Polarizing Microscope Techniques and Applications PDF

Polarizing Microscope Techniques and Applications Lecture 9 H.D.A. Reyes | Correlations 2 Polarizing Microscope • Used extensively to examine transparent minerals, fragments, grains and small crystals, as well as thin sections of minerals, rocks and other crystalline aggregates • Particularly...

Polarizing Microscope Techniques and Applications Lecture 9 H.D.A. Reyes | Correlations 2 Polarizing Microscope • Used extensively to examine transparent minerals, fragments, grains and small crystals, as well as thin sections of minerals, rocks and other crystalline aggregates • Particularly useful in the determination of the optical properties of individual crystals or aggregates and in the interpretations of textures, structures, growth patterns, and various relationship of natural or artificial substance. H.D.A. Reyes | Correlations 2 Polarizing Microscope • Parts of Polarizing Microscope and Uses Source: https://www.olympuslifescience.com/en/microscoperesource/primer/techniques/pol arized/polmicroalignment/ H.D.A. Reyes | Correlations 2 Polarizing Microscope • Parts of Polarizing Microscope and Uses (Base) Part a) Light source – with Halogen lamp (illuminator) housing bulb Line cord sliding control, lever, voltmeter b) Collector lens system c) Field iris diaphragm with field iris diaphragm ring d) Filter mount with blue filter H.D.A. Reyes | Correlations 2 Function source of light concentrates light controls light ray bundle at the source field approximates daylight Source: https://www.olympuslifescience.com/en/microscoperesource/primer/techniques/pol arized/polmicroalignment/ Polarizing Microscope • Parts of Polarizing Microscope and Uses (Sub-Stage Assembly) Part Function a) Polarizer (lower polar) With Polarizer scale, polarizer rotation ring, screw polarizes light in one direction b) Condenser with clamping screw, condenser centering screw, fixed back lens movable top and front lens with swing out knob c) Aperture Iris Diaphragm with lever and numerical aperture scale H.D.A. Reyes | Correlations 2 controls and illuminates light coming from the source field directed to the object field controls cone of light catering the objective (useful for R. I. determination) Source: https://www.olympuslifescience.com/en/microscoperesource/primer/techniques/pol arized/polmicroalignment/ Polarizing Microscope • Parts of Polarizing Microscope and Uses (Stage Assembly) Part Function a) Stage with stage clamping screw verniers platform for specimen b) Stage Clips fix specimen on stage c) Mechanical Stage with vernier scale click stops, stage centering screw for point locations and systematic traverse in a species along mutually perpendicular directions Source: https://www.olympuslifescience.com/en/microscoperesource/primer/techniques/pol arized/polmicroalignment/ H.D.A. Reyes | Correlations 2 Polarizing Microscope • Parts of Polarizing Microscope and Uses (Microscope Stand / Tube ) Part 1. Coarse Adjustment Knob Function For Focusing Image 2. Fine Adjustment Knob Source: https://www.olympuslifescience.com/en/microscoperesource/primer/techniques/pol arized/polmicroalignment/ H.D.A. Reyes | Correlations 2 Polarizing Microscope • Parts of Polarizing Microscope and Uses (Objective Assembly) Part Function a) Revolving Nosepiece with objective centering screw holds objectives b) Objectives initial magnifications 4x, 10x, 20x, 40x essential lenses of microscope for magnification and resolution Source: https://www.olympuslifescience.com/en/microscoperesource/primer/techniques/pol arized/polmicroalignment/ H.D.A. Reyes | Correlations 2 Polarizing Microscope • Parts of Polarizing Microscope and Uses (Intermediate Polarizing Assembly) Part a) Test plate insertion slot (accessory opening) b) Analyzer ( upper polar ) c) Bertrand lens with focusing ring Function for insertion of microscopic accessory plate polarizes light for observing interference figure Source: https://www.olympuslifescience.com/en/microscoperesource/primer/techniques/pol arized/polmicroalignment/ H.D.A. Reyes | Correlations 2 Polarizing Microscope • Parts of Polarizing Microscope and Uses (Ocular Assembly) Part a) Observation tube with clamping screw and light Function holds eyepieces b) Path selector knob c) Eyepieces with cross hair d) Diopter adjustment ring e) Photo Tube H.D.A. Reyes | Correlations 2 essential lenses of microscope for magnification or resolution conform with objectives for focusing eyepieces for camera attachment in photomicrography Source: https://www.olympuslifescience.com/en/microscoperesource/primer/techniques/pol arized/polmicroalignment/ Polarizing Microscope • Accessory Plates ❑ mica plate – cut to such thickness that it increases or decreases retardation of a section by about 1/4λ (sodium light) ❑ gypsum plate - It is used to determine fast and slow directions (electric vectors) of light polarization in crystals under view on the microscope stage by increasing or decreasing retardation of the light ❑ quartz wedge – ground to produce interference colors from the beginning of the first to the end of the third or fourth order. Equals 0.009 ❑ compensator H.D.A. Reyes | Correlations 2 Polarizing Microscope H.D.A. Reyes | Correlations 2 Polarizing Microscope Light Theories of Light ❑ Corpuscular Theory – beam of light consists of a stream of minute particles, or photons, given off at high velocity by a luminous body that travel through space in straight lines and eventually reach the eye. ❑ Wave Theory – advanced by Dutch scientist Christian Huygens which considered light to be transmitted by the vibration of particle in the waves. The phenomena of light such as reflection, refraction, diffraction and interference may be readily explained in accordance with this theory. H.D.A. Reyes | Correlations 2 Polarizing Microscope Light Theories of Light ❑ Electromagnetic Theory – proposed by Scottish physicist James Clerk Maxwell (1873) who considered light as made up of waves but said that waves are electromagnetic. A wave consists of rapidly alternating electric and magnetic fields normal to each other and normal to the direction of propagation of light. ❑ Quantum Theory – by Planck, assuming that radiating oscillators in a black body radiate energy discontinuously in units called quanta. H.D.A. Reyes | Correlations 2 Polarizing Microscope Light Light as wave Soirce: https://hemantmore.org.in/foundation/science/physics/wave-theory-light/461/ H.D.A. Reyes | Correlations 2 Polarizing Microscope Light Light as wave Source: https://physics.stackexchange. com/questions/194233/whatdoes-a-light-wave-look-like3d-model/ H.D.A. Reyes | Correlations 2 Polarizing Microscope Light Light as wave H.D.A. Reyes | Correlations 2 Polarizing Microscope Light Light as wave H.D.A. Reyes | Correlations 2 Polarizing Microscope Light Light as wave Source: https://brocku.ca/earthsciences/people/gfinn/optical/wav efron.htm/ H.D.A. Reyes | Correlations 2 Polarizing Microscope Light Light as wave: Definition of terms ❑ Displacement – may represent a curve combining movement around a circle with motion along a straight line. ❑ Vibration direction – lies in the wavefront and is perpendicular to the ray in isotropic media. In anisotropic media, it is only perpendicular in limited directions. ❑ Wavelength – distance between two successive crests or troughs, or any corresponding distance along the wave. ❑ Wavefront – surface determined at a given instant by all parts of a system of waves traveling along the same direction and in the same phase. In anisotropic, wavefront is perpendicular only in certain directions. H.D.A. Reyes | Correlations 2 Polarizing Microscope Light Light as wave: Definition of terms ❑ Wave normal – direction perpendicular to the wavefront. In isotropic, the wave normal and ray direction are the same. In anisotropic, they differ aside from certain directions. ❑ Frequency- number of vibrations in a given unit of time. ❑ Amplitude – maximum displacement of a wave from the line of transmission ❑ Period – time interval necessary for a wave to undergo a complete oscillation ❑ Crest – point of the wave with the maximum upward displacement ❑ Trough - point of the wave with the greatest downward displacement ❑ Beam – a group of light waves following along a sample path H.D.A. Reyes | Correlations 2 Polarizing Microscope Light Light as wave: Definition of terms ❑ Ray – straight-line path followed by light in moving from one point to another ❑ Refractive index – equal to the ratio of the wave-normal velocity in a vacuum to the wave-normal velocity in the medium whether isotropic or anisotrophic. ❑ Monochromatic light – light of a single wavelength ❑ Light vector – lies parallel to the plane of the wavefront. In isotropic, perpendicular to the direction of the propagation. In anisotropic, still parallel but not perpendicular to the direction of the propagation. Types of Vector: ✓ 1. electric – measures the electrical displacement ✓ 2. magnetic – measures the magnetic displacement or induction. H.D.A. Reyes | Correlations 2 Polarizing Microscope Light Light as wave: Definition of terms ❑ Speed of light – 186,284 miles per second ❑ White light – combination of all the different wavelengths visible to the eye. Maybe considered composed of seven different colors. Source: https://www.olympuslifescience.com/fr/microsco peresource/primer/lightandcol or/particleorwave/ H.D.A. Reyes | Correlations 2 Polarizing Microscope Manipulation of Microscope ❑ Orthoscopic observation – realistic virtual image with a flat field ➢ Plane polarized light (uncrossed Nicols ) ✓ Low to high magnification objective ✓ Analyzer out ✓ Condenser top lens out ✓ Bertrand lens out ➢ Crossed Nicols ✓ Low to high magnification objective ✓ Analyzer in ✓ Condenser top lens out ✓ Bertrand lens out H.D.A. Reyes | Correlations 2 Source: https://en.wikipedia.org/wiki/Petrographic_microscope Polarizing Microscope Manipulation of Microscope ❑ Conoscopic observation – yields interference figures which represent an optical pattern caused by the behavior of light in individual crystal ✓ ✓ ✓ ✓ ✓ High magnification objective (40x) Analyzer in Condenser top lens in Bertrand lens in Accessories in Source: http://jmderochette.be/Conoscopy/beryl_1.htm H.D.A. Reyes | Correlations 2 Sample Preparation 1. Cutting - at least 1”x 2” size using the cutting machine 2. Grinding – to a 240 mesh abrasive them to 800 mesh. 3. Heating – both the thin section and the sample in a hot plate put Canada Balsam in thin section and heat it for about 30 minutes. 4. Mounting - put the sample in thin section 5. Cutting – using the diamond saw cutter 6. Grinding – to a 300 mesh abrasive up to at least 0.03 mm thickness 7. Covering – cover the thin section with the sample using the cover slip. 8. Washing – clean the thin section using the xylol solution. H.D.A. Reyes | Correlations 2 Source: https://thinsection.com.au/ Source: https://www.net32.com/ec/xylol-xylenesoftening-gutta-percha-points-1-d-42368 Optical Mineralogy ▪ Study of the interaction of light with minerals ▪ Most commonly limited to visible light and usually further limited to the non-opaque minerals ▪ Opaque minerals commonly studied in reflected light (study is generally called ore microscopy) ▪ Most general application of optical mineralogy is to aid in the identification of minerals, either in rock thin sections or individual mineral grains H.D.A. Reyes | Correlations 2 Optical Mineralogy Optical Properties ▪ Observation of Transparent Minerals ❑ Orthoscopic (Uncrossed) ▪ Low power objective (10x) ▪ Bertrand lens is optional ▪ Properties can be observed ➢ ➢ ➢ ➢ Color and Pleochroism Cleavage Shape and Form Relief/ Refractive Index (R.I.) H.D.A. Reyes | Correlations 2 Optical Mineralogy Optical Properties ▪ Observation of Transparent Minerals ❑ Orthoscopic (Uncrossed) ➢ Color and Pleochroism Color - observed w/ plane prolonged light; not always the same as megacopic color Pleochroism - change in color of a mineral in varying degrees as the stage is rotated due to differences in light absorption & extraordinary rays generally expressed as a formula H.D.A. Reyes | Correlations 2 Optical Mineralogy Optical Properties ▪ Observation of Transparent Minerals ❑ Orthoscopic (Uncrossed) ➢ Cleavage - the ability of a mineral to separate into smaller particles bounded of faces of possible crystal form. Expressed or best explained in terms of direction ▪ Categorized into four qualities: ✓ Perfect ✓ Good ✓ Poor ✓ None H.D.A. Reyes | Correlations 2 Directions of Cleavage One Direction Two Directions Three Directions Four Directions Six Directions All Directions Optical Mineralogy Optical Properties ▪ Observation of Transparent Minerals ❑ Orthoscopic (Uncrossed) ➢ Cleavage Cleavage is much easier to see in thin section than in hand specimen. Cleavage along the length of the grain is exhibited by many minerals (A). Pyroxenes viewed end on (B) usually show the characteristic 87-degree cleavage, while crosssections of amphibole show the characteristic 56degree cleavage (C). What you see will depend on the orientation of the grain. A true cross-section of an amphibole will show 56-degree cleavages but an oblique section will show other angles and a longitudinal section will show longitudinal cleavage as in (A). H.D.A. Reyes | Correlations 2 Optical Mineralogy Optical Properties ▪ Observation of Transparent Minerals ❑ Orthoscopic (Uncrossed) ➢ Shape and Form - can be expressed by using the terms eubedral subhedral anhedral; lath-shape, bladed etc Grains that show no recognizable crystal form are said to be anhedral (A). Grains that show imperfect but recognizable crystal form are said to be subhedral (B). Grains that show sharp and clear crystal form are said to be euhedral (C). 6 - sided hornblende 8 - sided pyroxene H.D.A. Reyes | Correlations 2 Optical Mineralogy Optical Properties ▪ Observation of Transparent Minerals ❑ Orthoscopic (Uncrossed) ➢ Relief- degree of visibility of a transparent mineral in an immersion medium ➢ A function of the difference between n mineral and n medium ➢ R.I of Canada balsam = 1.53 H.D.A. Reyes | Correlations 2 Optical Mineralogy Optical Properties ▪ Observation of Transparent Minerals ❑ Orthoscopic (Uncrossed) ➢ Relief 1. HIGH RELEF (+ relief) - index of refraction (R.I) of the mineral is Higher than the medium 2. LOW RELIEF (- relief) - R.I of the mineral is lower than the medium 3. ZERO RELIEF - almost the same with the medium 4. CHANGE of RELIEF - varies as the stage is rotated, takes place if one n mineral is near n balsam, and the other n mineral H.D.A. Reyes | Correlations 2 Optical Mineralogy Optical Properties ▪ Observation of Transparent Minerals ❑ Orthoscopic (Uncrossed) ➢ Relief Relief is the contrast between a mineral and its surroundings due to difference in refractive index. The four grains shown here show increasing relief clockwise from left. Relief is positive when the grain has higher refractive index than its surroundings, negative if lower. Negative relief compared to quartz, feldspar and normal slide mounting media is relatively rare. A few silicates show small negative relief, but strong negative relief is limited mostly to non-silicates like fluorite. H.D.A. Reyes | Correlations 2 Optical Mineralogy Optical Properties ▪ Observation of Transparent Minerals ❑ Orthoscopic (Uncrossed) ➢ Refractive Index (RI) ✓ is the ratio of the velocity of light in a vacuum to its velocity in the medium ✓ For isotropic substances: R.I, constant velocity in all directions ✓ For anisotropic : more than one R.I, light velocities vary with direction H.D.A. Reyes | Correlations 2 Optical Mineralogy Optical Properties ▪ Observation of Transparent Minerals ❑ Orthoscopic (Uncrossed) ➢ Refractive Index (RI) ✓ Central Illumination Method or Becke Line Method ✓ A Becke line is a band or rim of light visible along a grain/crystal boundary in plane-polarized light. It is best seen using the intermediate power lens (or low power in some cases), on the edge of the grain, with the diaphragm stopped down a bit H.D.A. Reyes | Correlations 2 Optical Mineralogy Optical Properties ▪ Observation of Transparent Minerals ❑ Orthoscopic (Uncrossed) ➢ Refractive Index (RI) - Central Illumination Method or Becke Line Method A Becke line is the result of two things, both related to refraction along the boundaries of mineral grains: 1. The fact that minerals in thin sections tend to be thicker in the centre and thinner towards the edges, thus they act as lenses (if the refractive index is higher than the mounting medium the rays converge toward the center of the grain; if the refractive index is lower, the rays diverge towards the edge of the grain); and 2. The internal reflection of light within the mineral due to the presence of vertical grain boundaries. H.D.A. Reyes | Correlations 2 Optical Mineralogy Using a Becke line to determine the relief (either positive or negative) 1. Make sure the polars on the microscope are uncrossed and you have plane-polarized light. 2. Pick a grain that has sharp edges along the edge of the thin section (where you can see the mineral against the Canada Balsam or epoxy). 3. Focus on medium power (or low power in some cases), on the edge of the grain. 4. Shut the diaphragm down a bit. 5. IMPORTANT - SLOWLY, slightly increase the distance between the thin section and the objective (defocus by lowering the stage). 6. You will see 2 thin lines appear along the grain boundary, one bright (bright or white Becke line) and one dark (dark Becke line). The bright Becke line moves into the medium of higher refractive index. 7. An important special case: If ngrain approximately equals the balsam , the bright and dark Becke lines are colored. H.D.A. Reyes | Correlations 2 Optical Mineralogy H.D.A. Reyes | Correlations 2 Optical Mineralogy Optical Properties ▪ Observation of Transparent Minerals ❑ Orthoscopic (Crossed) ➢ ➢ ➢ ➢ Isotropism & Anisotropism Interference colors Birefringence Twinning H.D.A. Reyes | Correlations 2 Optical Mineralogy Optical Properties ▪ Observation of Transparent Minerals ❑ Orthoscopic (Crossed) ➢ Isotropism ▪ Those w/ uniform physical properties in all directions ▪ Remain dark in all positions even if the stage is rotated. ▪ No change in color ➢ Anisotropism ▪ Those that display colors in varying degrees as the stage is rotated ▪ Produces interference colors. H.D.A. Reyes | Correlations 2 Optical Mineralogy Optical Properties ▪ Observation of Transparent Minerals ❑ Orthoscopic (Crossed) ➢ Interference colors ▪ For anisotropic minerals only. ▪ observed w/ reference to the color chart (Michel – Levy Chart) ▪ Vary w/ the thickness of section, nature of ml.; direction in which the ml ▪ section is cut and the light employed. ▪ Colors displayed by a birefringent mineral in crossed polarized light. ▪ The term “order” is used in describing interference colors. H.D.A. Reyes | Correlations 2 Optical Mineralogy Optical Properties ▪ Observation of Transparent Minerals ❑ Orthoscopic (Crossed) Retardation A ➢ Interference colors Order Colors Notes 0 Zero Black 0-5500 First Gray, White, Yellow, Red Neutral colors are cold, yellows dull. 5500-11000 Second Violet Through Spectrum to Red Purest colors, though not totally pure 11000-16500 Third Violet Through Spectrum to Red Have a "fluorescent" appearance 16500 and up Fourth and higher Mostly greens and pinks Colors become more washed out with increasing retardation H.D.A. Reyes | Correlations 2 Optical Mineralogy Optical Properties ▪ Observation of Transparent Minerals ❑ Orthoscopic (Crossed) ➢ Birefringence ▪ This maximum colour is often diagnostic of an anisotropic mineral and it is observed in sections that display simultaneously the maximum and minimum refractive indices. H.D.A. Reyes | Correlations 2 Optical Mineralogy Optical Properties ▪ Michel-Levy Chart H.D.A. Reyes | Correlations 2 Optical Mineralogy Optical Properties ▪ Observation of Transparent Minerals ❑ Orthoscopic (Crossed) ➢ Twinning ▪ Formation of rational symmetry intergrowth of 2 or more grains of crystalline species. Source: http://www.labotka.net/310/Atlas/Plates/Twinning.html H.D.A. Reyes | Correlations 2 Optical Mineralogy Optical Properties ▪ Observation of Transparent Minerals ❑ Orthoscopic (Crossed) ➢ Twinning ▪ Carlsbad Twining Form of penetration twinning where two crystals form as penetration twins Source: http://www.labotka.net/310/Atlas/Plates/Twinning.html H.D.A. Reyes | Correlations 2 Optical Mineralogy Optical Properties ▪ Observation of Transparent Minerals ❑ Orthoscopic (Crossed) ➢ Twinning ▪ Lamellar Twining - common within the plagioclase feldspars, in places where two adjoining twin slabs or lamellae are mutually reversed with respect to each other and every alternate twin 'plate' or 'slab' has an identical atomic structure. H.D.A. Reyes | Correlations 2 Source: https://www.pinterest.ph/pin/190558627963248722/ Optical Mineralogy Optical Properties ▪ Observation of Transparent Minerals ❑ Orthoscopic (Crossed) ➢ Twinning ▪ Cross-hatched twinning shows two kinds of repeated twinning in thin section, with one set of twins arranged at 90° to the other set. The lamellar twins overlap each other and have 'fuzzy' edges, giving a 'tartan' appearance. H.D.A. Reyes | Correlations 2 Source: https://lifeinplanelight.wordpress.com/2011/03/01/photomicrographtuesdays-k-feldspar/ Optical Mineralogy Optical Properties ▪ Observation of Transparent Minerals ❑ Orthoscopic (Crossed) ➢ Zoning ❖ Refers to solid solution which do not have uniform composition. Source: http://www.alexstrekeisen.it/english/vulc/calcite.php H.D.A. Reyes | Correlations 2 Optical Mineralogy Optical Properties ▪ Observation of Transparent Minerals ❑ Orthoscopic (Crossed) ➢ Types of Zoning in Plagioclase ✓ NORMAL ZONING - center is more calcic becoming more sodic toward the margin ✓ REVERSE ZONING - center is more sodic becoming more calcic toward the margin. normally steplike ✓ OSCILLATORY ZONING progressions for more calcic interior to more sodic margins w/ local reversals in adjacent zones H.D.A. Reyes | Correlations 2 Optical Mineralogy Optical Properties ▪ Observation of Transparent Minerals ❑ Orthoscopic (Crossed) ➢ Extinction & extinction angle ❖ Parallel Extinction - when a mineral becomes dark parallel to the crosshairs. ❖ Inclined/Oblique - at an angle with the direction of polars. ❖ Symmetrical planes of mineral to the diagonal; vibration direction is diagonal. H.D.A. Reyes | Correlations 2 Optical Mineralogy Optical Properties ▪ Observation of Transparent Minerals ❑ Conoscopic - To get interference figure o High Power Objective ( 40x) o Condenser top lens In o Analyzer In (crossed polars) o Bertrand Lens In o Accessories In (gypsum plate) ➢ Information from interference figure ❖ Number of optic axis ( uniaxial or biaxial) ❖ Optic sign (positive or negative) ❖ Optic angle (2v) ❖ Dispersion H.D.A. Reyes | Correlations 2 Optical Mineralogy Optical Properties ▪ Observation of Transparent Minerals ❑ Conoscopic ➢ Uniaxial ✓ Interference Figure ✓ Optic Sign (+,-) ➢ Biaxial ✓ Interference Figure Source: http://academic.brooklyn.cuny.edu/geology/powell/courses/eesc2100/ ✓ Optic Sign EESC%202100-LAB2-PLM2-F11.pdf ✓ Optic Angle (2V) Dispersion H.D.A. Reyes | Correlations 2 Optical Mineralogy Optical Properties ▪ Observation of Transparent Minerals ❑ Conoscopic Source: https://brocku.ca/ earthsciences/pe ople/gfinn/optical /unioafig.htm H.D.A. Reyes | Correlations 2 Optical Mineralogy Optical Properties ▪ Observation of Transparent Minerals ❑ Conoscopic ➢ Uniaxial ✓ Interference Figure ✓ Optic Sign (+,-) ➢ Biaxial ✓ Interference Figure ✓ Optic Sign ✓ Optic Angle (2V) Dispersion H.D.A. Reyes | Correlations 2 Optical Mineralogy Optical Properties ▪ Observation of Transparent Minerals ❑ Conoscopic ➢ Sign of elongation ❖ based on retardation ❖ x’ (show) - yellow ( negative retardation) ❖ Z’ (fast) - blue (positive retardation) refer to maximum interference color H.D.A. Reyes | Correlations 2 Optical Mineralogy Optical Properties ▪ Observation of Transparent Minerals ❑ Conoscopic ➢ Sign of elongation ❖ Steps in getting the sign of elongation: ✓ move to extinct position ✓ move to 45° ✓ insert the compensator (gypsum plate) o Note the change in color (whether they add or subtract) H.D.A. Reyes | Correlations 2 Optical Mineralogy Optical Properties ▪ Observation of Transparent Minerals ❑ Conoscopic ➢ Sign of elongation ❖ If addition takes place, (+) elongation is produced & ❖ It is LENGTH SLOW; slow ray is // to the axis of elongation ❖ If subtraction takes place, a negative (-) elongation is ❖ produced & it is LENGTH FAST. H.D.A. Reyes | Correlations 2 Staining Techniques H.D.A. Reyes | Correlations 2 Staining Techniques H.D.A. Reyes | Correlations 2 Courtesy of: Reveral, R.R. (2018) Petrography H.D.A. Reyes | Correlations 2 Petrography Petrography ▪ The identification or interpretation of framework mineralogy and textures leading to classification of wallrock as sedimentary, metamorphic or igneous. ▪ Identification of replacement mineralogy and paragenesis, and interpretation in terms of histories of diagenesis, metamorphism, hydrothermal alteration and/or weathering. H.D.A. Reyes | Correlations 2 Petrography Petrography ▪ Texture – refers to degree of crystallinity, grain size or granularity, and the fabric or geometrical relationship between the constituents of a rock. ❑ Degree of crystallinity ➢ Holocrystalline – consist wholly of crystals ➢ Holohyaline – consist entirely of glass ➢ Hypocrystalline/Merocrystalline – Contain both crystals and glass H.D.A. Reyes | Correlations 2 Source: http://zasoby.open.agh.edu.pl/~0 9sjzdera/english/wprowadzenie/ budowa.html Source: http://www.alexstrekeisen.it/engli sh/vulc/holohyaline.php Source: http://www.alex strekeisen.it/en glish/vulc/leucit e.php Petrography Petrography ❑ Degree of crystallinity ➢ Microlites – Extremely minute, incipient crystals, provided they are birefringent. ➢ Crystallite - Smaller, spherical, rod- and hair-like isotropic from. Source: https://www.rockptx.co m/fkm-126-to-fkm-150/ H.D.A. Reyes | Correlations 2 Petrography Petrography ❑ Grain size or Granularity ➢ Aphanitic or Eucrystalline – Fine grained ➢ Phaneritic or Dyscrystalline – Coarse Grained ➢ Cryptocrystalline – Very-fine and undistinguishable under petrographic microscope ➢ Pegmatitic – Very-coarsed grained H.D.A. Reyes | Correlations 2 Petrography Petrography ❑ Fabric ➢ Euhedral / Idiomorphic / Automorphic bounded completely by crystal faces. Crystals were ➢ Anhedral / Allotriomorphic / Xenomorphic - Crystals were not bounded by crystal faces. ➢ Subhedral / Hypidiomorphic bounded by crystal faces. Crystals were partially Source: http://www.rc.un esp.br/igce/petr ologia/nardy/mo habi.html H.D.A. Reyes | Correlations 2 Petrography Petrography ❑ Fabric ➢ Allotriomorphic Granular / Xenomorphic Granular / Aplitic/ Sugary/ Saccharoidal ✓ Chiefly minerals are anhedral. ✓ Common in aplites ➢ Panidiomorphic Granular / Automorphic Granular / Lamprophyric ✓ If chiefly minerals are euhedral ✓ Common in dark hypabyssal rocks or lamprophyres ➢ Hypidiomorphic Granular Granitic ✓ If all faces are present H.D.A. Reyes | Correlations 2 / Hypautomorphic Granular / Petrography Petrography ➢ Petrographic rules of genesis ▪ When one mineral is surrounded by another mineral, the enclosing mineral if the younger. ▪ Early crystals are generally euhedral or at least more nearly than those of later crystals ▪ If both large and small crystals occur together, the larger one are the first to develop. H.D.A. Reyes | Correlations 2 Petrography Petrography ➢ Vitrophyric Texture ▪ Of the phenocrysts lie in a matrix of glass. ➢ Felsophyric Texture ▪ If the groundmass is a dense intergrowth of quartz and feldspar. ➢ Orthopyric Texture ▪ If the groundmass of feldspar are rectangular in form instead of slender lath crystals H.D.A. Reyes | Correlations 2 Source: http://web.pdx.edu/~ ruzickaa/G410/McS ween/glos19.html Source: http://www.npshistor y.com/publications/g eology/pp/604/sec3 a.htm Petrography Petrography ➢ Glomeroporphyritic Texture ▪ Glomeroporphyritic or Glomerophyric is a term used to describe a porpyritic texture in which phenocrysts are clustered into crystal clots. aggregates called glomerocrysts or Glomeroporphyritic textures are common and often included plagioclase and pyroxenes in basic rocks. They form by a processes known as synneusis, where accumulation of crystals occurs by surface tension and fixing by interpenetration due to crystal growth. ▪ Glomerocrysts are an important consideration in crystal fractionation by crystal settling since the density of the glomerocryst is an average of that of its constituent phases. Formation of glomerocrysts may in part explain the settling of plagioclase in basic intrusions in which plagioclase crystals are less dense than the surrounding magma. H.D.A. Reyes | Correlations 2 Petrography Petrography ➢ Glomeroporphyritic Texture Pyroxene and plagioclase glomerophyre in a Basalt Source:http://www.alexstrekeisen.it/e nglish/vulc/glomerophyric.php H.D.A. Reyes | Correlations 2 Petrography Petrography ➢ Poikilitic Texture ▪ Refers to crystals, typically phenocrysts, in an igneous rock which contain small grains of other minerals. In igneous rocks Poikilitic texture is widely used to determine order of crystallization; if one mineral is enclosed by another then the enclosed grain must have been the first to crystallize. This may sometime be true , but it is certainly not always so Source: http://www.alexstrekeisen.it/english/vulc/poikilitic.php H.D.A. Reyes | Correlations 2 Petrography Petrography ➢ Ophitic Texture ▪ A variant the Poikilitic texture, is one where random plagioclase laths are enclosed by pyroxene or olivine. If plagioclase is larger and encloses the ferromagnesian minerals, then the texture is subophitic and the laths typically impinge on one another to form sharp angles. Note that the from intergranular change through subophitic to ophitic textures in basaltic rocks results from slower cooling and slower nucleation rates H.D.A. Reyes | Correlations 2 Augite Crystals with plagioclase inclusions Source: http://www.alexstrekeisen.it/english/vulc/poikilitic.php Petrography Petrography ➢ Sub-ophitic Texture ▪ A variant the Poikilitic texture, plagioclase is larger and encloses the ferromagnesian minerals, then the texture is subophitic and the laths typically impinge on one another to form sharp angles. Augite Crystals with plagioclase inclusions Source: https://www.cefns.nau.edu/geology/naml/Meteorite/Book-Textures.html H.D.A. Reyes | Correlations 2 Petrography Petrography ➢ Hyalophitic Texture ▪ Angular interstices between feldspars filled with glass instead of pyroxene Source: https://www.researchgate.net/figure/Photomicrograph-of-a-hyaloophitic-texture-and-b-sub-ophitic-texture-in-Basalt-Glass_fig2_320711155 H.D.A. Reyes | Correlations 2 Petrography Petrography ➢ Corona Texture/ Reaction Rim / Opacitic Rim A genetic term for a border of secondary minerals formed at the margin of a primary grain in an igneous or metamorphic rock. Its use implies that the secondary minerals have formed as a result of reactions between the primary grain and the adjacent primary grain, a fluid, or a melt. ▪ Common structure of hydrated mineral in volcanic rocks such as biotite or amphiboles. ▪ Formed from post-magmatic reaction. ▪ H.D.A. Reyes | Correlations 2 Hornblende crystal with Opacitic rim. Source: http://www.alexstrekeisen.it/immagini/vulc/bordoopaciticoanfibolo(5).jpg Petrography Petrography ➢ Kelyphitic Rim ▪ Secondary rim/coronas Source: http://www.labotka.net/310/Atlas/Plates/Kelyphitic-Rims-on-Garnet.html H.D.A. Reyes | Correlations 2 Petrography Petrography ➢ Intergranular Texture ▪ Intergranular is a textural term indicating that a crystal occupies the angular space between at least two larger crystals. Intergranular crystals have crystallised later than the crystals that control their shape. A common case of Intergranular texture is in which the space between plagioclase crystals is occupied by one or more granules of pyroxene (± olivine and opaque oxides) not in optical continuity between them. H.D.A. Reyes | Correlations 2 Basalt with intergranular groundmass Source: http://www.alexstrekeisen.it/immagini/vulc/intergranulare(2).jpg Petrography Petrography ➢ Intersertal Texture ▪ A textural term used to denote that the angular spaces between larger crystals is occupied by glass, or glass and small crystals. The glass may be devitrified or altered to other phases. Spaces between crystals occupied by glass in a Basalt. Source: http://www.alexstrekeisen.it/immagini/vulc/intersertale(3).jpg H.D.A. Reyes | Correlations 2 Petrography Petrography ➢ Pilotaxitic Texture ▪ Texture of the groundmass of a holocrystalline igneous rock in which lath-shaped microlites (typically plagioclase) are arranged in a glass-free mesostasis and are generally interwoven in irregular unoriented fashion. Source: https://www.thinglink.com/scene/623001739691294722 H.D.A. Reyes | Correlations 2 Petrography Petrography ➢ Trachytic Texture ▪ A texture of extrusive rocks in which the groundmass contains little volcanic glass and consists predominantly of minute tabular crystals, namely, sanidine microlites. The microlites are parallel, forming flow lines along the directions of lava flow and around inclusions. Trachytic texture occurs in rocks that are rich in alkalies; hence the vitreous mass of the rocks has a relatively low viscosity. H.D.A. Reyes | Correlations 2 Source: http://www.alexstrekeisen.it/english/vulc/trachytic.php Petrography Petrography ➢ Myrmekitic Texture ▪ An intergrowth of branching rods of quartz set in a single crystal of plagioclase, neighboring rod of quartz have the same lattice orientation and extinguish together. Myrmekite may also occur at grain boundaries of Kfeldspar crystals. Source: http://www.alexstrekeisen.it/english/pluto/myrmekite.php Schematic illustration of K-Feldspar porphyroclast in a deformed rocks, with myrmekite growing on sides facing the shortening direction (black arrows). From David Shelley (1983) H.D.A. Reyes | Correlations 2 Petrography ➢ Seive Texture ▪ Petrography Plagioclase commonly exhibits a variety of disequilibrium textures in volcanic rocks, especially in orogeny andesites. These textures often include combinations of complex zoning patterns and resorption feature in plagioclase that record changing physical conditions in magmatic system. Sieve texture is common in plagioclase or in pyroxene crystals in extrusive volcanic rocks; It is interpreted by some authors as the result of mixing processes H.D.A. Reyes | Correlations 2 Source: http://www.alexstrekeisen.it/english/vulc/sieve.php Petrography ➢ Seive Texture Petrography Diagram 1Results of mixing plagioclase crystals of various compositions with magma of composition M that is in equilibrium with a plagioclase of composition 3. A): Crystals of composition 1, mixed with M; crystals may melt, but in any case dissolve to give rounded shapes. If M penetrate the dissolving crystals (1), or if partial melting occurs internally, sieve texture is produced as (1) react with the melt to form more calcic rim (no more calcic than 3). B.): Crystals of composition (2) cannot melt completely and cannot simply dissolve in M, but they will react during partial solution or melting, to give a sieve texture and more calcic rim (no more calcic than 3). The former euhedral outline is preserved. C.): Crystals of composition (4) continue to grow with euhedral overgrowth of composition (3). From Shelley H.D.A. Reyes | Correlations 2 Petrography ➢ Exsolution Petrography ▪ A process by which a solid solution phase unmixes into two separate phases in the solid state. occurs only in ▪ Exsolution minerals whose compositions vary between two or more pure endmember compositions. Source: http://www.alexstrekeisen.it/english/pluto/exolutionlamellaepyroxene.php H.D.A. Reyes | Correlations 2 Petrography ➢ Perthitic Texture ▪ Petrography Is an intimate intergrowth of sodic and potassic feldspar resulting from subsolidus exsolution Source: www.alexstrekeisen.it/english/pluto/antiperthite.php H.D.A. Reyes | Correlations 2 Petrography ➢ Anti-PerthicTexture ▪ Petrography is an intergrowth arising due to exsolution where potassic feldspar is present as blebs or lamellae within a sodic feldspar. Source: www.alexstrekeisen.it/english/pluto/antiperthite.php H.D.A. Reyes | Correlations 2 Selected References • • • • • Williams, H. et. al. (1954). Petrography; An Introduction of the Study of Rocks and Thin Sections. W. H. Freeman and Company, San Francisco. http://www.alexstrekeisen.it/english/index.php https://www.mindat.org/glossary/gypsum_plate https://hemantmore.org.in/foundation/science/physics/wave-theorylight/461/ http://www.open.edu/openlearn/science-mathstechnology/science/introduction-minerals-and-rocks-under-themicroscope/content-section-3.6.2 H.D.A. Reyes | Correlations 2