Chapter 10 - A

Identification of minerals with the petrographic

microscope

Content

Sample preparation Microscope alignment Determination of the refractive

index Use of interference colors Conoscopic observation of

interference figures

Microscopy Transmitted light microscopy

Transparent crystals Light transmits through mineral grains Common rock-forming minerals

Reflected light microscopy Opaque crystals Light reflects from highly polished surface Usually ore minerals

This course: transmitted light microscopy

Sample preparation:Transmitted light

microscopy Grain mount:

Finely ground fragments; immersed in oil and scattered on glass plate; covered by thin sheet of glass

Thin section: Cut slab from rock sample –

area of interest Bottom - polished and

cemented onto glass slide Top - ground to desired

thickness; covered with balsam and thin cover glass

Rock-forming minerals now transparent

Why use a cover glass?

Microscope alignment Important in order to:

have light going through the center of all lenses, of the stage, the condenser

get two polarizers filtering light at vibration directions perpendicular to each other

Oculars – one or both adjusted for each eye; cross-hair in focus

Stage – center exactly in the optic axis; object not to move during stage rotation

Condenser – when switched on light beam should be centered around cross-hair

Polarizer – one set at 0º and one at 90º

Other settings Brightness of light – comfortable for your eyes –

very bright will give headaches and burns out filaments

Iris – determines the diameter of the light beam coming from the source – different setting for different magnifications

Condenser lens – use to get high resolution at high magnification

Focusing – to avoid collision: first bring sample close to objective lens (not against) and increase distance until sample in focus

Determination of the refractive index

Grain mount Edges of crystal – act as small

prisms which concentrate light as a ring of light – the Becke line

When increasing the distance from sample to objective (defocusing), the Becke line is always refracted in the direction of a medium of higher RI

In practice: Change liquids until two adjacent

liquids defines the range for the index of the mineral

Determination of refractive Index

Birefringence (δ) When a ray of light is split into two

separate polarized rays – each with a single vibration direction perpendicular to that of the other ray

True maximum birefringence value (δ) of mineral Isotropic: δ = n – n = 0 Uniaxial: δ = nε – nω

Biaxial: δ = nγ – nα

Under the microscope: Observed under crossed polarized light as:

Interference colors Only in anisotropic minerals

Birefringence/double refraction

Doubly refracted waves are polarized but separate, vibrating in different planes – no interaction

Need interference – study interference colours and properties To get interference – a second polarizer

inserted – the analyzer: Crossed polarizer/upper polarizer/crossed

nichols Used to analyze the interference effects of light

in minerals

Interference colours

First order colors Second order colors Third order colors

Birefringence A characteristic that all anisotropic minerals

have, intensity differs High birefringent minerals – third/fourth order

interference colours Med birefringent minerals – second order

interference colours Low birefringent minerals - first order interference

colours

For specific mineral birefringence depends on orientation: Maximum birefringence - orientation of grain

shows highest possible interference colour for the specific mineral

Minimum or no birefringence – orientation of grain shows lowest or no interference colour for specific mineral

Intermediate birefringence – orientation of grains shows interference colours intermediate between minimum and maximum

Interference colours

Determine order of colour and so value for birefringence – interference color chart

Use of interference colorsTrue birefringence

In sample: crystals in random orientations each grain different interference colors, each with corresponding birefringence

Minimum birefringence Circular section (perpendicular to optical axis) give lowest

order or no interference colours – refractive indices on both axes equal or almost equal

Also referred to as the isotropic section

True birefringence Longest elliptical section (parallel to optical axis) give highest

order colors Refractive index on major axis = largest; on minor axis =

smallest

THUS: to determine the true birefringence of mineral – choose grain with highest interference colors and read of the value of birefringence from the color chart

Use of interference colors:Accessory plates (compensators)

Accessory plate is a crystal with known birefringence and orientation

Determine unknown mineral optical orientation by comparing with known crystal plate orientation

Crystal orientation in plate parallel with mineral orientation Plate colors interfere constructively with colors of

mineral Addition – Positive (Red plate + color of mineral = blue)

Crystal orientation in plate perpendicular with mineral orientation Plate colors interfere destructively with colors of

mineral Subtraction – Negative (Red plate - color of mineral =

yellow)

Use of interference colors:Accessory plates (compensators)POSITIVE NEGATIVE

Use of interference colors:

Extinction As an anisotropic crystal is rotated a

full turn under crossed polarized light, it goes into extinction 4 times I.e. – at every 90° rotation the mineral

goes dark

This happens every time the two perpendicular vibrating directions falls parallel with the two polarizer directions

Use of interference colors:Extinction angle

When optical axis vertical (circular section) – mineral dark during rotation

When inclined – mineral go dark once every 90º

Angle of extinction can be measured for elongated minerals or minerals with strong cleavage Parallel extinction Inclined extinction Symmetrical extinction No extinction angle

Observation of interference figures using convergent light

– conoscopic view Insert condenser

lens Gives convergent

light Enters sample at

50º - 90º angles See image of light

source

Interference effects atdifferent angles

Conoscopic observation of interference figures

Isotropic No image

Conoscopic observation of interference figures

Uniaxial Perpendicular to

optical axis

Conoscopic observation of interference figures

Uniaxial At an angle to the

optical axis

Conoscopic observation of interference figures

Uniaxial Parallel to the

optical axis