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Optical Properties of Minerals

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Optical Microscopy

  • Study of how light passes through thin sections – rock cut and polished to about 0.3 mm thickness
  • Use properties of light absorption and propogation through a mineral à affected by atomic arrangement and composition
  • Learn the properties of light associated with techniques governing the use of a petrographic microscope

Why use the microscope??

  • Identify minerals (no guessing!)
  • Determine rock type
  • Determine crystallization sequence
  • Document deformation history
  • Observe frozen-in reactions
  • Constrain P-T history
  • Note weathering/alteration
  • Fun, powerful, and cheap!

Minerals and propogation of light

  • Opaque minerals – minerals in which light does not go through à always black even in thin sections.  Typically these have molecules with higher atomic density (which includes many ore minerals).  How light reflects off of these minerals is used to identify them with a reflected light microscope.
  • Nonopaque minerals – minerals in which light does go through à use these properties to identify them with the petrographic microscope

The petrographic microscope

  In order to use the scope, we need to understand a little about the physics of light, and then learn some tools and tricks…
In order to use the scope, we need to understand a little about the physics of light, and then learn some tools and tricks…

Thin section

Thin rectangular slice of rock that light can pass through.  One side is polished smooth and then stuck to a glass slide with epoxy resin  The other side is ground to 0.03 mm thickness, and then polished smooth.  May be covered with a thin glass cover slip

Thin section

Properties of Light

Light travels as an electromagnetic wave In a solid, liquid or gaseous medium the electromagnetic light waves interact with the electrons of the atom.

Properties of Light

Plane Polarized light (PPL)

In air, light normally vibrates in all possible directions perpendicular to the direction of travel (A). Plane Polarized Light vibrates in one plane (B). PPL is produced by substage polarizer which stops all other vibration directions

Crossed Polars

A second polarizer can be inserted above the stage, perpendicular to the substage polarizer. In air or an isotropic medium, it will stop light from first polarizer

Crossed Polars

Passage of Light

(1) Reflection from an external or internal surface. Angle of incidence (i) = angle of reflection (r)

Reflection

The velocity of light depends on the medium through which it passes.Light is an electromagnetic wave which interacts with electrons.The distribution of electrons are different for each material and sometimes for different directions through a material.When light passes from one medium to another there is a difference in velocity. Light rays apparently bend at the contact

Angle of incidence ≠ Angle of Refraction.

Passage of Light

Refractive Index

The amount of refraction is related to the difference in velocity of light in each medium.Refractive index (R.I.) for air is defined as 1

The absolute refractive index for a mineral (n) is the refraction relative to that in air.

  •   depends on the atomic/crystal structure
  •   is different for each mineral
  •   is constant for a mineral
  •   is a diagnostic property of the mineral
  •   between 1.3 and 2.0

There may be one, two or three values of R.I. depending on the atomic structure of the mineral.

Opaque Mineral

Sulphides and oxides minerals PPL does not pass through So Minerals looks black in PPL regardless of orientation of mineral or polarizers.

Mineral cannot be identified in transmitted light; needs reflected light

Opaque mineral in granite
Rotated 45o in PPL

Transparent mineral

PPL passes through the 30μm thickness of the thin section. The electromagnetic light waves interact with the electrons in the minerals and slow down.The higher the density of electrons the slower the light wave travels

CPX in gabbro
PPL

Becke Line

A white line of light between two minerals allows the Relative Refractive Index (R.R.I.) to be measured. This is relative to an adjacent medium which can be glass, epoxy, or another mineral

R.I. epoxy: 1.54 to 1.55 • •

The edge of the grain acts like a lens distorting the light
Perthite:
Microcline with exsolved albite
showing Becke Line between the two minerals
(PPL)

To measure relative refractive index of two touching minerals or mineral / epoxy

Use PPL (upper polarizer out)

  • Partly close the substage diaphragm, reducing light by 50-75%
  • Slightly raise and lower the microscope stage, observing the movement of the Becke Line at boundary of grain.
  • When decreasing the distance between the ocular and the stage, (raising the stage) the line moves into the material of lower R.I.
To measure relative refractive index of two touching minerals or mineral / epoxy

Relief

Apparent topographic relief of mineral grains caused by differences in R.I.

  • Positive relief – high R.I.
  • Negative relief – low R.I.
  • R.I. epoxy = 1.54 to 1.55 
Apatite

Cleavage

Parallel cracks in mineral related to crystal structure, often diagnostic of a mineral. In thin sections cleavage is developed during grinding of thin section. Note how many directions of cleavages are present. Measure the angle between cleavages or between cleavage and some mineral feature e.g. edge of grain, extinction.

Amphiboles
e.g. hornblende ~ 54o/126o
Pyroxene e.g. augite ~ 90o;

Fracture

Irregular cracks not related to atomic structure e.g. olivine

Olivine in gabbro (PPL)

Metamict Texture

Intense fracturing cause by radiation. Disruption of crystal lattice can decrease optical properties. The mineral may appear isotropic


Zircon and Allanite

Colour in PPL

Due to absorption of selective wavelengths of light by electrons e.g absorption of red gives a green colour. May be diagnostic of the mineral e.g. green chlorite. Beware: biotite and hornblende may be either brown or green

Isotropic Minerals

Isometric (cubic) minerals e.g. garnet, halite

  • Amorphous materials: glass, epoxy resin, air
  • Atomic structure is the same is all directions
  • Light travels through the mineral with equal velocity in all directions
  • Refractive Index: one value (n) regardless of orientation

Between crossed polars

Isotropic minerals always look black regardless of orientation of crystal or rotation of stage

Between crossed polars

Indicatrix

An imaginary figure which indicates the vibration directions and size of refractive index. The length of a semi-axis shows the size of R.I. in that direction through the mineral. For isotropic minerals, R.I. (n) and hence the length of the indicatrix semi-axes are the same for all directions through the mineral. Therefore, the indicatrix for isotropic minerals is a sphere with only one value of R.I. (n)

Isotropic Indicatrix

Isotropic Minerals

Colour in PPL may be diagnostic. Absorption of light is the same in all directions so the colour will be the same regardless of orientation of crystal and remains constant when stage is rotated. Cleavage: rare but fracture common. Always in extinction between crossed polars

Garnet minerel under the microscope

Anisotropic minerals:

Uniaxial – light entering in all but one special direction is resolved into 2 plane polarized components that vibrate perpendicular to one another and travel with different speeds

Biaxial – light entering in all but two special directions is resolved into 2 plane polarized components…

Along the special directions (“optic axes”), the mineral thinks that it is isotropic – i.e., no splitting occurs

Uniaxial and biaxial minerals can be further subdivided into optically positive and optically negative, depending on orientation of fast and slow rays relative to xtl axes

1) Light passes through the lower polarizer

Color & Pleochroism

  Color is observed only in PPL

  •   Not an inherent property – changes with light type/intensity
  •   Results from selective absorption of certain l of light
  •   Pleochroism results when different l are absorbed   differently by different crystallographic directions –  rotate stage to observe
-Plagioclase is colorless
-Hornblende is pleochroic

Index of Refraction (R.I. or n)

  • n is a function of crystallographic orientation in anisotropic minerals
    • isotropic minerals: characterized by one RI
    • uniaxial minerals: characterized by two RI
    • biaxial minerals: characterized by three RI
  • n gives rise to 2 easily measured parameters: relief & birefringence

Relief

  • Relief is a measure of the relative difference in n    between a mineral grain and its surroundings
  • Relief is determined visually, in PPL
  • Relief is used to estimate n


What causes relief?

Difference in speed of light (n) in different materials causes refraction of light rays, which can lead to focusing or defocusing of grain edges relative to their surroundings

2) Insert the upper polarizer

Insert the upper polarizer

3) Now insert a thin section of a rock

Now insert a thin section of a rock

Conclusion has to be that minerals somehow reorient the planes in which light is vibrating; some light passes through the upper polarizer

4) Note the rotating stage

Most mineral grains change color as the stage is rotated; these grains go black 4 times in 360° rotation – exactly every 90o

rotating stage

Some generalizations and vocabulary

All isometric minerals (e.g., garnet) are isotropic – they cannot reorient light.  Light does not get rotated or split; propagates with same velocity in all directions. These minerals are always black in crossed polars.All other minerals are anisotropic they are all capable of reorienting light (transmit light under cross polars).

All anisotropic minerals contain one or two special directions that do not reorient light.

  • Minerals with one special direction are called uniaxial
  • Minerals with two special directions are called biaxial

How light behaves depends on crystal structure

‘Splitting’ of light à what does it mean?

  • For some exceptionally clear minerals where we can see this is hand sample this is double refraction à calcite displays this
  • Light is split into 2 rays, one traveling at a different speed, and this difference is a function of thickness and orientation of the crystal à Norden Bombsight patented in 1941 utilized calcite in the lenses to gauge bomb delivery based on speed, altitude of plane vs target
  • ALL anisotropic minerals have this property, and we can ‘see’ that in thin sections with polarized light!

Difference between our 2 rays

  • Apparent birefringence – d – difference in refractive index (speed) between the 2 rays
  • Retardation – D à distance separating the 2 rays
  • Retardation therefore is a function of the apparent birefringence and the thickness of the crystal à ideally all thin sections are 0.3 mm, but mistakes do happen…

Polarized light going into the crystal splits ® into two rays, going at different velocities and therefore at different wavelengths (colors)

  •   one is O-ray with n = w
  •   other is E-ray with n = e

When the rays exit the crystal they recombine. When rays of different wavelength combine

Interference Colors
Michel-Lévy Color Chart – Plate 4.11

Estimating birefringence

1) Find the crystal of interest showing the highest colors (D depends on orientation)

2) Go to color chart

  • thickness = 30 microns
  • use 30 micron line + color, follow radial line through intersection to margin & read birefringence

Suppose you have a mineral with second-order green

Extinction

  • When you rotate the stage à extinction relative to the cleavage or principle direction of elongation is extinction angle
  • Parallel, inclined, symmetric extinction
  • Divided into 2 signs of elongation based on the use of an accessory plate made of gypsum or quartz (which has a retardation of 550 nm) which changes the color à for a grain at 45º from extinction look for yellow (fast) or blue (slow).

Twinning and Extinction Angle

  • Twinning is characteristic in thin section for several common minerals – especially feldspars
  • The twins will go from light to dark over some angle
  • This is characteristic of the composition
  • Stage of the petrographic microscope is graduated in degrees with a vernier scale to measure the angle of extinction precisely
Quartz and Microcline Birefringence
Olivine mineral under the PPl and XPL

Appearance of crystals in microscope

  • Crystal shape how well defined the crystal shape is
    • Euhedral – sharp edges, well- defined crystal shape
    • Anhedral – rounded edges, poorly defined shape
    • Subhedral – in between anhedral and euhedral
  • Cleavage – just as in hand samples!
  • Physical character – often note evidence of strain, breaking, etching on crystals – you will notice some crystals show those features better than others…
Cite this article as: Mahmut Mat, "Optical Properties of Minerals", in Geology Science, [online] Accessed 18th July 2019, Available at: http://geologyscience.com/general-geology/optical-properties-of-minerals/
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