A SEMINAR PRESENTETIONON

Optical Properties of MineralsPresented by – Student’s name

Contents IntroductionPolarised lightDifferent types of transmitted microscope studiesProperties under plane polarised lightProperties under cross nicol conditionBirefringenceRetardationIsotropic indicatrixBiaxial indicatrix Interference figures

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.

INTRODUCTION

Slightly modern petrological microscope

conoscope

Internal light source, polarized

Trinocular head

Reflected light source

Vernier scale

Analyzer, upper polarizer, nicols lens

Accessory plate

Objectives

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 & pleochroismForm, inclusion, alteration

Isotropic or anisotropicPolarisation coloursExtinction & Extinction angle

Uniaxial or biaxialInterference figure

Cleavage, twinkling Elongation Optic sign ( + or -)

Refractive indices, Relief

Sign of elongation (length slow or length fast)

2v

Pleochroic haloes Twinning & zoning

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 serpentine

Fluid inclusions in quartz in alkali granite

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 conditions

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.

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.

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.

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

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 - Quartz

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

n

n a=X

c=Z

b=Y

c

a

b

Z

X

Y

extinction angle

UCN

PPL

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 feldspar

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.

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

(-) crystal: w > e oblate

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

QuartzCalcite

Positive & Negative uniaxial mineral

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 mineral

Terminalogy in Biaxial indicatrix OPTIC AXES – through which no double refraction

occurs OPTIC PLANE – includes optic axis/axes OPTIC NORMAL – perpendicular to optic plane 2V or OPTIC ANGLE – angle between optic axes Axis 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 POSITIVE 2V is present only in BIAXIAL MINERALS! 2V is always <90o. If 2V = 90 then mineral is optically neutral.

Uniaxial interference Figures

To 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.

Centred Off Centred

SIGN DETERMINATION USING ACCESSORY PLATE

Biaxial interference Figures

They 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 Birefringence Thickness Grain orientation Four 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.

Bi axial acute bisectrix interference figure

Optic plane is parallelto polarizer

Optic plane is not parallelto polarizer CROSS SEPARATESINTO 2 ISOGYRES (NW-SE)

Separation of melatopes is a measure of 2V

2V determination in acute bisectrix figure

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 figure