Geol 2311 11/28/06 Lab 18 – Optical Mineralogy – Optical Properties of Biaxial Minerals Due Date: Tues, December 4. Total Points 35 Objective: Become familiar with the optical properties of biaxial minerals. Procedure: View the thin sections underlined below and answer the related questions A) Optic signs slide with the Phlogopite mineral; Obtain an optic figure for this phase. What type of figure is it? __________________________ (1 pt) What is the optic sign? ______ (1 pt) What is the approximate 2V angle ________ (1 pt) What vibration axis are you looking down with this mineral orientation? __________ (1 pt; see Fig. 1) B) Optic signs slide with the Epidote mineral: Obtain an optic figure for this mineral. What type of figure is it? __________________________ (1 pt) What is the approximate 2V angle ________ (1 pt) C) Andalusite Bxo and Bxa figures slide: Obtain an optic figure for each orientation. What optic sign do you get from the Bxa figure? ________ (1 pt) What optic sign do you get from the Bxo figure? _______ (1 pt) They should be different; which is the correct one? _________ (1 pt) This exercise demonstrates that when the 2V angle gets close to 90º a Bxa is difficult to distinguish from a Bxo figure and therefore optic signs can be erroneous if the wrong figure is used. D) Sanidine Bxa or ⊥001-010: Obtain an optic figure for this phase. What is the optic sign? ______ (1 pt) What is the approximate 2V angle? ________ (1 pt) E.1) Celestite Bxa: Obtain an optic figure. What is the optic sign? ______ (1 pt) What is the approximate 2V angle? ________ (1 pt) E.2) Celestite Optic Plane: What two principal refraction indicies (α, β or γ) will occur in a section through the optic plane of the biaxial indicatrix? _____ & ______ (1 pt) What is the maximum birefringence of Celestite (see Klein p. 427)? ____________ (1 pt) Why aren’t the maximum interference colors consistent with this birefringence? ___________________________________________________________ (1 pt)E.3) Celestite Bxo: Obtain a Bxo optic figure. Note that the positions of the melatopes are centered outside the field of view and ringed by yellow isochromes. The 2V of this mineral is less than that for andalusite and thus the Bxo figure is more distinct than the Bxa figure. What is one of the more obvious difference between the Bxo and Bxa figures for Celestite? __________________________________________________________________________(1 pt) What does the “o” in Bxo stand for?___________________ (1 pt) F.1) Topaz Bxa: Obtain an optic figure. What is the optic sign? ______ (1 pt) What is the approximate 2V angle based on isogyre separation? ________ (1 pt) F.2) Topaz ⊥ One Optic Axis: Obtain an optic figure. What is the approximate 2V angle based on isogyre curvature? ________ (1 pt) Draw a picture of the isogyre curvature and the change in isochrome colors that indicate the optic sign (1 pt) G.1) Tremolite Bxa: Obtain an optic figure. What is the optic sign? ______ (1 pt) What is the approximate 2V angle based on isogyre separation? ________ (1 pt) G.2) Tremolite Optic Plane: What is the extinction angle between (110) cleavage and Z vibration direction? _______________(1 pt) Is the Z vibration direction the fast or slow vibration direction? ____________(1 pt) H.1) Hornblende ||100: Obtain an optic figure. What is the optic sign? ______ (1 pt) What is the approximate 2V angle? ________ (1 pt) H.2) Hornblende ||001: Note the 120° cleavage which is most visible when looking down the Z axis. Hornblende has three distinct pleochroic colors associated the three principle vibration directions X(or α) – yellow green; Y(or β) olive green; Z(or γ) dark green. What kind of extinction does the section display? ________________________ (1 pt) What two vibration directions are present in this section? _________& _________ (1 pt)Which is the fast ray? ________________ (1 pt) I. Sonju Lake Intrusion samples (SL-1 to SL-16). Select one of the sections from the thin section box and view with the instructor microscope. Scout for a pyroxene or olivine grain that displays a nearly centered biaxial optic axis figure. Save the image on the MAC and label with your name and identifying charaters (e.g., Miller OL-BOA) What is the approximate 2V? ________________ (2 pts) J. Granite Samples D-37. Select one of the sections from box 7 from last weeks lab and view with the instructor microscope. Scout for a biotite grain that displays a nearly centered acute bisectrix (Bxa) figure. Save the image on the MAC and label with your name and identifying charaters (e.g., Miller BIO-Bxa) What is the optic sign? ________________ (2 pts) Figure 1. Changes in optic sign resulting from changes in indices of refraction (γ, β, α). Positive - γ>>β>α Acute 2V angle bisected by Z axis Negative - γ>β>>α Acute 2V angle bisected by X axisD U - + Figure 4. Determination of the optic sign of a biaxial mineral from a centered acute bisectrix figure (Bxa) Figure 3. Estimation of 2V from the amount of separation of isogyres in a centered acute bisectrix figure (Bxa) Figure 2. Estimation of 2V from the curvature of isogyres in a biaxial optic axis (BOA) figure D D D D D D U U U U U