Optical mineralogy Flashcards
What is the refractive index of a mineral?
n=1/V, where V is the velocity of light passing through a mineral. Minerals refract [or bend] light, so a mineral that has a high RI value will slow light considerably, as the RI is inversely proportional to the velocity.
What are the two groups transparent minerals can be divided into?
Anisotropic and isotropic
What is the definition of an isotropic mineral?
No double refraction occurs and only one ray of light passes through the mineral. The ray [the ordinary ray] has a velocity that will be the same in all directions throughout the crystal. An example is cubic minerals.
What is the definition of an anisotropic mineral?
Light will be split and doubly refracted, so two rays are produced. These may travel at different speeds in the crystal, depending on their direction of travel.
What are three key observations that can be made of minerals in plane polarised light?
Colour, pleochroism, relief
What is pleochroism? What is a mineral that clearly exhibits this feature?
The ability of minerals to change colour as the stage is rotated. At 90 degrees from the original position, the mineral will change colour. At 180 degrees from the original position, the mineral will return to its original colour.
Biotite shows this property strongly.
How can we distinguish between low and high relief minerals?
High relief minerals will have black edges, appearing as if they are popping out of the rock.
Differences in the speed of light in different minerals cause the refraction of light Rays which leads to focusing/defocusing of grain edges. This is subjective [compares to surrounding minerals].
What is the uniaxial indicatrix?
This is used to represent the optical properties of a mineral. It is constructed using refractive indices of minerals as the principle axes. For uniaxial minerals, the RIs are the extraordinary and ordinary rays [e ray is parallel to the c axis].
It is ellipsoid in shape, like a rugby ball.
What is double refraction?
Where a mineral causes the light passing through it to split into two rays
What axis are most minerals elongated along?
The c crystallographic axis.
What is the difference between positive and negative elongation and optic sign?
Positive - the slow ray is parallel to the elongation direction, so is length slow. It is optically positive and has positive elongation.
Negative - the fast ray is parallel to the elongation direction, so it is length fast. It is optically negative and has negative elongation.
What are accessory plates? Name three common ones.
These are separate plates which can be inserted into the microscope to change the optic configuration. By convention, the slow and fast rays are perpendicular.
Three examples include sensitive tint or gypsum plates, quartz wedges and mica plates.
How does interference occur in minerals?
During travel through the crystal, two rays are formed with one ray slowed relative to the other. Rays travel out of the crystal and a phase difference has been created. When they pass through the analyser the light rays are reresolved into one plane so interference will occur.
What is the difference between addition and compensation?
If the interference colours of the mineral move to higher order colours [with the insertion of an accessory plate] this is addition; the slow ray of the mineral is parallel to the slow of the plate.
If the colours move to lower order colours, this is compensation; the fast ray of the mineral is parallel to the slow of the plate.
