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Atibrarian (talk | contribs) Adding short description: "A pattern of birefringence colors" |
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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 an 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 [[Microscope slide|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 [[Extinction (optical mineralogy)|optical extinction]] (apparent isotropy) would be seen. In other words, the interference figure presents all possible birefringence colours for the mineral at once.
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== 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 [[Maltese cross|"Maltese" cross]] shape to its isogyres. If you are looking perfectly down the optic axis, the pattern will remain completely unchanging as the stage is rotated. However, if the viewing angle is slightly away from the optic axis, the centre of the cross will revolve/orbit around the central point as the stage is rotated. The form of the cross will stay constant as it moves.
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The maximum separation between isogyres occurs when the slide is rotated exactly 45 degrees from one of the orientations where the isogyres come together. The point where the isogyres is most tightly curved represents the position of each of the two optic axes present for a biaxial mineral, and thus the maximum separation between the two curves is diagnostic of the angle between the two optic axes for the mineral. This angle is called the '''optic angle''' and often notated as '''"2V"'''. In some cases, knowing the optic angle can be a useful diagnostic tool to discriminate between two minerals which otherwise look very similar. In other cases, 2V varies with chemical composition in a known way for a given mineral, and its measured value can be used to estimate ratios between elements in the [[crystal structure]] – for example, Fe/Mg in [[olivine]]s. However, in these cases it becomes important to also be sure of the ''optic sign'' of the mineral (essentially, this tells you how the optic angle is orientated with respect to the whole [[optical indicatrix]] describing the refractive indices of the mineral in 3D). The optic sign and optic angle can be determined together by combining interference pattern microscopy with use of a [[sensitive tint plate]].
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.
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