Lab_13_15_optics_qz-feldspar_amph_px_2023

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Dec 6, 2023

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Name: Labs 13 – 15 : Optics: Quartz and Feldspars, Pyroxenes and Amphiboles Due Monday October 30 by noon on bCourses At the end of these labs, you should understand the following aspects of optics: i) Refractive index, and how it is important for determining relief (including + and –) ii) Interference colors and how birefringence is used in mineral identification iii) How to obtain and analyze an interference figure. How to determine + sign and – sign iv) How to use the type of interference figure and its sign to identify minerals. Important information: Isotropic: cubic Uniaxial: tetragonal, trigonal, hexagonal Biaxial: orthorhombic, monoclinic, or triclinic When asked to describe optical properties of a mineral, include: ● Crystal morphology (general description, e.g., equidimensional, rod shaped, etc.) ● Relief (high or low, positive or negative) ● Cleavage, and any relation of cleavage to morphology (crystal shape) if the morphology well defined ● Is it isotropic or anisotropic? If isotropic, all crystals in all orientations remain dark under cross-polarized light as the stage is rotated. If not isotropic (anisotropic), then crystals will go black as you rotate the stage under plane polarized light. ● Uniaxial or Biaxial? o Distinguish between these based on an interference figure (see below) o If biaxial, estimate 2V (see below) ● What is the maximum birefringence? (1) Thin section 104-397 containing garnet i) Is garnet isotropic or anisotropic? To answer this you need to view a selection of crystals under cross-polarized light and rotate the stage. ii) What is the crystal system of this mineral? 1
iv) What is the shape of the indicatrix? (The shape that describes how RI (refractive index) varies with direction in the crystal): v) Does it have a high or low RI value? To answer this, compare the relief to that of epoxy or quartz. If possible, state positive or negative relief relative to quartz/epoxy. 2) The interference figure is used to constrain the crystal system and provide information useful for identification. To obtain an interference figure : Survey the thin section: identify a grain very with low birefringence (one viewed down an optic axis) – it should stay dark as your stage is rotated (not isotropic, has just been cut down the optic axis). a) Focus your view. Now CAREFULLY insert the 40x objective (rotate the turret, don’t touch the objective) and CAREFULLY focus using the inner focus knob (turn it gently!). b) Insert the condenser lens (below the stage) and make sure this lens is as close to the thin section as possible without touching by using the knob on the left of the stage to adjust. As shown below, this will condense light into a spot on the thin section. c) The 2 nd polarizer should already be inserted d) Insert the Bertrand lens (round knob between the eye pieces, labeled “B”). e) Look. You should see an illuminated circle with a dark band or a cross. If ‘ uniaxial ’ you should see isogyres (dark lines in the interference figure) that form a cross, as shown in (A). The center of the cross may move around as you rotate the stage but there should always be a cross. After inserting the gypsum plate : for a uniaxial positive mineral you will see yellow in the first (top left) and third (counting clockwise) quadrants, blue in the second and fourth quadrants, as shown in (B); and for a uniaxial negative mineral you will see the opposite color pattern, shown in (C) 2
If “biaxial” you should EITHER see something similar to (A), but as you rotate the stage the isogyres that make the cross will separate and rejoin (small 2V as shown in (D)) OR you will just see one curved isogryre (large 2V as shown in E). Insert the gypsum plate and use the same information as for B and C to determine if biaxial positive or negative. You can estimate the “2V” of biaxial minerals from the curvature of the isogyres using this chart Framework silicates: quartz and the feldspars (1) A commonly occurring silica-rich mineral is quartz. The transition from α to β quartz upon heating is reversible and rapid, so β -quartz is never found below the transition temperature. 3
Examine the hand samples of quartz. Note that color is not a good way of telling quartz from other minerals. The lack of cleavage (conchoidal fracture) is a very useful characteristic for distinguishing quartz from feldspar. Examine well-formed crystals of quartz. How many “forms” do the crystals display? Thin section 104-299 (quartz-rich rock called quartzite) Obtain an interference figure for quartz to determine whether quartz is uniaxial or biaxial, positive or negative. Follow the instructions above. Sketch the interference figure before and after inserting the gypsum plate (annotate your sketch, and using color may help): Thin section 102-54 (granite rock). Identify quartz based on its lack of color, low relief, low birefringence and lack of twinning. Find a grain that remains dark as you rotate the stage under cross-polarized light and obtain an interference figure (this is a grain viewed down the optic axis). Sketch the interference figure. Is it the same as for 104-299 ? Examine the models for α - (low) and β - (high) quartz , the low and moderately high temperature polymorphs, respectively. Note how the SiO 2 tetrahedra are arranged in each. For each, sketch one tetrahedron and all the tetrahedra to which it is linked (showing the connections). 4
α - (low) quartz β - (high) quartz, Examine the model for a high-pressure form of SiO 2 , coesite. Identify the rings consisting of 4-tetrahedra (two pointing up, two down) in chains. Sketch how they are connected to form chains of 4-membered rings. (2) Examine a phase diagram for SiO 2 (lecture notes or internet). What would you conclude about the conditions experienced by a rock if it contains coesite? Two examples of processes that might create these conditions are: What polymorphs would you expect to find in volcanic lavas (very hot, low pressure rocks)? 5
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