This Lab will not be marked so make sure to get anything you are unsure about checked by your TA!

Transcription

1 Lab One. Microscope Refresher and Advanced Microscope Techniques Learning Goals. After this Lab, you should be able: Demonstrate the operation of the petrographic microscope in plane and crossed polarized light Determine how many minerals are present in a thin section and describe their optical properties Identify minerals based on their optical properties Distinguish between the positively and negatively elongated minerals Adjust your microscope into conoscopic mode to observe interference figures Distinguish between uniaxial and biaxial minerals Determine optic sign of anisotropic minerals This Lab will not be marked so make sure to get anything you are unsure about checked by your TA! I. The scope! It s been a long summer For these labs we have one type of microscope which has the same common features as the ones used for EOSC 221, familiarize yourself with this type. You will be assigned a locker and microscope for the term, you are responsible for these ~ $10,000 instruments, make sure they are safely locked away when you have finished using them and report all problems to your TA s immediately. The microscopes will be available throughout the term for out of hours lab practice, however, most labs should be completed in lab time using the TA s as a resource!! Using the diagram below and your own microscope as a guide, work out what all the parts of the microscope are and label them. NB you may want to check back to your EOSC 221 notes for the correct names for all the parts! 1

2 All parts of your microscope should function smoothly and easily. NEVER USE FORCE! Ask your TA for help if your microscope is not operating properly. It is your responsibility to ensure that your microscope is in good working order at the start of all labs. 2

3 Activity II. Mineral Refresher For this course we have provided a series of reference thin sections containing some of the more common minerals you will encounter. Before we tell you what minerals they contain take a look at two thin sections from the list and use the following tables to identify how many minerals are present, what they are and how abundant they are. Thin sections: 37; 101; 87; 108; 105; 135; 56; 208; 150; 148; 1535; 166; 75; P2371; 45; 79; 59; 158; 49; 42; 55; 210; 23; 72; 14 Thin Section number Min. Relief Colour Pleochroism Cleavage Birefringence Extinction Elongation Mineral %, size, features

4 Thin Section number Min. Relief Colour Pleochroism Cleavage Birefringence Extinction Elongation Mineral %, size, features

5 Activity III. Evaluate the sign of elongation in minerals. Sign of elongation. A sign of elongation is an optical property useful for identification of prismatic minerals. Minerals for which the sign of elongation is a good distinguishing feature have hexagonal or tetragonal symmetry. Such minerals commonly have parallel extinction. How to determine the sign of elongation: 1, Find a grain whose cross-section is prismatic (rod-like, or needle-like) and goes extinct when the elongation is parallel to the cross hairs 2. Find the slot for the accessory plate and note its orientation (usually SW- NE) 3. Rotate the stage so the grain is parallel to the slot. In this position, the grain should have the brightest interference colour. 4. Write down the interference colour of the grain at this orientation 5. Insert the accessory plate and see if the interference colour of the mineral becomes higher or lower. You can calculate the two colours expected (higher and lower) by adding or subtracting 550 nm (the retardation of the accessory plate) from the value of birefringence estimated for the mineral. To do this, use the Michel-Levy chart on page If interference colours are of higher order as compared to the initial interference colour of the mineral, we call this mineral negatively elongated. 7. If interference colours are of lower order as compared to the initial interference colour of the mineral, we call this mineral positively elongated. The theory that explains these observation is based on the fact that when anisotropic crystals exhibit a prismatic habit (i.e. shows elongation in one direction), polarized light passing through the crystal with polarization parallel to the long dimension will not travel at the same velocity as light polarized in other directions. The direction parallel to the longest axis of the crystal (axis c) is clearly unique. This distinction allows the prismatic minerals to be divided into two types: length fast (faster light vibrates parallel to long dimension, negative elongation) and length slow (slower light vibrates parallel to the long dimension, positive elongation). Determine the sign of elongation in glaucophane or in the elongate mineral pleochroic from blue to green in T/s M What type of extinction does it have? 5

6 Activity IV. The method of conoscopy and observing interference figures of minerals. Anisotropic minerals under the microscope can be characterized by assigning them into two classes-- biaxial and uniaxial minerals. These characteristics depend on the symmetry of the minerals and are usually listed in mineral identification flow charts, like the one on page 40 of your lab textbook. For uniaxial minerals, we need two indices of refraction to describe the mineral s refractive index. For biaxial minerals, we need three. An optic axis is a direction through a crystal along which no double refraction occurs. One can determine whether a mineral has 1 or 2 optic axes by observing an interference figure of the mineral. Furthermore, conoscopy classifies minerals into optically positive and optically negative. Optic sign describes the relationship between indices of refraction with respect to the symmetry of the crystal. To observe interference figures, you should: 1. Find a grain oriented with the optic axis vertical. Such grains will appear isotropic (black or almost black in XPL) because optical properties in all directions in this cross-section are similar. These grains will also have the lowest order interference colours. Initially, grains in this orientation will be provided for you in an oriented thin section, but eventually you will have to find these in thin sections yourself. 2. Focus on the grain in low power (crossed polars), move up to medium power and check focus, then move up to high power and double check focus (make sure you are not focused on a crack or impurity in the grain). Be *extra* careful when focusing in high power! Always use the fine adjustors! Go slow and watch what you re doing! It is very easy to shatter thin sections this way! 3. Insert the Bertrand lens and the condensing lens. A centered optic axis figure should appear. 4. Insert the accessory plate to determine the optic sign of the mineral. If the NW-SE corners turn yellow, the mineral has a positive optic sign, if the NW- SE corners turn blue, the mineral has a negative optic sign. 6

7 a. Quartz c. Look at Quartz c in orthoscopic mode, low power, crossed polars. How can you tell that these mineral cross-sections are good for conoscopic observations? (hint: they should all appear isotropic as all grains oriented c). b. Examine Quartz c using conoscopic illumination to find an interference figure. Make two coloured drawings showing (1) the interference figure and (2) the interference figure with the accessory plate inserted. Compare your drawings with figures on page 21, 22 and 26 of the textbook to determine if quartz is uniaxial or biaxial and what its optic sign is. c. Barite. Find an interference figure using the techniques described above. Determine the following: biaxial or uniaxial? positive or negative? Sketch the (1) interference figure and (2) the interference figure with the accessory plate inserted. NOTE: in the real world (i.e. as a geologist) you will not be given oriented thin sections of minerals such that you will easily find perfect interference figures. You will *mostly* find that interference figures may not appear centered. A useful tip from a trained petrologist: When viewing an uncentered interference figure, if you see parallel movement of the isogyre, the mineral is uniaxial, if you see a change in curvature of the isogyre, the mineral is biaxial. 7

8 Activity V. Fingerprinting minerals with observations of multiple optical properties Examine another of the reference thin sections. Using your newly developed optical property skills, determine how many different minerals there are in this thin section and complete the following table. Concentrate on the rock forming and/or phenocrystal minerals and ignore any fine-grained groundmass. Thin Section number Min. Relief Colour Pleochroism Cleavage Birefringence Extinction Elongation 1 Uniaxial or biaxial Optically Positive or negative

9 Based on your observations write a full description for this thin section including mineralogy, abundance, grain size, textural features etc. What is the proper name for this rock? Sketch a field of view to illustrate the key features of the rock. Magnification Field of view width Description: Rock name: 9