DEPARTMENT OF APPLIED GEOLOGY - PowerPoint Presentation

Slide1

DEPARTMENT OF APPLIED GEOLOGY

A SEMINAR PRESENTETION

ON

Optical Properties of Minerals

Presented by –

Student’s name Slide2

Contents IntroductionPolarised

lightDifferent types of transmitted microscope studiesProperties under plane polarised lightProperties under cross nicol conditionBirefringence

RetardationIsotropic indicatrixBiaxial indicatrix Interference figuresSlide3

optical mineralogy– branch of mineralogy dealing with optical properties of minerals.Optical properties of minerals are important for their identification. Optical properties are determined with the help of polarising microscope.

Double refraction -Light separates into two rays which makes images seen through the crystal appear to be doubled.

Ordinary light – ordinary light travels in straight lines with a tranverse motion. It vibrates in all directions at right angles to the direction of propagation.Polarized light – when the vibrations of the wave motion is confined to a single plane only, the light is called polarised light.

introductionSlide4

Slightly modern petrological microscope

conoscope

Internal light source, polarized

Trinocular

head

Reflected light source

Vernier

scale

Analyzer, upper polarizer, nicols lens

Accessory plate

ObjectivesSlide5

Different types of transmitted microscope studies

Determined under Plane polarized light

Determined under crossed polars & orthoscopic illumination

Determined from interference colours obtained under crossed polars & conoscopic illumination

Colour & pleochroism

Form, inclusion, alteration

Isotropic or anisotropic

Polarisation colours

Extinction & Extinction angle

Uniaxial or biaxial

Interference figure

Cleavage, twinkling

Elongation

Optic sign ( + or -)

Refractive indices

, Relief

Sign of elongation (length slow or length fast)

2v

Pleochroic haloes

Twinning & zoningSlide6

Properties under plane polarised light-Colour – reflection of light from any surface of mineral or any object.Form – The shapes of commonly occuring crystals and/or of aggregates of crystalline grains

.Inclusion – It is the smaller minerals within the larger host minerals.Alteration – when mineral subjected to weathering & it altered into secondary mineral.Eg. Olivine – Serpentine,

Olivine altered in serpentineFluid inclusions in quartz in alkali graniteSlide7

Relief - Relief is a measure of the relative difference between a mineral grain and its surroundings.

Quartz has low relief

Garnet has high relief Cleavage – This is the property that some minerals exhibit of breaking along definite smooth planes.

In hornblende 2 set cleavage in ppl & ucn conditionsSlide8

Twinkling – Twinkling effect is observable in anisotropic minerals with widely differing refractive indices on rapidly rotating the stage under plane polarised light.Eg. Calcite.

Refractive indices – is a ratio between the sine of the angle of incidence and the sine of the angle of refraction, which is always constant for the two media concerned. E.g. Quartz – 1.55, Halite – 1.54

In biotite twinkling is present.Slide9

Pleochroism - Some anisotropic coloured minerals change their colour (quality & quantity), upon rotation of stage in plane polarized light (Pleochroism or diachroism)

Pleochroic

haloes – these are curious little circular spots characteristically present in a few minerals that tend to be strongly pleochroic. Eg. Biotite, tourmaline, amphiboles muscovite.

The mineral biotite changes color from dark brown to black when the thin section is rotated.Slide10

Properties under cross nicol conditions

Isotropic minerals – the interaction of light with minerals in every direction is constant.Eg. Garnet, diamond.

Anisotropic minerals - the minerals which changes their optical properties when oriented in different direction.Polarisation colour - When an anisotropic mineral is placed between crossed nicols, it exhibits vivid colours as a consequence of light being split into two rays on passing through the mineral. These are interference

colours. Slide11

Birefringence – It is difference between the refractive indices of two rays i.e., ordinary & extraordinary rays. Slide12

Extinction – When minerals are seen under the cross nicol position and when the field remain dark is called the extinction. Types – Straight extinction - orthopyroxene Inclined extinction – clinopyroxene

wavy extinction - QuartzExtinction Angle – Crystal edges or prominent cleavages are used to find the angle at which extinction occurs and is known as extinction angle.

extinction angle

UCN

pplSlide13

Twinning – Two or more crystals

intergrow in each other. Polysynthetic twinning in calcite

Zonning - Some plagioclase feldspars will have one composition in the interior of the crystal, and a gradually or sharply changing composition toward the outer edge of the crystal,This is called zoning.

Zoned feldsparSlide14

Retardation -

When slow ray emerges from a anisotropic crystal, fast ray must have already emerged & travelled some distance. This DISTANCE is called Retardation (

∆)Retardation is proportional to thickness (t

) of the crystal and to the birefringence () in the direction light is travelling:∆ = t x 

Isotropic indicatrix

All

minerals belonging to the

cubic crystal system

are isotropic with respect to their optical properties.Slide15

In the tetragonal,

trigonal

, and hexagonal crystal systems (a=b =c; or a1=a2=a3=c ).

R.I. in the plane perpendicular to the main symmetry axis (z) must be constant. R.I. parallel to C can be different.

Uniaxial indicatrix Slide16

(-) crystal:

w > e

 oblate

(+) crystal: e (c)> w (a)

Quartz

Calcite

Positive & Negative

uniaxial

mineralSlide17

Orthorhombic, monoclinic, and triclinic crystal systems have a

triaxial

ellipsoid indicatrix, defined by semi axes with length,

a

b, and g

.Elongated along Z axis but flattened along X axis. By convention we define

a

<

b

<

g

.

Z

Y

X

a

(short)

b

(medium)

Biaxial mineralSlide18

Terminalogy in Biaxial indicatrixOPTIC AXES – through which no double refraction occursOPTIC PLANE – includes optic axis/axes

OPTIC NORMAL – perpendicular to optic plane2V or OPTIC ANGLE – angle between optic axesAxis is ACUTE BISECTRIX – if 2V is bisected by it (Z or X)Axis is OBTUSE BISECTRIX – if obtuse angle between optic axes is bisected by Z or X.If ACUTE BISECTRIX IS X mineral is NEGATIVE

If ACUTE BISECTRIX IS Z mineral is POSITIVE2V is present only in BIAXIAL MINERALS!2V is always <90o.If 2V = 90 then mineral is optically neutral.Slide19

Uniaxial interference FiguresTo determine OPTIC SIGN the best position is to look down optic axis .

How to Know that position?Mineral appears isotropic in that position. Grains should be oriented so that optic axis should be vertical or near vertical.Resulting figure is called OPTIC AXIS UNIAXIAL figure.A typical figure has a BLACK CROSS which do not move when stage is rotated. Centre of cross is MELATOPE (direction of optic axis).

Dark bands are ISOGYRES (=orientation of LP & UP).Surrounding colour rings (if present) are ISOCHROMES.They are interference colours of equal retardation.Minerals with low birefringence do not show ISOCHROMES.THIS IS CALLED A CENTRED OPTIC AXIS FIGURE.Slide20

Centred

Off CentredSlide21

SIGN DETERMINATION USING ACCESSORY PLATESlide22

Biaxial interference FiguresThey are obtained in the same way as uniaxial

figures.Very complicated & difficult to find grains oriented as desired.Interpretation is difficult. The interference colours shown by them are dependent on

BirefringenceThicknessGrain orientationFour types of biaxial interference figures can be obtained:Optic normal figure (max interference colours)Obtuse

bisectrix figure (high interf. Colours)Acute bisectrix figure (

relativ low int.Col)Optic axis figures (no interference colors)Last two give max. Optical information.Slide23

Bi axial acute bisectrix interference figure

Optic plane is parallel

to polarizer

Optic plane is not parallel

to polarizer CROSS SEPARATES

INTO 2 ISOGYRES (NW-SE)Slide24

Separation of

melatopes

is a measure of 2

V2V determination in acute

bisectrix figureSlide25

It is difficult to identify crystals in the correct orientation to give an acute

bisectrix figure.

It is much easier to obtain an optic axis figure, since in this orientation the mineral appears isotropic (or very low order birefringence).Optic Axis figureSlide26

Thank you