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Reflected Light
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List of definitions
The 2V angle refers to the angle between the two optic axes in biaxial minerals. This angle is zero for uniaxial minerals since there is only one optic axis.
This is determined by looking at an interference figure.
If it's an interference figure in the cut perpendicular to the optic axis 2V, the 2V angle is determined according to the curve of the isogyre.
If it's a interference figure in the cut perpendicular to the acute bisectrix, the 2V angle is determined according to the distance between the bands of the isogyres.
Igneous rocks with a high proportion (4-16 wt%) of Potassium and Sodium (hence Alkali-) and deficient in Silica and Alumina in relation to other rocks. Thus leading to them containing Potassium and Sodium-rich minerals, such as of nepheline or sodalite.
Variation in colour or intensity of a mineral depending on orientation, viewed under crossed polarized light. Make sure to increase your light intensity if you struggle to see this phenomenon.
Minerals which are pleochroic or bireflectant are generally also anisotropic. Isotropic minerals and the basal sections of hexagonal, tetragonal, and trigonal minerals do not tend to show any anisotrophism.
An anisotropic mineral is one where light travels through it at different speeds, depending on the mineral's orientation vis-à-vis the light ray incidence direction. This is related to the intrinsic structure of the mineral. Most observed minerals are anisotropic.
The opposite of isotropic minerals.
Anomalous birefringence means the birefringence changes dramatically within a mineral, which produces anomalous interference colours.
A texture in a rock where a larger, rounded mineral or a cluster of minerals is surrounded by finer grained material stretched out in one axis. This makes the overall structure look like an eye, hence the name.
Biaxial crystals have 2 optic axes as opposed to uniaxial crystals which only have one.
All minerals that crystallize in the orthorhombic, monoclinic, or triclinic crystal systems are biaxial.
Specific to minerals observed under reflected light.
The change in brightness of the ore mineral as you rotate the stage in PPL. This is due to the change in the amount of light reflected from the mineral sample as the stage is rotated.
Think of the visual effect being similar to that of pleochroism, but for reflected light.
In practical terms, when people talk about low or high birefringence for minerals in thin section, they refer to the interference colours that you observe. ie. the colour or colour intensity changing as you rotate the stage in crossed polars.
The key colours to note are: The first order colours range from black to reddish violet. Blue only appears in the 2nd and 3rd order colours and higher orders have mainly pink and green shades and become more pastel/pale-bright (ie. not vivid colours).
These colours are plotted on that rainbow looking birefringence interference chart, also known as the Michel-Levy Birefringence chart. With an observed colour and knowing the thickness of the thin section, we can figure out the birefringence.
The interference colours can be split into orders and the numbers on the x-axis refer to the amount of retardation; the greater the order, the greater the retardation is.
Birefringence, also known as double refraction, is the optical property of the mineral having a refractive index that depends on the polarization and propagation direction of light. When a ray of unpolarised light enters a mineral, it tends to split into two rays, hence the name double refraction. The two rays are the ordinary ray, which continues straight, and the extraordinary ray, which is refracted at an angle.
The refractive index of all anisotropic minerals varies continuously (between a maximum value and a minimum value) depending on the vibration direction of the light within the crystal. Birefringence is the maximum possible difference in refractive indices of the mineral.
Mineral texture where the mineral looks like a fused together bunch of grapes.
Simple twinning, where the mineral only has 2 twins. As opposed to multiple twins, such as cross-hatched or the multiple polysynthetic/albite twinning common in Plagioclase.
From a crystal structure point of view, each twin is a 180° rotation of the other.
The result from the innate propensity of minerals to fracture in a regular and parallel manner (this forms cleavage planes). This fracturing is often due to the underlying crystal structure of the mineral. For example, sheet silicates will often have thin plates in one cleavage direction.
Under the microscope, this looks like the mineral is fractured by straight, parallel cracks.
A mineral can have multiple sets of cleavages in different directions. In which case, the angles between the cleavages are characteristic of the mineral. Eg. Amphiboles are known for having cleavages at 120°.
Rock texture which shows layers, resulting from minerals growing in a radiating and concentric fashion. Not to be confused with zoning, which is only within a mineral.
Two sets of twins at different angles, resulting in a grid like effect, akin to plaid. Also called Tartan twinning.
Crystals can be classed into seven groups (systems) depending on their symmetry and the atomic-level dimensions of the various axes of the unit cell (Bravais lattices). The seven systems are: cubic (aka. Isometric), monoclinic, triclinic, orthorhombic, trigonal (aka. rhombohedral), tetragonal and hexagonal.
Used to describe crystals with sharp edges and well formed faces. Opposite of anhedral.
Exsolution is a process when a mineral "unmixes" and reveals the separate compositional phases within the same mineral. Exsolution occurs only in minerals whose compositions vary between two or more pure endmember compositions.
Under a microscope, the (generally) two separate phases have different optical properties and thus the difference is clearly visible. Exsolution lamellae occur as oriented intergrowths that display parallel and herringbone textures, a bit like artistic representation of flames. These lamellae are often planar and their orientation is crystallographically controlled.
The point at which the mineral goes black when you turn the stage in crossed polars. Depending on the mineral, this point can be an instantaneous change to black and the mineral will stay black for a few degrees of stage rotation. Alternatively, it can fade into black over a few degrees of stage rotation. The extinction can be straight (also called parallel), inclined or undulose.
If the crystal goes into extinction when the long axis of the crystal or the cleavage planes are parallel to the North-South or East-West cross-hairs; then the extinction is straight.
Granulites are a high grade metamorphic rock, of the granulite facies, that has been subject to high temperatures and moderate pressures.
Series of minerals with broadly similar crystal structures and chemical composition. There are main groups (ie. silicates, native elements, oxides etc.) and subgroups (ie. Amphibole group in silicate groups). NB. There is no hard and fast rule governing these groups, it's merely indicative.
Eg. Most of the groups in thin section minerals are subgroups of the silicate main group.
The shape of the mineral. Minerals have a tendency to appear in certain geometric shapes, which correspond to simple or combined crystallographic shapes.
This macroscopic tendency is the result of the atomic-level structure of the crystals.
Mineral deposit processes that are deep within the earth (around the crust) and from hydrothermal fluxes coming from below.
Generally used for ore deposits. The opposite of supergene.
When extinction is not straight. If extinction occurs when the mineral's cleavage fracture line is oriented parallel to the cross hairs, the extinction is straight. If not, the extinction is inclined.
The extinction angle, or angle of extinction, refers to the rotational angle between when the mineral goes extinct and the nearest straight feature such as the edge of the mineral or a cleavage plane. ie. How many degrees do you have to spin the stage to get the mineral from looking extinct to it being aligned with a straight edge or cleavage plane. This Youtube video explains it in detail.
Formally known as the conoscopic interference pattern, the interference figure is the colourful pattern separated by isogyres (black areas) you get when you look at your sample through the condenser, a high magnification, Bertrand lens and a 550nm quartz compensator. The best patterns are the ones when looking directly down the crystal's optic axis.
For uniaxial minerals, the isogyres appear at right angles to each-other and are straight (like a cross Pattée or Iron cross) and do not change as the stage is rotated.
For biaxial minerals, one or two curved isogyres appear and change drastically as you rotate the stage.
The interference figure is used to work out the 2V angle, whether the mineral is uni- or biaxial, the optic sign and the orientation of the cut of the mineral during preparation.
Specific to minerals observed under reflected light.
The glittery, faint space nebula -esque colours you see within a mineral under crossed polars.
Often seen in the more transparent minerals under reflected light; this is due to light entering the mineral, bouncing around inside and interacting with cracks and cleavage planes.
An isotropic mineral is one where light travels through it at the same uniform speeds, regardless of the mineral's orientation vis-à-vis the light ray incidence direction. This is related to the intrinsic structure of the mineral.
Amorphous minerals (such as glass) and minerals with a cubic crystal system are isotropic and they appear black under transmitted light thin sections in crossed polars.
The opposite of anisotropic minerals.
Igneous rock from deep within the earth’s mantle, often brought up by volcanic processes. They are known for containing diamonds.
Some crystals tend to grow in elongated habits and they can be elongated parallel to their slow vibration direction or parallel to their fast vibration direction.
Crystals elongated parallel to the slow direction have positive sign of elongation or are said to be length slow.
Crystals that are elongated parallel to their fast direction have a negative sign of elongation or are said to be length fast.
In brief, to determine the sign of elongation, place the crystal so that it is in a position where the long direction of the crystal is parallel to the slow direction in the 550nm compensator. Pop in the compensator, find an area with gray interference colours and cross the polars. If that area shows a 2nd order blue, then the slow direction of the crystal is aligned with that of the compensator. It is length slow.
If the gray turns yellow, the fast direction in crystal is aligned with the slow direction of the compensator. It is length fast.
Metamorphosed mafic rock (rich in Mg and Fe; dominated by pyroxene, amphibole, olivine, mica; generally dark rock) that has lost all traces of its original texture and mineralogy owing to complete recrystallization.
Intergrowths that look like one mineral has bunch of ''worms'' of another mineral inside it. Often due to hydrothermal alteration.
Indicates the type of double refraction/birefringence in a mineral.
In a practical sense, the optic sign is positive when the interference figure with a 550nm compensator has a blue coloured top right quadrant. It is negative when the top right is yellow.
For uniaxial minerals, if the extraordinary ray has a higher refractive index than the ordinary ray, the optic sign is positive. If the ordinary ray has the greater index, the optic sign is negative.
For biaxial minerals, in the optic indicatrix, if the refractive index of the intermediate/beta ray is nearer that of the low or alpha ray, the optic sign is positive. If it is nearer the high or gamma ray, the optic sign is negative.
The optic axis is the direction in the crystal where the rays of transmitted light suffer no birefringence. ie the direction where the refractive index is the same and the light doesn’t split. An optical axis is a direction rather than a single line: all rays that are parallel to that direction exhibit the same lack of birefringence.
Light travelling parallel to the optic axis makes the crystal behave isotropically.
The same thing as exsolution lamellae, but the specific term applied to the intergrowth of Potassium and Sodium rich feldspars. Typically, the host grain is orthoclase or microcline, and the lamellae are albite.
Pleochroism is the ability of a mineral to absorb different wavelengths of transmitted light depending upon its crystallographic orientations in relation to light going through it.
Under the microscope in PPL, this is seen as the mineral changing colour and/or colour intensity as the stage is rotated.
Minerals that have the same chemical formula, but different crystal structures are called polymorphs.
Polymorphs are usually the result of differing formation conditions, such as varying temperatures and pressures. Different crystal structures will be more stable at different specific conditions.
Intergrowths that are visually similar to exsolution lamellae, resulting from several (two or more) minerals co-precipitating from melt.
A mineral having the shape / habit of another, usually as a result of the original mineral being dissolved and replaced with another mineral that fills the same shape and outline.
Specific to minerals observed under reflected light.
The reflectivity is the ratio of the amount of light reaching the mineral over how much bounces back up to the observer. ie. if reflectivity is 50%, then half of the light shone on the mineral gets reflected back into your eyes.
The higher the reflectivity, the brighter the mineral looks. The lower the reflectivity, the dimmer/duller it looks.
Refractive index is a number that tells you how fast light travels through that medium. Higher refractive indices also have greater dispersion.
A difference in refractive index between minerals means a change in the speed of the light travelling through it, ie. refraction. Think of that classic example of the "broken" straw in a glass of water (Snell’s law).
A large difference in refractive index between two minerals will mean a greater difference between the angle of refraction and that of incidence. In practical terms, this translates to a sharper boundary. This is known as relief.
Relief is an indicator of how well a material and its outline can be seen and distinguished from the surroundings and background. The amount of relief is proportional to the absolute value difference between the refractive indices of the mineral and its surrounding.
In practical terms, the greater the difference between the refractive indices, the wider and darker the mineral outlines. And thus the higher the relief.
Note that relief is a consequence of the difference between the refractive indices of the two mediums in contact. When there is only slight relief, the difference is small even when both minerals have large refractive indices. When the relief is sharp, there exists a significant difference between the indices of the mineral and the medium.
Ore deposits that are near the surface of the Earth and are formed from surface interaction mechanisms (as opposed to deep crustal/mantle processes). Eg. Ore deposits due to meteoric water circulation and chemical weathering near the surface.
The opposite of hypogene.
This is a texture for irregular, fine-grained (<micrometer scale) mineral intergrowths that form as a result of a reaction that did not go to completion. It is when one or more phases within a mineral, or adjacent minerals, becomes unstable and recrystallises into more stable constituents.
These inter-growths are often recognized by their wormy appearance and often occur along the boundaries of reacting minerals (or not in equilibrium). Kind of like exsolution lamellae but with a more fine grained texture.
Symplectitic intergrowths are more commonly seen in high temperature rocks like rocks from the granulite facies where it is also a textural evidence of decompression.
Also known as wavy extinction, is when different parts of a mineral that would normally go extinct at the same time, do not. ie. When some parts of the minerals are not extinct when others are.
NB. The extinction or lack thereof is random and not systematic (not to be confused with twinning).
This is often evidence that the mineral has been subject to deformation, which has slightly altered and warped its crystal lattice, thus causing the inconsistent extinction pattern.
Crystal with one optic axis, as opposed to two in biaxial crystals.
When polarized light enters an anisotropic crystal from below, and neither of the privileged directions in the crystal are parallel to the polarizer, the light is broken up into two mutually perpendicular polarized waves that travel through the crystal.
One of these waves will be vibrating in the direction of high refractive index whereas the other will be vibrating in the direction of the lower refractive index. As refractive index is inversely proportional to the velocity of the wave, the wave vibrating in the direction of the larger refractive index will travel more slowly in the crystal than the wave vibrating in the direction of the lower refractive index.
Thus, the wave vibrating in the higher refractive index direction is known as the slow wave, and the wave vibrating in the lower refractive index direction as the fast wave.
A hardness test that involves loading and pressing a pyramid shaped diamond into the material we want to test the hardness of. The resulting hardness (in HV unit) is the area of the dent in the material left by the diamond divided by the amount of load applied to it.
Also known as Thomson structures, these textures of interwoven bands of two types of Fe and Ni alloys (Kamacite and Taenite). The gaps between the bands are filled with a mixture of these two allows.
Typically found in Iron meteorites.
