Earth Materials Lecture 5: Introduction to Petrography Flashcards
Define Petrography
Laboratory based technique for studying
rocks and minerals in thin section.
How is Petrography conducted?
By use of tools:
− Petrological (polarizing) microscope – for making
routine observations using transmitted light that is
polarized = plane polarized light (PPL).
− Rocks cut into thin section = 30 μm thick.
− Upper analyser in place = cross-polarized light (XPL)
What is the purpose of petrography/main uses
Allows us to observe rocks and minerals in a level of detail not available in hand specimens. Features and colours of minerals are made more obvious under microscope.
Used in rock classification.
Describe lights role and nature in petrography
Light is a transverse waveform
Light leaving a source vibrates in all directions.
Minerals can polarize light.
How do electrons affect the behavior of light in minerals
Electric component of light is affected by electron
clouds surrounding the atoms that comprise the crystal
lattice.
Density and distribution of clouds affects the behaviour
of light in that mineral.
In these crystal lattices, light can vibrate better in some directions than others. (Permitted ray vibration directions).
Define Refraction
Incident light passing from one
substance to another is bent at the interface if the two substances are made from materials with different
properties.
Incidence angle
Refractive angle
Describe and explain Snells law (1621)
Ratio of incidence angle to Refractive angle
/
Ratio of Sine i to Sine R
This ratio is the RI (Refractive Indices)
Define an isotropic mineral
Isotropic minerals are minerals where their refractive properties do not vary with direction. Light is only reflected in one way.
Isotropic minerals lack interference colours so appear black on XPL, this is permanent extinction.
Define an anisotropic mineral
An Anisotropic mineral are minerals where their refractive properties vary with direction.
In isotropic mienrals there is double refraction. This is where one light ray splits into two once refracted;
One unchanged ordinary ray
One ray which is changed once refracted
What and how do you calculate the Birefringence
Birefringence is the is the difference between the highest and lowest Refractive indices of the multiple Refracted rays of light in a anisotropic mineral.
Max RI - Min RI = Birefringence
Define a Uniaxial mineral
A Uniaxial mineral is a mineral which is anisotropic ( a mineral with multiple refractive properties depending on which it is viewed ) however when it is viewed through in thin section, if the C axis (Height/Up or down) is parallel to the axis of the microscope, only one refractive property can be shown/all light appears to be refracted the same way/have the same RI.
Define extinction
Extinction in minerals is how the light/colours change in perception when the mineral is rotated. Permanent extinction is when the mineral appears black.
Isotropic minerals are always in permanent extinction as it only has one refractive property.
Define Pleochrosim
Pleochroism is when a mineral’s colour changes when rotated in PPL (Plain Polarised Light). This is only present Anisotropic minerals.
Define Anhedral
Crystals with no crystal faces, crystals are underdeveloped
Define Subhedral
Crystal faces with some faces, crystal partially developed