Study of Rocks 1) Field outcrop observe relationship between rocks preliminary identification of large minerals generalized rock composition and type take samples 2) Microscopic determination mineralogy textural relationships rock composition, type origin and history 3) Other analytical techniques such as Electron Microprobe, ICPMS, Scanning Electron Microscope X-ray diffraction Isotopic analysis Mineral spectroscopy More detailed understanding of origin and history of rock PPM -84.0 -92.0 -100.0 NMR

Petrographic Microscope Ocular Lens Petrographic Microscope Upper Polarizer Objective Lens Stage Focus Substage Assembly Including lower polarizer Light and blue filter

Thin section Thin rectangular slice of rock that light can pass through. One side is polished smooth and then stuck to a glass slide with epoxy resin The other side is ground to 0.03 mm thickness, and then polished smooth. May be covered with a thin glass cover slip 0.03 mm

Properties of Light Light travels as an electromagnetic wave In a solid, liquid or gaseous medium the electromagnetic light waves interact with the electrons of the atom. Direction of Travel (wavelength) (Amplitude)

Plane Polarized light (PPL) In air, light normally vibrates in all possible directions perpendicular to the direction of travel (A) Plane Polarized Light vibrates in one plane (B) PPL is produced by substage polarizer which stops all other vibration directions

Crossed Polars A second polarizer can be inserted above the stage, perpendicular to the substage polarizer. In air or an isotropic medium, it will stop light from first polarizer Isotropic garnet in XPL Isotropic garnet in PPL

Passage of Light (1) Reflection from an external or internal surface. Angle of incidence (i) = angle of reflection (r) i r

(2) Refraction The velocity of light depends on the medium through which it passes Light is an electromagnetic wave which interacts with electrons The distribution of electrons are different for each material and sometimes for different directions through a material When light passes from one medium to another there is a difference in velocity Light rays apparently bend at the contact Angle of incidence ≠ Angle of Refraction. i r i r

Refractive Index The amount of refraction is related to the difference in velocity of light in each medium. Refractive index (R.I.) for air is defined as 1 The absolute refractive index for a mineral (n) is the refraction relative to that in air. depends on the atomic/crystal structure is different for each mineral is constant for a mineral is a diagnostic property of the mineral between 1.3 and 2.0 There may be one, two or three values of R.I. depending on the atomic structure of the mineral.

Deer, Howie and Zussman Refractive Indices are listed for rock- forming minerals in D.H.Z. as n (isotropic), ε ω (uniaxial) or α β γ (biaxial). δ (birefringence) is the maximum difference between values of R.I. Garnet Group

Opaque Mineral Sulphides and oxides PPL does not pass through Minerals looks black in PPL regardless of orientation of mineral or polarizers Mineral cannot be identified in transmitted light; needs reflected light Opaque mineral in granite Rotated 45o in PPL

Transparent mineral PPL passes through the 30μm thickness of the thin section The electromagnetic light waves interact with the electrons in the minerals and slow down The higher the density of electrons the slower the light wave travels CPX in gabbro PPL

Becke Line A white line of light between two minerals allows the Relative Refractive Index (R.R.I.) to be measured This is relative to an adjacent medium which can be glass, epoxy, or another mineral R.I. epoxy: 1.54 to 1.55 The edge of the grain acts like a lens distorting the light Perthite: Microcline with exsolved albite showing Becke Line between the two minerals (PPL)

To measure relative refractive index of two touching minerals or mineral/epoxy Use PPL (upper polarizer out) Partly close the substage diaphragm, reducing light by 50-75% Slightly raise and lower the microscope stage, observing the movement of the Becke Line at boundary of grain. When decreasing the distance between the ocular and the stage, (raising the stage) the line moves into the material of lower R.I.

Relief Apparent topographic relief of mineral grains caused by differences in R.I. Positive relief - high R.I. Negative relief - low R.I. R.I. epoxy = 1.54 to 1.55 Apatite R.I.= 1.624, 1.666 In quartz R.I. = 1.544, 1.553 PPL

Cleavage Parallel cracks in mineral related to crystal structure, often diagnostic of a mineral In thin sections cleavage is developed during grinding of thin section Note how many directions of cleavages are present Measure the angle between cleavages or between cleavage and some mineral feature e.g. edge of grain, extinction. Amphiboles e.g. hornblende ~ 54o/126o Plagioclase: ~90o Pyroxene e.g. augite ~ 90o;

Fracture: Irregular cracks not related to atomic structure e.g. olivine Olivine in gabbro (PPL)

Metamict Texture Intense fracturing cause by radiation Disruption of crystal lattice can decrease optical properties The mineral may appear isotropic Zircon Allanite

Colour in PPL Due to absorption of selective wavelengths of light by electrons e.g absorption of red gives a green colour May be diagnostic of the mineral e.g. green chlorite Beware: biotite and hornblende may be either brown or green Brown biotite in granite Green chlorite in granite Green/blue hornblende in amphibolite

Isotropic Minerals Isometric (cubic) minerals e.g. garnet, halite Amorphous materials: glass, epoxy resin, air Atomic structure is the same is all directions Light travels through the mineral with equal velocity in all directions Refractive Index: one value (n) regardless of orientation NaCl a1 a2 a3 a1 = a2 = a3 α = β = γ = 90o

Between crossed polars Isotropic minerals always look black regardless of orientation of crystal or rotation of stage Garnet rotated in XPL

Indicatrix An imaginary figure which indicates the vibration directions and size of refractive index The length of a semi-axis shows the size of R.I. in that direction through the mineral For isotropic minerals, R.I. (n) and hence the length of the indicatrix semi-axes are the same for all directions through the mineral Therefore, the indicatrix for isotropic minerals is a sphere with only one value of R.I. (n) Isotropic Indicatrix n

Isotropic Minerals Colour in PPL may be diagnostic Absorption of light is the same in all directions so the colour will be the same regardless of orientation of crystal and remains constant when stage is rotated Cleavage: rare but fracture common Always in extinction between crossed polars PPL Garnet in metasediment XPL