Geoscience 240 – Lab 4 Sedimentary Minerals and Petrography
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LAB 4: SEDIMENTARY MINERALS AND THE PETROGRAPHIC
MICROSCOPE
Sediments and sedimentary rocks consist of frameworks, grains, matrix and cements that are
predominantly solid inorganic materials either from prior rock and mineral detritus like granite
clasts or quartz, feldspar and mica grains or biogenic materials such as shells and coral (carbonate
minerals) or teeth and bones (apatite). Because of this, to identify source terrains, degree of
transport, processes and paleo-environments it is useful
There are three parts to this lab:
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Part 1: Introduction to the Petrographic Microscope
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Part 2: Grain Mounts
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Part 3: Optical Mineralogy
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Part 4: Textures of common sedimentary materials, minerals and rocks in thin section
PART 1: INTRODUCTION TO THE PETROGRAPHIC MICROSCOPE
Obtain a petrographic microscope being sure to carry it by its base and the curved upright member
of the stand. Before you pick up and move a microscope ensure there are no loose accessory plates,
glass slides or thin sections on its turntable or base! Also obtain a transformer for the light source,
preferably one with an on off switch. Familiarize yourself with the parts of the microscope, their
names and correct adjustments and handling.
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Geoscience 240 – Lab 4 Sedimentary Minerals and Petrography
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Ocular-eyepiece – usually 10X or 12X with cross hairs and a barrel focus controlled by the knurled
ring. Bring the cross hairs into focus first, then raise or lower the stage to focus on the slide. Do not
touch the flat surface of the eyepiece lens with your greasy fingers or eyeballs! The former will
smudge the image and the latter will hurt or damage your cornea!
Bertrand Lens – A knob below the eyepiece that is used for taking optical interference figures using
a small conoscopic lens or pinhole. The normal position has an open bore with this optic rotated out
of the optic train. This is used only within single grains, when the objective is fully cenetered, under
hiogh power magnification. Other than for optic sign determinations (+ or -), it is not used.
Analyser or Upper Nicols Prism – This is a lever or a sliding insert at the base of the upper portion
of the scope usually where the scope angles for convenient viewing. This alternately inserts or
removes a thin slice of mineral or other polarizing material which constrains the emitted light only
to vibrate E-W (across the scope). This permits either Plane polarized PPL or cross polarized XPL
viewing of mineral specimens to identify their basic internal structure (Isotropic or anisotropic) by
how they interact with the polarized transmitted light from below. Viewing in PPL permits easy
recognition of grain boundaries, relief, texture, cleavages, fractures, inherent colouration, zonation,
pleochroism alteration, and inclusions. Viewing in PPL permits the recognition of isotropic phases,
pore space, birefringent minerals, Z angle c (cleavage to optic axis or optic angle).
Accessory Plate Slot – Permits the insertion of a ¼ wavelength mica or gypsum plate, a rot-1 plate
or a quartz wedge for adding or subtracting known amounts of retardation to measure
birefringence and optic sign in minerals with known orientation. The accessory plates cost $200-500
apiece so please do not drop them!
Stand – this is your handle for carrying the microscope and it provides alignment and rigidity for
the optic train. It is not a Klingon Bat’leth!
Objectives and Turret – The objectives, generally in 2.5X (brown), 10X (yellow) and 40X (blue)
magnification allow different field of view versus degree of detail. E.g. overall texture of grains
versus cements or structures within a single grain or microfossil. On our scopes these give field of
view (FOV) of 7 mm, 1.4 mm and 0.4 mm respectively for the diameter of the circular area
illuminated. This permits you to estimate grain sizes and to name sediments by their particle size
distributions, sandstone versus siltstone versus wacke etc. Too change objectives ensure the stage
is suitably dropped before rotating the turret and do not go from medium to high power until the
stage is backed off a bit to avoid breaking a thin section $25 each or damaging the delicate objective
lens. Only rotate the turret by the large knurled ring! Do not grab the individual objective lenses by
their cylindrical barrels to change powers! Near the head of each objective are 2 small delicate
knurled rings which permit centering of the objective. This is a delicate, difficult and time
consuming procedure for a technician. Please do not grab by these rings and throw things out of
aim or you will not be able to rotate the stage and stay on the grain of interest!
Stage and clips – This platform is bored with holes to hold stage clips which can fasten down slides
to ensure area of interest is not lost. Stage clips will also stop slides from flinging across the lab if you
are too zealous in rotating the stage! The stage can be rotated to measure angles between grain
elongation, cleavages and optic directions to help identify minerals. It has a 360° ruled scale on its
circumference along with a locking screw and vernier toi measure angles to less than a degree.
When the stage is roated under PPL some iron bearing minerals change colour, displaying
pleochroism. This can help to identify and distinguish various common ferromagnesian minerals
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Geoscience 240 – Lab 4 Sedimentary Minerals and Petrography
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such as: Biotite (brown-red-tan), Hornblende (Brown-green-tan), Titaniferous Augite (Pale pinkBrown) and other pyroxenes and amphiboles.
Condenser lens or illuminator and polarizer – There is a rack and pinion mounted small cylinder
on your scopes with a frosted grey sub-stage polarizer and a light source that can be raised and
lowered to change intensity of light. For relief and general texture observations under PPL, keep it
lowered somewhat and a bit on the dim side. Some allow bulbs to be centered only with a tool and 3
offset screws, others have a small joystick allowing for finger alignment to aim the light up the
center of the optic train. For dark specimens that are not very transparent, to illuminate a single
grain or for taking optical interference figures under high power objective, raise this up close to the
stage. On more expensive microscopes the light source is below inside the base and this is a flip in/
flip our substage condenser lens that provides focused conoscopic illumination for the above
mentioned purposes. The polarizer passes PPL Plane polarized light that is constrained to vibrate
N-S (in the vertical plane of the microscope’s symmetry). This provides light of know orientation for
measurement of angles and vector optical properties within anisotropic mineral lattices.
Focus – This is a double barreled knob on the sides of the scope which permits the stage to be
raised and lowered to bring specimens within focus. The larg ring travels quickly while the smaller
inset knob is for delicate final adjustment. It is good practice to start on low power with the
objective turret (brown lens) in position and look from the side while you raise the stage up as far
as it goes before dropping it to find the focus. Ensure the thin sections are right side up and that
grain mounds are made on single glass slides with a thin cover slip. Do not use slides upside down or
use another glass slide for a cover slip as the higher power objective cannot get close enough to focus
without bumping and scratching the objective or breaking the slide. To join my bozo club and have to
sit out the lab in the corner with abucket on your head ignore the previous instructions!
Illumination System – Transformer – Our scopes have a delicate little female 2 pin socket plug
which insets gently at eh side of the sub stage light source and polarizer barrel. The other end is a
grounded plug. Some have on-off switches. Keep them turned off or unplugged when not in use as the
tiny light bulbs are expensive, burn out and are a pain in the patoot to replace and refocus. Some
allow 2 levels of illumination depending on the pug direction into the illuminator housing.
1-a. Acquaint yourself with the components above and discuss their various functions and
adjustments with a lab partner or instructor. For most purposes you do not need to use the
high power objectives.
In transmitted light most materials, glass, air, water, oil, epoxy, minerals etc are transparent when if
they are cloudy, bubbly with inclusions or inherently coloured. By contrast denser things made of
metals (gold, copper, platinum), metal oxides (magnetite, hematite, ilmentite, pyrolusite, cuprite),
metal sulfides (pyrite, galena, chalcopyrite, argentite, arsenopyrite), graphite or graphite like
carbonized organic matter are opaque and appear black or another solid colour (brown, red, golden
etc.) when viewed from above. The ones you cannot see through in PPL are termed opaque. For the
transparent ones, there are 2 basic optical types: isotropic meaning cubic or a single refractive
index in all directions regardless of orientation and anisotropic which means they have 2 or 3
different principal refractive indices that are a function of a grains internal structure and
orientation. For isotropic or cubic substances: glasses, halite, garnet, tar etc. when you cross the
analyser (upper polarizer) clear things go black and remain black on 360° rotation. For anisotropic
things: most minerals including quartz, feldspars, calcite, epidote, micas, amphiboles, pyroxenes,
corundum, apatite etc. they might appear transparent in PPL but crossing the upper polarizer gives
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Geoscience 240 – Lab 4 Sedimentary Minerals and Petrography
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them false birefringence colours that vary between whites or greys at the low end to hot rainbow
(psychedelic!) colours alternating with 4 black extinction positions as you rotate the stage through
360°. This is because the passage of light is a vector refraction property of the crustal lattice and
there are only 4 special positions when the optic directions in the crystal are rotated into
coincidence with the polarizer or analyser direction of the scope that permits light to pass through
unrotated and to be blocked out by the upper analyser. All crystal systems other than cubic are
anisotropic : tetragonal (zircon, thorite, anatase, rutile, scapolite, scheelite) and hexagonal (quartz,
calcite, dolomite, corundum, apatite) minerals have 2 principle refractive indices and are termed
uniaxial. Most minerals are more complex and less symmetric in their structures and have 3
principle refractive indices and are termed biaxial including: orthorhombic (olivine, hypersthene,
grunerite, zoisite, aragonite), monoclinic (micas, hornblende, augite) or triclinic (feldspars,
turquoise, kyanite, wollastonite, brittle micas). Obtain a thin section of a porphyritic volcanic or a
plutonic igneous rock and find and describe each of the items below and explain briefly how you
know it has this type of optical behavior:
1-b. (2) Rock specimen name and thin section number: _____________________________________________
1-c. (2) Opaque minerals/materials - can you identify them by their reflectance using another
light source or shape? –
1-d.(2) Isotropic materials – How can you tell these materials are isotropic and what does this
mean concerning their internal structure?
1-e. (2) Anisotropic materials - How can you tell these materials are anisotropic by observations
in PPL and what does this mean concerning their internal structure?
PART 2: GRAIN MOUNTS
Obtain clean unused long format glass slides, cover slips and Group A: sieved silt sized (<60
microns) volcanic ash, quartz or garnet and Group B: mineral materials having a cleavage:
Calcite, Gypsum, Muscovite, Biotite, K-Feldspar, Plagioclase. Gently take a few grains (10-15)
of material from one of each of the 2 groups, put a drop of water on them with a disposable
pipette and gently place a cover slip along one edge of the drop, lowering it across the grains. If
you have too much or too little water adjuct with the dropper or kimwipes accordingly.
Examine each grain mount under PPL and XPL. Draw, label, include a scale bar and describe
one specimen from each group, then examine each other’s slides. Relief is a relative term to
describe how different the refractive index (indices) are from each other (when viewed in PPL)
from the glass slide itself or mounting medium, water, oil, epoxy etc. Relief depends on grain
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Geoscience 240 – Lab 4 Sedimentary Minerals and Petrography
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thickness as well as refractive indes and even position of the crystal if there is more than one
RI. The volcanic glass, quartz, feldspar and mica will have relatively low relief while the garnet
and biotite will appear bolder as if they really have cliff like edges and they also go in and out
of focus with very little fine focus adjustment. Note if any of these materials have inclusions,
cleavage, fracture, twinning or inclination of the extinction direction with respect to grain
edges or cleavage directions.
2-a. Group A – Sample name ______________________________
Sketch and description (5):
2-b. Group B – Sample name ______________________________
Sketch and description (5):
PART 3: OPTICAL MINERALOGY AND CRYSTALLOGRAPHY
Minerals crystallize in one of 6 general classes of internal structures that are distinguished
by their internal symmetry and external crystal morphology. Depending on how they pass
versus reflect or block incident light, minerals are either Transparent (in thin enough slices)
like most silicate, carbonate and phosphate minerals or Opaque, like most native metals (Au,
Cu, Ag, Pt, Sb), transition metal oxides (Magnetite, Chromite, Pyrolusite, Cuprite), sulfides
(Galena, Chalcopyrite, Pyrite, Argentite) or arsenides (Arsenopyrite).
Isotropic – Goes extinct on 360° rotation in cross polarized light (with analyser clicked in). Looks
transparent in PPL but Black in XPL. All minerals that do this are Cubic-Isometric. These materials
have one refractive index (slowness of light) and a spherical indicatrix.
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Geoscience 240 – Lab 4 Sedimentary Minerals and Petrography
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Cubic = Isometric – These substances or minerals have only 1 refractive index. Includes
highly ordered, regular crystl structures that are equal in all dimensions such as: Halite,
Sylvite, Fluorite, Analcite, Garnet, Diamond, Spinel, Cubic Zirconia as well as disordered or
microcrystalline structures with random orientations including: air, water, oil, pyrobitumen,
volcanic glass, opal, poorly crystalline clays and metamict minerals damaged by radiation
haloes around pitchblende, zircon, monazite and other minerals with significant U or Th
content more than a few %. Common external crystal forms include: cubes, octahedral,
tetrahedral, dodecahedra, pyritohedra, gyroids etc. Some cubic minerals are 3-D tough and
only have irregular or conchoidal fracture, while others have longer or weaker bonds in 3, 4
or 6 different directions and have regular cleavages and cleavage fragments: 3-cubes (Halite,
Sylvite) , 4-tetrahedral or octahedral (Fluorite, Diamond), or 6-dodecahedra (Sphalerite).
Anisotropic - All other crystalline substances, minerals are not isotropic and have 2 or 3
different refractive indices in 3 perpendicular directions. These minerals exhibit unusual or
extraordinary refractive qualities in cross polarixed light which allows gemologists to
determine what they are by simple visual examination in doubly polarized light (or falsify
them if you just bought coloured glass!). Under cross polarized light XPL, these substances
have 4 perpendicular extinction positions and in between these, they exhibit false
interference colours or birefringence. The external manifestation of this and jewelers term
is dichroism (2 colours or change of colours upon rotation).
Uniaxial minerals - Substances with 2 refractive indices have a Uniaxial Indicatrix that
resembles a prolate or oblate ellipsoid of revolution. Prolate ones have a smaller ordinary RI
and are termed optically positive and usually length fast. Oblate ones have a larger ordinary
RI and are termed optically negative and are usually length slow. All Uniaxial minerals
belong to either Hexagonal (Quartz, Calcite and other Rhombohedral Carbonates, Corundum,
Apatite, Nepheline and Ice-I) or Tetragonal (Anatase, Cassiterite, Chalcopyrite, Scheelite,
Scapolite, Thorite, Vesuvianite, Zircon). Of these more symmetric minerals having either a
4-fold or a 6-fold rotational axis of symmetry, Those with only faint 1st grey to pale white
birefringence include: Corundum, Apatite, Nepheline and Vesuvianite. By contrast, Zircon
has high birefringence with hot colours even for the smallest grains and calcite has extreme
6th or 7th order birefringence that twinkles, looks salmon pink in XPL and is called “high
white” as the rainbow colours are so close together they almost merge back into white light.
The other onesmentioned above have maximum birefringence colours around 1st white to
pale yellow but can look grey or nearly black when viewed down the 4 or 6 fold-symmetry
direction and single optic axis. Watch how quartz varies when you lay the prism down
versus look down the pyramid, or calcite laid on a rhomb cleavage versus looking down the 3
fold corner.
Biaxial Minerals – Substances with 3 different principal refractive indices (perpendicular to
each other are termed biaxial. Their indicatrix has 3 different elliptical cross sections and 2
circular sections when viewing down either of the 2 optic axes. Minerals in the
Orthorhombic, Monoclinic and Triclinic Crystal classes are all biaxial. By number,
Orthorhombic and Monoclinic minerals are the most abundant. A few are of interest in
sedimentary petrography. Orthorhombic minerals include: Olivine, Topaz, Aragonite (orthocarbonate group), Barite (ortho-sulphate group), Enstatite-Hypersthene (orthopyroxene
group), Grunerite (orthoamphibole group) and Zoisite. Monoclinic minerals include: Augite,
Biotite, Celsian, Chlorite, Epidote, Feldspar (low), Gypsum, Jadeite, Muscovite, Talc and
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Geoscience 240 – Lab 4 Sedimentary Minerals and Petrography
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Tremolite. Triclinic minerals include: Albite through Anorthite Plagioclase Feldspars,
Chloritoid (Brittle micas), Kyanite, Microcline K-Feldspar, Pectolite, Rhodonite, Turqoise,
Wollastonite. Aragonite is the carbonate of sea shells with needles perpendicular to the shell
surfaces for added strength. Fe-rich olivine survives in desert environments. Epidote has
high birefringence colours. Augite has high birefringence and prismatic nearly right angle
cleavage. Biotite, hornblende and chlorite are frequently pleochroic (changing colours of
green brown tan in PPL. Hornblende has 56/124° cleavage and is splintery. Feldspars have
nearly right angle cleavage, low greys and whites for birefringence and diverse twin laws
plus concentric zonation in volcanics. Plagioclase can alter to calcite and clays. Alkali
feldspars frequently alter to clays and have perthite exsolution. Epidote, chloritoid, jadeite,
olivine, augite and hornblende can all be green in PPL.
Instructions:
Use the large bench top light sources from physics with polarizing films and a few thin
transparent mineral specimens of: halite, fluorite, prismatic quartz, clear or optical calcite
rhombs, gypsum rhombs (var, selenite) and muscovite sheets or thin books. Note whether
each mineral is isotropic or anisotropic and draw crystals and describe or label the
relationship between their crystal growth or cleavage shapes and special optical directions
according to polarizing directions. Note that unlike your petrographic microscopes, these
light stands and filters are polarized in the 45 degree positions NW-SE and NE-SW.
3-a Halite (3)
3-b Fluorite (3)
3-c Calcite (3)
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Geoscience 240 – Lab 4 Sedimentary Minerals and Petrography
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3-d Quartz (3)
3-e Gypsum (3)
3-e Muscovite (3)
PART 4: PETROGRAPHIC DESCRIPTION OF SANDSTONES
PROCEDURE
Obtain in turn, a thin section of each type of the following coarse grained sandstones: Quartz
sandstone (Potsdam or Navajo) #_______________, arkose #_____________, greywacke #_______________,
glauconitic #______________. Draw, label scales, label grain types, matrix (if present), cement(s)
and any special structures or textures visible in thin section: lamination, cross beds,
fractures, fossils, oil staining etc., then fill out a petrographic data sheet from the lab manual
p 42 including all observations requested. For colours refer to a paired hand specimen. The
Quartzite is mature and either a long system or multi cycle sandstone. The arkose is a 1st
cycle or short system continental sediment derived from a convergent margin mountain belt
or an eroded shield area. The Greywacke should contain rock fragments of volcanic or
sedimentary material and represents 1st cycle and short system deposition of either an arc
or passive margin. The glauconite is a sub-tropical to higher latitude marine shelf lag deposit
typical of a sediment starved or reworked storm shelf where upwelling and biological debris
are significant components.
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Geoscience 240 – Lab 4 Sedimentary Minerals and Petrography
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4-a. Quartzite #_________________________ (5)
4-b Arkose #_____________________________(5)
4-c. Greywacke #________________________(5)
4-d Glauconitic Sandstone #_____________ (5)
4-e Magnetite rich placer sandstone, Jurassic, Crowsnest Pass# ____________ (5)
4-f Magnetite rich “Black Sand” from Sidney Spit# _______________ (5)
4-g Placer gold concentrate in vial for exam under binocular scope. # ________________ (5)
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