ESS 439 Lab 2 Examine Optical Properties of Minerals

Transcription

1 ESS 439 Lab 2 Examine Optical Properties of Minerals

2 The optical properties depend on the manner that visible light is transmitted through the crystal, and thus are dependent on mineral s Crystal Structure

3 Orthosilicates (isolated tedrahedra [SiO4]) Olivine Garnet Single Chain silicates Pyroxene Double Chain silicates Amphibole Sheet silicates Biotite Muscovite Framework silicates Orthoclase Quartz

4 The optical properties depend on the manner that visible light is transmitted through the crystal, and thus are dependent on mineral s Crystal Structure Crystal Symmetry

5 Garnet Isometric Olivine Orthopyroxene Clinopyroxene Orthorhombic Amphibole Biotite Monoclinic Muscovite K-feldspar Plagioclase Quartz Calcite Triclinic Trigonal

6 The optical properties depend on the manner that visible light is transmitted through the crystal, and thus are dependent on mineral s Crystal Structure Crystal Symmetry Chemical Composition

7 Orthosilicates (isolated tedrahedra [SiO4]) Olivine (Mg,Fe) 2 SiO 4 Garnet (Mg,Fe,Ca,Mn) 3 Al 2 (SiO 4 ) 3 Single Chain silicates Double Chain silicates Sheet silicates Framework silicates Pyroxene (Ca,Mg,Fe) 2 Si 2 O 6 Amphibole hydrous Na-Ca-Mg-Fe-Al Biotite K(Mg,Fe) 3 AlSi 3 O 10 (OH) 2 Muscovite KAl 3 AlSi 3 O 10 (OH) 2 Orthoclase KAlSi 3 O 8 Quartz SiO 2 Mafic minerals Mg, Fe (ferric) dark-colored dense high T (1000 C) Felsic minerals Fsp, silica (Qtz) light-colored less dense low T (600 C)

8 Which to buy? Polarized vs. Non-Polarized sunglasses? Non-polarized lenses have a dark shade and reduce the intensity of light. However, these lenses do not consider the direction in which light is coming. Polarized lenses are made with vertical polarization (thus only allow vertical waves to pass through). They are designed to reduce the glare by absorbing horizontal light waves, as sunlight reflected from flat surfaces (e.g. land, water) is often reflected back horizontally. Polarized sunglasses are a better choice!

9 How to test if your sunglasses are polarized? Only light with vibrational component that aligns with the orientation of the polarizers can pass. Plane Polarized Light (PPL) Cross Polarized Light (XPL)

10 How does petrographic microscope work? Crystals on a thin section can re-orient light waves.

11 Crystal symmetry leads to two broad division of minerals Isometric minerals (e.g., Garnet) are isotropic (same in all directions) They cannot re-orient light. They are always black in XPL. Anisotropic materials They are capable of re-orienting light.

12 Splitting of light in anisotropic medium When a ray of light enters an anisotropic medium, it is almost always split into two polarized waves perpendicular to each other. Both partial rays are characterized by different propagation rates due to different refraction indices.

13 Splitting of light double refraction Since the angle of incidence of the light is 0, there should be no refraction according to Snell s Law. But one of the two rays violates this law and got refracted. It hence is referred to as the extraordinary ray, or e-ray. The other light follows this law and did not get refracted. It is called the ordinary ray, or o-ray.

14 Refractive index Light is refracted ( bends ) when passes from one medium to another, which is accompanied by a change in velocity. R.I. = n = V vacuum /V mineral n will always > 1.0 since V mineral can never be greater than V vacuum. Most transparent minerals have n between 1.3 to 2.4. In general, V mineral decreases with increasing density of the mineral. Thus, higher density minerals will have higher n. Mineral s density strongly depends on chemical composition and crystal structure. For silicate minerals, n increases with increasing Ca, Ti, Cr, Fe, Mn contents. n is also higher in minerals packed tightly (e.g. orthosilicate vs framework silicate).

15 Orthosilicates (isolated tedrahedra [SiO4]) Single Chain silicates Double Chain silicates Sheet silicates Framework silicates Olivine (Mg,Fe) 2 SiO 4 Pyroxene (Ca,Mg,Fe) 2 Si 2 O 6 Amphibole hydrous Na-Ca-Mg-Fe-Al Biotite K(Mg,Fe) 3 AlSi 3 O 10 (OH) 2 Muscovite KAl 3 AlSi 3 O 10 (OH) 2 n = Orthoclase KAlSi 3 O 8 n = Quartz SiO 2 n = Garnet (Mg,Fe,Ca,Mn) 3 Al 2 (SiO 4 ) 3 n = 1.74 Mafic minerals Mg, Fe (ferric) dark-colored dense high T (1000 C) n = Felsic minerals Fsp, silica (Qtz) light-colored less dense low T (600 C) n =

16 Relief Relief refers to the relative difference in n between neighboring crystals. Examine the grain boundaries for the relief of a crystal. Minerals with relief higher than the resin have positive relief and vice versa. Quartz: n = Epoxy: n = ~1.54 Garnet: n = 1.74 Epoxy: n = ~1.54 Left: low relief of quartz. It can hardly be distinguished from the resin due to their similar n values. Right: high relief of garnet. It boundary appears extremely distinct and thick.

17 Mineral color Minerals that contain transition metal (Ti, V, Cr, Mn, Fe, Co, Ni, Cu) commonly display a strong color [multiple valance states facilitate electron transfer] Cr 3+ : green in diopside, emerald, grossular Fe 2+ : light green in olivine, blue in sapphire Fe 3+ : dark red, e.g. hornblende, almandine, biotite grossular emerald olivine diopside sapphire almandine

18 Biotite Amphibole OH- enhances the effects of Fe, resulting in strong color and pelochroism e.g. Biotite, Amphibole vs. Pyroxene does not contain OH-, it is near colorless.

19 Pleochroism (many colors) A mineral s absorption color changes when the stage is rotated. Caused by different crystal orientations. Common in minerals that display strong color in hand specimen.

20 Interference color The superposition of the two waves is called interference. The color mineral shows under XPL. It changes as we rotate the stage (PPL) (XPL)

21 The highest interference color of a mineral max birefringence (Δn) quartz calcite white/grey/black in Quartz and feldspar Mafic minerals including amphibole, pyroxene, olivine pearly grey/white shades of calcite

22 Extinction It describes when cross-polarized light dims (so it is observed under XPL) Every 90 rotation of the stage, anisotropic mineral will go through the maximum (brightest) and minimum (black) transmission of light. We call the minimum extinction.

23 Types of extinction Parallel extinction when the crystal s long direction/cleavage lines is oriented N-S or E-W, the mineral is extinct.

24 Types of extinction Orthorhombic orthopyroxene shows parallel extinction. (PPL) (XPL)

25 Types of extinction Inclined extinction mineral get extinct when their long direction/cleavage lines at an angle to N-S or E-W directions.

26 Types of extinction Monoclinic clinopyroxene shows inclined extinction. (PPL) (XPL)

27 Types of extinction Symmetrical extinction mineral goes extinct at the same angle to two sides of a mineral or its cleavage planes (e.g., amphibole, pyroxene).

28 Cleavage Most easily observed in PPL, but visible in XPL as well No cleavage: quartz, olivine, garnet Planer cleavage: mica

29 Cleavage Prismatic cleavages (amphiboles and pyroxene ) You have to be looking down the prism to see 2 crossing sets of cleavages, in many views you will see only one set of cleavages. Cleavage angle: 60/120 Cleavage angle: ~90

30 Twinning Twinning is a very common phenomenon in feldspars. It varies according to the composition and the crystal system. Carlsbad twinning: A pair of individual crystals separated by a single plane. Most common in K-feldspar, may also be present in plagioclase. Carlsbad penetration twin

31 Albite Polysynthetic (multiple) twinning Abundant in plagioclase Parallel sets of albite twins between the composition planes (010). Albite polysynthetic twin

32 Carlsbad-Albite compound twinning Common in plagioclase. The two halves of the Carlsbad twins may show albite twinning. The albite twins are oriented parallel to the Carlsbad twins. 32

33 Albite Pericline compound twinning (cross hatch twinning) Particularly frequent in microcline because the triclinic low temperature microcline often forms by transformation from monoclinic high temperature orthoclase, associated with progressive ordering of Al and Si during cooling. Albite polysynthetic twin Pericline polysynthetic twin

34 Zoning Caused by compositional change during crystallization. Common in solid solutions (e.g. plagioclase).