Science & Art 1st session SVT: Rock Art

Photo : E. Lacouture

This picture shows a thin section of eclogit observed with polarized light. Polarising microscope is a great tool for rock studies. It also reveals beauty of crystals. How do scientists use this microscope? How can we identify that way minerals and rocks? Work session objectives 1- Watch the video about polarisation and answer the questions 2- Complete the schematics on your paper and use the two filters to explain to your partner the physical principle of polarisation. 3- Watch a rock thin section with polarizing petrographic microscope 4- Identify main minerals with teacher's help and take a photo with a tablet or a mobile 5- Write a short text (in English) to explain what you're observing (name of the rock, group of the rock, main minerals it contains) and how you got it (process used) Scientific report: 3 of you will be chosen randomly to prepare a short video report about rock polarization with ipad. This group will present his work to the others during the last 15' and will have to use props given by the teacher (2 polarizing filters, one calcite crystal, a piece of sheet, a pen, a petrographic microscope and a rock thin section). No more than 1'30 for the video but no less than 1'. In a group, you have to cooperate (a cameraman and 2 reporters sharing time speaking). Organisation: in group of 3 / for 1h30 NB : don't forget to save your work on a memory stick in order to print it for the exhibition ...Did you know 2014 is the year of crystallography?

Document 1: polarizing microscope

The petrographic microscope is a wonderful analytical tool and has been around for 140 years. An innovative, wealthy English gentleman by the name of Henry Sorby (above), invented this microscope and spent much of his leisure time looking at paper-thin slices of mostly meteorites glued to glass slides (rock thin sections). Naturally, he is also known as the "Father of Petrography," the study of rocks in thin section. How does it work? As an analogy, the polarizers work like Polaroid sunglasses. Light actually vibrates in all directions perpendicular to its pathway. Light passing through the Polaroid lens is allowed to vibrate in only one direction (is plane-polarized), which reduces the sunlight glare. If you take the lenses out of the frames and place one lens over the other and rotate one lens 90°, no light will pass through and everything is dark. In the case of the microscope, plane polarized light from the first lens is passed through the crystals in a very thin section of rock (25-30 micrometers). As it does so, the plane-polarized light is twisted a bit and breaks up into two directional components before it enters the upper polarizer. One of the directional components is slower than the other one and they become "out of phase" when they are resolved in the upper polarizer. This difference in velocity is called retardation and results in the production of colors as shown in the photomicrographs labeled "crossed polarized light" or XPL. From http://www4.nau.edu/meteorite/Meteorite/Book-Tools.html Text vocabulary: to be around for: existe depuis wealthy: riche thin section: lame mince glare: éclat lens: lentille (filtre) twisted: dédoublé to break up: se séparer, se diviser out of phase: déphasé Microscope vocabulary: to magnify: grossir (au microscope) to get closer: rapprocher to get away: éloigner to hold: tenir a glass slide: une lame de verre to focus on: faire la mise au point sur to adjust: régler

Document 2: what are rocks, minerals and elements? Rocks are a composed of one or more minerals. A rock can be made up of only one mineral or, as shown in the figure, a rock can be made up of a number of different minerals. So, rocks are composed of one or more minerals and minerals are composed of one or more elements.

From : http://ratw.asu.edu/aboutrocks_whatarerocks.html

Vocabulary to split into: diviser matter: matière stacked: entassé, empilé, aggloméré neatly: parfaitement

Main minerals observed

With one polarizing lens

With two polarizing lenses set at right angles to each other.

Quartz

white

Rather stocky crystals, from white to dark when turning the stage

Feldspar

white

Rather long crystals with parallel lines, from white to dark when turning the stage

Biotite

Light brown, pleochroic

From light brown to dark brown, some radiohalos

Olivine

White with dark crackles

Various colours (blue, red, green, pink...)

Pyroxen

Durty white

Generally orange with variations

Grenat

White with dark crackles

Ever dark even if you turn the stage

Oxides

Dark

Remain dark

Document 3: main groups of rocks

http://www.stonesurfaces.ie/userfiles/images/rock_genesis.png

Questions about the video 1) Air and water are isotropic: what does it mean?

2) When can a light ray be refracted?

3) What's birefringence phenomenon? When does it occur?

4) Why birefringence is a diagnostic property for minerals?

5) Without any rock section onto the stage, what should we see in the ocular if both polarizing filters are set at right angle to each other? Why?

Fill in the frames of the schematic above with the following words: Polarizer / Analyzer / Anisotropic crystal / White light / Birefringence / Retardation / Plane polarized light

http://www.polaroidsunglasses.co.uk/blog/2009/07/polarized-sunglasses-explained/