A SEMINAR PRESENTETION ON Optical Properties of Minerals Presented by Students name Contents Introduction Polarised light Different types of transmitted microscope studies ID: 581477
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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