maximum separation between isogyres occurs when the slide is rotated exactly 45 degrees from one of the
On either side of the "saddle" formed by the isogyres, birefringent rings of colour run concentrically around two eye like shapes called ''
A [[Interference colour chart|Michel-Levy Chart]] is often used in conjunction with the interference pattern to determine useful information that aids in the identification of minerals.
==See also==
* [[Wave interference]]
==References==
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{{DEFAULTSORT:Conoscopic Interference Pattern}}
[[Category:Optical mineralogy]]
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