Optical Mineralogy Answer any additional questions. as many questions without us

Question

Optical Mineralogy Answer any additional questions. as many questions without using your book. Use your book to check your answers and answer 1. Using the information provided for nepheline, answer the following questions. a. Crystal system: Hexagonal b. Optical class: c. noo: 1.529 d. ne: 1.526 e. 8: f. Optic sign: L How did you determine this? g. What crystallographic axis coincides with the optic normal? X What type of interference figure would you see in a grain with: 8-0.003? i. 8 0.001? 1. sketch what this would look like, labelling the following: a. ? vibration direction b. ovibration direction c. a-axis d. c-axis e. optic axis/melatope t. Locations of color addition (A) and compensation (C) Sketch the associated indicatrix section and label the applicable vibration directions. 2.

Explanation / Answer

Nepheline:

Optic class- pyramidal

Birefringence- 0.003-0.004

Optic sign- negative

It is determined by using and accessory plate or gypsum which is inserted into the microscope at 45° from the cross wires. The interference colours are observed with and without the accessory plate. And the rise and fall of colour is observed in each quadrant. When the fast direction of the crystal coincide with the fast direction of the accessory plate , it causes rise of colour and sign is positive. For negative, the fast direction of the crystal coincide with the slow direction of the crystal and thus causes fall of colour.

For uniaxial crystals like that of nepheline optic axis is the vertical C axis and perpendicular to it lies horizontal axes, in hexagonal system a1,a2, a3.

Hornblende:

Optic class- 2/m

Birefringence in optic plane section will be 0.02-0.14.

Optic sign- +ve , because 2Vz =35° it means z direction is the acute bisectrix of the crystal, so the optic sign is positive.

Optic normal coincide with beta axis.

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