Optical Mineralogy a Review

A review of mineral properties under the petrographic microscope

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Why use the microscope?? •Identify minerals •Determine rock type •Determine crystallization sequence •Document deformation history •Observe ‘frozen-in’ reactions •Constrain P-T history •Weathering/alteration

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west (left)

east (right)

Light vibrating E-W PPL - plane polarized light

Light vibrating in many planes and with many wavelengths

Unpolarized light

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thin section west (left)

east (right)

Light vibrating E-W Light vibrating in many planes and with many wavelengths

Unpolarized light

Light and colors reach eye!

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some optical characteristics…

Cleavage – number and orientation of cleavage planes

Twinning – type of twinning, orientation Extinction angle – parallel or inclined? Angle? Habit – characteristic form of mineral – euhedral, etc., and specific shape

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Cleavage

Most easily observed in PPL (upper polarizer out), but visible in XPL as well

• No/poor cleavages: quartz, olivine (irregular fracture) • 1 perfect cleavage: micas • 2 good cleavages: pyroxenes, amphiboles

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Cleavage

random fractures, POOR cleavage: olivine

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2 cleavages intersecting at ~90° (87-88°): pyroxene

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2 cleavages intersecting at ~90° (87-88°): pyroxene

60° 120° 2 cleavages

intersecting at ~60°/120° (56 & 124°): amphibole

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Twinning

Presence and style of twinning can be diagnostic

Twins are most obvious in XPL (upper polarizer in)

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Clinopyroxene (augite)

Plagioclase

— Simple twin on {100}

— Simple (Carlsbad) twin on (010)

— Pericline twin on (001)

— Polysynthetic albite twins on (010)

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Microcline

Plagioclase

Crosshatch (tartan) twinning

Simple (Carlsbad) twinning Pericline twins Polysynthetic albite twinning

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Colour & pleochroism: rotate the stage in plane polarized light

Many coloured mineral grains change color as the stage is rotated; note the range of colours

A few coloured minerals stay the same in all orientations

These minerals are pleochroic

Note the colour AND whether the colour changes upon rotation

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Mineral properties: color & pleochroism • Color is observed only in PPL • Not an inherent property - changes with light type/intensity • Results from selective absorption of certain λ of light • Pleochroism results when different λ are absorbed

differently by different crystallographic directions - rotate stage to observe

plag

hbl

plag

hbl

-Plagioclase is colorless -Hornblende is pleochroic in olive greens

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rotate the stage with crossed polars Most mineral grains change color as the stage is rotated; these grains go black 4 times in 360°

rotation-exactly every 90o

Glass and a few minerals stay black in all orientations

These minerals are anisotropic

These minerals are isotropic

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Some generalizations and vocabulary • All isometric minerals (e.g., garnet) are isotropic – they cannot reorient light. These minerals are always black in crossed polars. • All other minerals are anisotropic – they are all capable of reorienting light. • All anisotropic minerals contain one or two special directions that do not reorient light. – Minerals with one special direction are called uniaxial – Minerals with two special directions are called biaxial

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Isotropic Uniaxial Biaxial

How light behaves depends on crystal structure

Isometric – All crystallographic axes are equal

Orthorhombic, monoclinic, triclinic – All axes are unequal

Hexagonal, trigonal, tetragonal – All axes ⊥ c are equal but c is unique

This information helps us identify minerals!

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Extinction angle

Extinction behaviour is a function of the relationship between indicatrix orientation and crystallographic orientation

parallel extinction inclined extinction 19

Extinction angle – parallel extinction

• All uniaxial minerals show parallel extinction • Orthorhombic minerals show parallel extinction

(this is because the crystal axes and indicatrix axes coincide)

PPL XPL

orthopyroxene orthopyroxene

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Extinction angle - inclined extinction

Monoclinic and triclinic minerals: indicatrix axes do not coincide with crystallographic axes

These minerals have inclined extinction (and extinction angle helps to identify them)

clinopyroxene

extinction angle

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Habit or form

blocky

acicular

bladed

prismatic

anhedral/irregular

elongate

rounded

fibrous

tabular

euhedral

subhedral

equant

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Habit or form

blocky

acicular

bladed

prismatic

anhedral/irregular

elongate

rounded

fibrous

tabular

euhedral

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Mineral properties: relief • Relief is a measure of the relative difference in n

between a mineral grain and its surroundings • Relief is determined visually, in PPL • Relief is used to estimate n

olivine

plag

olivine: n=1.64-1.88 plag: n=1.53-1.57 epoxy: n=1.54

- Olivine has high relief - Plag has low relief

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Mineral properties: interference color/birefringence

Olivine - high

Clinopyroxene - moderate-high

Calcite - v. high washed out colours

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Mineral properties: interference colors/birefringence

Orthopyroxene - low-moderate, 1st order grey-orange

Quartz -low, -1st order grey-yellow

•Plagioclase - v. low

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Mineral properties: interference colors/birefringence • Colors one observes when polars are crossed (XPL) • Color can be quantified numerically: δ = nhigh - nlow

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Use of interference figures In conoscopic light with Bertrand lens in: You will see a very small, circular field of view with one or more black isogyres -- rotate stage and watch isogyre(s)

uniaxial If uniaxial, isogyres define cross; arms remain N-S/E-W as stage is rotated

biaxial

or

If biaxial, isogyres define curve that rotates with stage, or cross that breaks up as stage is rotated

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determining optic sign

blue in NE = (+)

Gypsum plate has constant Δ of 530 nm = 1st-order pink Isogyres = black: Δ=0 Background = gray: Δ=100 Add or subtract 530 nm: 530+100=630 nm = blue = (+) 530-100=430 nm = yellowish = (-) Addition = slow + slow Subtraction = slow + fast

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Biaxial interference figures

There are lots of types of biaxial figures… we’ll concentrate on only two

1. Optic axis figure - pick a grain that stays dark on rotation

Will see one curved isogyre

estimate 2V from curvature of isogyre

90° 60° 40°

See Nesse p. 103

determine sign w/ gyps (+) (-)

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Use of interference figures, continued… Now determine the optic sign of the mineral: 1. Rotate stage until isogyre is concave to NE (if biaxial) 2. Insert gypsum accessory plate 3. Note color in NE, immediately adjacent to isogyre -- § Blue = (+) § Yellow = (-)

uniaxial

biaxial

(+)

(+)

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Mineral abundances •  Modal Estimate •  Once you have

identified the minerals, estimate their relative abundances

•  Plotting the mineral abundances (e.g., on a ternary diagram) will help in determining the rock type

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Crystallization Sequence

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•  Now that we have identified the minerals and determined the rock type…

•  It is time to figure out how the rock formed!

Crystallization Sequence

What minerals came first?

Paragenesis - a formational order of equilibrium assemblages of mineral phases.

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Deformation Sequence

What minerals came first? What condition did the minerals crystallize in?

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Deformation Sequence

What minerals came first? What condition did the minerals crystallize in?

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Deformation Sequence

What minerals came first? What condition did the minerals crystallize in?

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Deformation Sequence

What minerals came first? What condition did the minerals crystallize in?

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Review – techniques for identifying unknown minerals

Start in PPL: • Color/pleochroism • Relief • Cleavages • Habit

Then go to XPL: • Birefringence • Twinning • Extinction angle • Uniaxial or biaxial? • 2V if biaxial • Positive or negative?

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Go to Nesse (or similar book…)

• Chemical formula • Symmetry • Uniaxial or biaxial, (+) or (-) • RIs: lengths of indicatrix axes • Birefringence (δ) = N-n • 2V if biaxial

Diagrams: * Crystallographic axes * Indicatrix axes * Optic axes * Cleavages * Extinction angles

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Another example

Then read text re: color, pleochroism, habit, cleavage, twinning, distinguishing features, occurrence – make sure properties match your observations. If not, check another mineral…

Crystallographic axes: a, b, c

Indicatrix axes: X, Y, Z or ε, ω

Optic axes

Cleavages

Extinction angles

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On to real rocks…

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Important Lab Information •  Be on time •  Order of lab:

– Hand in assignment (current definitions, previous lab exercise)

– Current lab intro (5-15 mins) – Complete lab assignment (due at the beginning

of next lab) •  Arrive prepared (read lab assignment, bring

textbooks, etc.) •  If you are going to miss a lab, let all TA’s

and Prof. D L-M know ahead of time"

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Important Lab Information •  Required materials

• Winter (2002 or 2009) text: An intro to igneous and metamorphic petrology • Nesse: Intro to optical mineralogy •  Current lab handout (print or on computer): available on course website •  Recommended: Camera SD card (for storing photomicrographs) – these are cheap (<$10.00 on Amazon)

•  Quizzes: Will not be every week – only 2-3 throughout the semester

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Important Lab Information •  First assignment due next week – mantle

xenolith definitions •  All assignments must be submitted in a

digital format (pdf, doc) to all TA’s • Mike Gadd: mggadd3@gmail.com •  Zoe Braden: zoe.braden@gmail.com •  Justin Drummond: jbrdrummond@hotmail.com

•  Late assignments will not be accepted!

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