A Pendahuluan Minop

Mineral optik2 SKS teoriby:hill. gendoet hartonoSemester 3, 2009-2010

Selasa, jam 09.50 10.40 jam 10.40 11.35

The Petrological Microscope

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The use of the Petrological Microscope

The use of the microscope allows us to examine rocks in much more detail. For example, it lets us :-

examine fine-grained rocks examine textures of rocks distinguish between minerals that are otherwise difficult to identify in hand-specimen (e.g. the feldspars)

Deer,W.A., Howie,R.A. & Zussman, J., 1978, An Introduction to the Rock Forming Minerals, Longman Group Ltd., London, 528 p.

Philpotts A.R., 1989, Petrografi of Igneous and Metamorphic Rocks, Prentice-Hall, Inc, Engewood Cliffs, New Jersey, 179 p.

Williams, Turner, F,J. & Gilbert, C.M., 1954, Petrography : An Introduction to the Study of Rocks in Thin Sections, W.H. Freeman & Co., San Francisco, 406 p.

Hatch, F.H; Wells, A.K., & Wells, M.K., 1972, Petrology of the Igneous Rocks, George Allen & Unwin (Publishers) Ltd., 13 Ed. 551 p.

Jones, N.W., & Bloss, F.D., 1980, Laboratory Manual For Optical Mineralogy, Burgess Publishing Company.

Pustaka

A petrological microscope

The petrological microscopediffers from an ordinary microscope in two ways:

it uses polarised lightand the stage rotates

There are two sheets of polaroid: the one below the stage of the microscope is the polariser, the other, above the stage, is the analyser. The analyser can be moved in and out.

Most rocks cut and ground to a thickness of 0.03mm become transparent.lenseyepiecefocuslight sourceanalyserpolariserrotating stagefine focus

Preparing thin sections

Rock specimens are collected in the field, then cut into smallthin slabs. These are glued on to glass slides and grounddown to 0.03mm thickness. At this thickness all rocksbecome transparent. Only a few minerals, mainly oreminerals, remain opaque, i.e. stay black under PPL.

If the sections are too thick, the polarisation colours areaffected. Quartz is used to check thickness for this reason see the next slide

The colours appear in a series of repeated rainbows across the chart and amineral may show any colour up to a maximum, reading from the left.quartzfeldsparcalciteolivineamphibolepyroxenebiotite muscoviteRead along 0.03mm line to the highest order colour seen in the mineralRead along diagonal to top for mineral name

Identifying MINERALS in thin section

When a slide is examined under the microscope, it is important to identify any mineral properties under plane polarised light (PPL) first (analyser out); then proceed to crossed polars (XPL) where the two polaroid sheets are at right angles to each other (analyser in).

Mineral properties under PPL

colour (natural colour)transparency (clear, cloudy or opaque)relief (high or low)crystal or fragment shapecleavage fracturepleochroism (colour change when stage is rotated)

1) Light passes through the lower polarizerUnpolarized light Jane Selverstone, University of New Mexico, 2003

2) Insert the upper polarizerwest (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) Jane Selverstone, University of New Mexico, 2003

Note how the olivine with its high relief stands out from the surrounding low relief plagioclaseRELIEFPPLplagioclaseolivine

Mineral properties in PPL: 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

garnet:n = 1.72-1.89quartz:n = 1.54-1.55epoxy:n = 1.54Quartz has low reliefGarnet has high relief

Mineral properties in PPL: 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

Jane Selverstone, University of New Mexico, 2003

What causes 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 Jane Selverstone, University of New Mexico, 2003

Two sets of cleavage are seen in this amphibole crystal; note the 120o angle between the cleavagesCLEAVAGEPPLamphibole1st set run parallel to line2nd set of cleavage

The olivine here shows uneven fractures which appear dark grey in the crystalFRACTUREPPLolivine

The biotite shows its distinct brown shades under PPL against the clear colourless quartz and feldsparCOLOURPPLbiotiteamphibole

PLEOCHROISMTwo views under PPL showing colour change in biotite on rotating the stage. PPLbiotiterotated 90o

Mineral properties under XPL

interference colours

(under XPL the colours seen are not the natural colours of the mineral but those caused by the interference of two refracted beams of light passing through an anisotropic mineral ; they are called interference colours)extinction angle

(as the stage is rotated, each anisotropic mineral goes extinct every 90o; in cases where there is cleavage in the mineral it is possible to measure the angle of extinction relative to the crosswires)twinning

(may be seen in coloured minerals under PPL, but most obvious under XPL, especially with regard to the feldspars)

Now insert a thin section of a rock in XPLwest (left)east (right)Light vibrating E-WHow does this work??Unpolarized lightnorthsouth Jane Selverstone, University of New Mexico, 2003

Conclusion has to be that minerals somehow reorient the planes in which light is vibrating; some light passes through the upper polarizerBut, note that some minerals are better magicians than others (i.e., some grains stay dark and thus cant be reorienting light)olivineplagPPLXPL

Interference colourswhite/grey/black in quartz, microcline and plagioclasemuch brighter colours of ferro-magnesian minerals including amphibole, pyroxene, olivinepearly grey shades of calcitequartzamphibolecalcite

Liquids, gases, amorphous solids such as glass, and isotropic minerals (isometric crystal system) stay black in all orientationsReview: With any isotropic substance (spherical indicatrix), when the analyzer is inserted (= crossed-nicols or XPL) no light passes extinct, even when the stage is rotatedNote: the gray field should also be extinct (glass and epoxy of the thin section are also isotropic), but is left lighter for illustration

Rotating the stage

Anisotropic minerals with an elliptical indicatrix section change color as the stage is rotated; these grains go black 4 times in 360 rotation-exactly every 90o

Consider rotating the crystal as you watch:B = polarizer vibration direction parallel e only E-ray Analyzer in extinct

C = polarizer vibration direction || w only O-ray also extinct with analyzerewwe