Optical Properties of Minerals DEPARTMENT OF APPLIED GEOLOGY A SEMINAR PRESENTETION ON Optical Properties of Minerals Presented by – Student’s name

Contents Introduction Polarised light Different types of transmitted microscope studies Properties under plane polarised light Properties under cross nicol condition Birefringence Retardation Isotropic indicatrix Biaxial indicatrix Interference figures

introduction 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.

Slightly modern petrological microscope Trinocular head Reflected light source Analyzer, upper polarizer, nicols lens Accessory plate Objectives Vernier scale conoscope Internal light source, polarized

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 & 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, Fluid inclusions in quartz in alkali granite Olivine altered in serpentine

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. In biotite twinkling is present. 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

Pleochroism - Some anisotropic coloured minerals change their colour (quality & quantity), upon rotation of stage in plane polarized light (Pleochroism or diachroism) The mineral biotite changes color from dark brown to black when the thin section is rotated. 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.

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. extinction angle ppl UCN

Twinning – Two or more crystals intergrow in each other. 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. Polysynthetic twinning in calcite Zoned feldspar

Isotropic indicatrix 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  All minerals belonging to the cubic crystal system are isotropic with respect to their optical properties. Isotropic indicatrix

Uniaxial indicatrix 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.

Positive & Negative uniaxial mineral (+) crystal: e (c)> w (a) (-) crystal: w > e  oblate Calcite Quartz

Biaxial mineral Z X Y (short) (medium) 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. X Y a (short) b (medium)

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 not parallel to polarizer CROSS SEPARATES INTO 2 ISOGYRES (NW-SE) Optic plane is parallel to polarizer

2V determination in acute bisectrix figure Separation of melatopes is a measure of 2V

Optic Axis 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).

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