Mineralogy geophysics-optical-min-1

Mineralogy geophysics-optical-min-1

• 1.
  Mineral Optik Pemakaian mikroskoppetrografi Untuk identifikasi mineral I Wayan Warmada
• 2.
  Mengapa memakai mikroskop? •Identifikasi mineral (no guessing!) • Menentukan tipe batuan • Menentukan sekuen kristalisasi • Mendokumentasi sejarah deformasi • Mencata pelapukan/alterasi • Fun, powerful, and cheap!
• 3.
  Mikroskop petrografi Juga disebut sebagaimikros- kop polarisasi (PLM, polariz- ing light mi- croscope)
• 4.
  Apa yang terjadijika cahaya melalui “scope”? sumber cahaya mata anda sinar cahaya gelombang berjalan dari sumber ke mata panjang gel, amplitude, A sinar berja- lan sebagai gelombang
• 5.
  Microscope light iswhite light, i.e. it’s made up of lots of different wavelengths; Each wavelength of light corresponds to a different color Can prove this with a prism, which separates white light into its constituent wavelengths/colors Apa yang terjadi jika cahaya melalui “scope”?
• 6.
  light vibrates in allplanes that contain the light ray (i.e., all planes perpendicular to the propagation direction plane of vibration vibration direction propagation direction Apa yang terjadi jika cahaya melalui “scope”?
• 7.
  1) Cahaya lewatpada polarizer bagian bawah west (left) east (right) Plane polarized light PPL=plane polarized light Unpolarized light Only the component of light vibrating in E-W direction can pass through lower polarizer – light intensity decreases
• 8.
  2) Gunakan polarizerbagian atas west (left) east (right) Now what happens? What reaches your eye? Why would anyone design a microscope that prevents light from reaching your eye??? XPL=crossed nicols (crossed polars) south (front) north (back) Black!!
• 9.
  3) Sekarang masukkanasahan tipis suatu batuan west (left) east (right) Light vibrating E-W Light vibrating in many planes and with many wavelengths How does this work?? Unpolarized light Light and colors reach eye!
• 10.
  Conclusion has tobe that minerals somehow reorient the planes in which light is vibrating; some light passes through the upper polarizer But, note that some minerals are better magicians than others (i.e., some grains stay dark and thus can’t be reorienting light) Minerals act as magicians!!
• 11.
  • Isotropic minerals:light does not get rotated or split; propagates with same velocity in all directions • Anisotropic minerals: • Uniaxial - light entering in all but one special direction is resolved into 2 plane polarized components that vibrate perpendicular to one another and travel with different speeds • Biaxial - light entering in all but two special directions is resolved into 2 plane polarized components… • Along the special directions (“optic axes”), the mineral thinks that it is isotropic - i.e., no splitting occurs • Uniaxial and biaxial minerals can be further subdivided into optically positive and optically negative, depending on orientation of fast and slow rays relative to xtl axes A brief review…
• 12.
  Isotropic Uniaxial Biaxial How light behavesdepends on crystal structure (there is a reason you took mineralogy!) 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 Let’s use all of this information to help us identify minerals
• 13.
  Sifat-sifat mineral: warna& pleokrisme • Color is observed only in PPL • Not an inherent property - changes with light type/intensity • Results from selective absorption of certain  of light • Pleochroism results when different  are absorbed differently by different crystallographic directions - rotate stage to observe plag hbl plag hbl -Plagioclase is colorless -Hornblende is pleochroic in olive greens Now do question 2
• 14.
  Sifat-sifat mineral: Indeksrefraksi (R.I. or n) Light is refracted when it passes from one substance to another; refraction is accompanied by a change in velocity n1 n1n2 n2 n2>n1 n2<n1 n = velocity in air velocity in mineral • n is a function of crystallographic orientation in anisotropic minerals  isotropic minerals: characterized by one RI  uniaxial minerals: characterized by two RI  biaxial minerals: characterized by three RI • n gives rise to 2 easily measured parameters: relief & birefringence
• 15.
  Sifat-sifat mineral: 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
• 16.
  Apa yang menyebabkanrelief? nxtl > nepoxy nxtl < nepoxynxtl = nepoxy Hi relief (+) Lo relief (+) Hi relief (-) Difference in speed of light (n) in different materials causes refraction of light rays, which can lead to focusing or defocusing of grain edges relative to their surroundings Now do question 3
• 17.
  Sifat-sifat mineral: warnainterferensi/birefringence • Colors one observes when polars are crossed (XPL) • Color can be quantified numerically:  = nhigh - nlow More on this next week… Now do question 4